Table of Contents

Micronaut Core

Core components supporting the Micronaut Framework

Version: 4.7.6

1 Introduction

The Micronaut Framework is a modern, JVM-based, full stack Java framework designed for building modular, easily testable JVM applications with support for Java, Kotlin, and Groovy.

The Micronaut framework was originally created by a team who had also worked on the Grails framework. The Micronaut framework takes inspiration from lessons learned over the years building real-world applications from monoliths to microservices using Spring, Spring Boot and the Grails framework. The core team continues to develop and maintain the Micronaut project through the support of the Micronaut Foundation.

The Micronaut framework aims to provide all the tools necessary to build JVM applications including:

  • Dependency Injection and Inversion of Control (IoC)

  • Aspect Oriented Programming (AOP)

  • Sensible Defaults and Auto-Configuration

With the Micronaut framework you can build Message-Driven Applications, Command Line Applications, HTTP Servers and more whilst for Microservices in particular Micronaut also provides:

  • Distributed Configuration

  • Service Discovery

  • HTTP Routing

  • Client-Side Load Balancing

At the same time, the Micronaut framework aims to avoid the downsides of frameworks like Spring, Spring Boot and Grails by providing:

  • Fast startup time

  • Reduced memory footprint

  • Minimal use of reflection

  • Minimal use of proxies

  • No runtime bytecode generation

  • Easy Unit Testing

Historically, frameworks such as Spring and Grails were not designed to run in scenarios such as serverless functions, Android apps, or low memory footprint microservices. In contrast, the Micronaut framework is designed to be suitable for all of these scenarios.

This goal is achieved through the use of Java’s annotation processors, which are usable on any JVM language that supports them, as well as an HTTP Server (with several runtimes Netty, Jetty, Tomcat, Undertow…​) and an HTTP Client (with several runtimes Netty, Java HTTP Client, …​). To provide a similar programming model to Spring and Grails, these annotation processors precompile the necessary metadata to perform DI, define AOP proxies and configure your application to run in a low-memory environment.

Many APIs in the Micronaut framework are heavily inspired by Spring and Grails. This is by design, and helps bring developers up to speed quickly.

1.1 What's New?

4.2.0

Kotlin base version updated to 1.9.20

Kotlin has been updated to 1.9.20, which may cause issues when compiling or linking to Kotlin libraries.

4.1.0

4.0.0

Core Changes

Java 17 Baseline

Micronaut 4 now requires a minimum of Java 17 for building and running applications.

GraalVM 23 support, GraalVM Metadata Repository and Runtime Initialization

The GraalVM Metadata Repository in Micronaut’s Gradle and Maven plugins is now enabled by default and Micronaut has been altered to by default primarily initialize at runtime to ensure consistency in behaviour between JIT and Native applications.

Kotlin 1.8

Micronaut framework 4.0 supports Kotlin 1.8

Experimental Support for Kotlin Symbol Processing (KSP)

Micronaut framework has offered support for Kotlin via KAPT. With version 4.0, Micronaut framework supports Kotlin also via Kotlin Symbol Processing (KSP) API.

Apache Groovy 4.0

Micronaut framework 4.x supports Apache Groovy 4.0.

Improved Modularity

The core of Micronaut has been further refactored to improve modularity and reduce the footprint of a Micronaut application, including:

  • Third-party dependencies on SnakeYAML and Jackson Databind are now optional and can be removed if other implementations are present.

  • The runtime and compiler code has been split, allowing the removal of the re-packaging of ASM and Caffeine and reduction of the runtime footprint.

  • The built-in Validation, Retry, Service Discovery, HTTP Session and WebSocket features have been split into separate modules allowing removal of this functionality if not needed.

Completed javax to jakarta Migration

The remaining functionality depending on the javax specification has been migrated to jakarta including the validation module (for jakarta.validation) and support for Hibernate 6 (for jakarta.persistence).

Expression Language

A new fully compilation time, type-safe and reflection-free Expression Language has been added to the framework which unlocks a number of new possibilities (like conditional job scheduling). It is expected that submodules will adopt the new EL over time to add features and capabilities.

Injection of Maps

It is now possible to inject a java.util.Map of beans where the key is the bean name. The name of the bean is derived from the qualifier or (if not present) the simple name of the class.

Arbitrary Nesting of Configuration Properties

With Micronaut 4 it is now possible to arbitrarily nest @ConfigurationProperties and @EachProperty annotations allowing for more dynamic configuration possibilities.

Improved Error Messages for Missing Configuration

When a bean is not present due to missing configuration (such as a bean that uses @EachProperty) error messages have been improved to display the configuration that is required to activate the bean.

Improved Error Messages for Missing Beans

When a bean annotated with @EachProperty or @Bean is not found due to missing configuration an error is thrown showing the configuration prefix necessary to resolve the issue.

Tracking of Disabled Beans

Beans that are disabled via Bean Requirements are now tracked and an appropriate error thrown if a bean has been disabled.

The disabled beans are also now visible via the Beans Endpoint in the Management module aiding in understanding the state of your application configuration.

HTTP Changes

Initial Support for Virtual Threads (Loom)

Preview support for Virtual Threads has been added. When using JDK 19 or above with preview features enabled you can off load processing to a virtual thread pool.

Experimental Support for HTTP/3

Experimental Support for HTTP/3 via the Netty incubator project has been added.

Experimental Support for io_uring

Experimental support for io_uring via the Netty incubator project has been added.

Rewritten HTTP layer

The HTTP layer has been rewritten to improve performance and reduce the presence of reactive stack frames if reactive is not used (such as with Virtual threads).

Annotation-Based HTTP Filters

Context Propagation API

Micronaut Framework 4 introduces a new Context Propagation API, which aims to simplify reactor instrumentation, avoid thread-local usage, and integrate idiomatically with Kotlin Coroutines.

1.2 Upgrading your Micronaut Application

Upgrading between Micronaut Framework versions

Check Micronaut Upgrade documentation to help you upgrade your Micronaut applications.

Breaking Changes

Review the section on Breaking Changes and update your affected application code.

2 Quick Start

The following sections walk you through a Quick Start on how to use the Micronaut framework to set up a basic "Hello World" application.

Before getting started ensure you have a Java 8 or higher JDK installed, and it is recommended that you use a suitable IDE such as IntelliJ IDEA.

2.1 Install the CLI

The Micronaut CLI is an optional but convenient way to create Micronaut applications. The best way to install Micronaut CLI on Unix systems is with SDKMAN which greatly simplifies installing and managing multiple Micronaut versions.

To see all available installation methods, check the Micronaut Starter documentation.

2.2 Creating a Server Application

Using the Micronaut CLI you can create a new Micronaut application in either Groovy, Java, or Kotlin (the default is Java).

The following command creates a new "Hello World" server application in Java with a Gradle build:

Applications generated via our CLI include Gradle or Maven wrappers, so it is not even necessary to have Gradle or Maven installed on your machine to begin running the applications. Simply use the mvnw or gradlew command, as explained further below.
$ mn create-app hello-world
Supply --build maven to create a Maven-based build instead

If you don’t have the CLI installed then you can also create the same application by visiting Micronaut Launch and clicking the "Generate Project" button or by using the following curl command on Unix systems:

curl https://launch.micronaut.io/hello-world.zip -o hello-world.zip
unzip hello-world.zip
cd hello-world
Add ?build=maven to the URL passed to curl to generate a Maven project.

The previous steps created a new Java application in a directory called hello-world featuring a Gradle build. You can run the application with ./gradlew run:

$ ./gradlew run
> Task :run
[main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 540ms. Server Running: http://localhost:28933

If you have created a Maven-based project, use ./mvnw mn:run instead.

For Windows the ./ before commands is not needed

By default, the Micronaut HTTP server is configured to run on port 8080. See the section Running Server on a Specific Port for more options.

To create a service that responds to "Hello World" you first need a controller. The following is an example:

import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;

@Controller("/hello") // (1)
public class HelloController {

    @Get(produces = MediaType.TEXT_PLAIN) // (2)
    public String index() {
        return "Hello World"; // (3)
    }
}
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller('/hello') // (1)
class HelloController {

    @Get(produces = MediaType.TEXT_PLAIN) // (2)
    String index() {
        'Hello World' // (3)
    }
}
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/hello") // (1)
class HelloController {

    @Get(produces = [MediaType.TEXT_PLAIN]) // (2)
    fun index(): String {
        return "Hello World" // (3)
    }
}
1 The @Controller annotation defines the class as a controller mapped to the path /hello
2 The @Get annotation maps the index method to all requests that use an HTTP GET
3 A String "Hello World" is returned as the response

If you use Java, place the previous file in src/main/java/hello/world.
If you use Groovy, place the previous file in src/main/groovy/hello/world.
If you use Kotlin, place the previous file in src/main/kotlin/hello/world.

If you start the application and send a GET request to the /hello URI, the text "Hello World" is returned:

$ curl http://localhost:8080/hello
Hello World
See the guide for Creating your First Micronaut Application to learn more.

2.3 Setting up an IDE

The application created in the previous section contains a main class located in src/main/java that looks like the following:

import io.micronaut.runtime.Micronaut;

public class Application {

    public static void main(String[] args) {
        Micronaut.run(Application.class);
    }
}
import io.micronaut.runtime.Micronaut

class Application {

    static void main(String... args) {
        Micronaut.run Application
    }
}
import io.micronaut.runtime.Micronaut

object Application {

    @JvmStatic
    fun main(args: Array<String>) {
        Micronaut.run(Application.javaClass)
    }
}

This is the class that is run when starting the application via Gradle/Maven or via deployment. You can also run the main class directly within your IDE.

2.3.1 IntelliJ IDEA

To import a Micronaut project into IntelliJ IDEA, open the build.gradle or pom.xml file and follow the instructions to import the project.

For IntelliJ IDEA, if you plan to use the IntelliJ compiler, enable annotation processing under "Build, Execution, Deployment → Compiler → Annotation Processors" by ticking the "Enable annotation processing" checkbox:

Intellij Settings

Once you have enabled annotation processing in IntelliJ you can run the application and tests directly within the IDE without the need of an external build tool such as Gradle or Maven.

2.3.2 Visual Studio Code

The Micronaut framework can easily be set up within Visual Studio Code by installing the following two extensions:

These extensions can be installed by clicking on the Install button in the banner of the pages linked above, or, by searching for the extensions within VS Code.

Installing Micronaut tools

Once they are installed they will give you access to a host of Micronaut specific features such as:

Once the extensions are installed just type code . in any Micronaut project directory and the project will be opened within VS Code.

For macOS, you need to install the code command by following these instructions.

2.3.3 Eclipse

To use Eclipse IDE, it is recommended you import your Micronaut project into Eclipse using either Gradle BuildShip for Gradle or M2Eclipse for Maven.

The Micronaut framework requires Eclipse IDE 4.9 or higher

Eclipse and Gradle

Once you have set up Eclipse 4.9 or higher with Gradle BuildShip, first run the gradle eclipse task from the root of your project, then import the project by selecting File → Import and choosing Gradle → Existing Gradle Project and navigating to the root directory of your project (where the build.gradle file is located).

Eclipse and Maven

For Eclipse 4.9 and above with Maven you need the following Eclipse plugins:

Once these are installed, import the project by selecting File → Import and choosing Maven → Existing Maven Project and navigating to the root directory of your project (where the pom.xml file is located).

Then enable annotation processing by opening Eclipse → Preferences and navigating to Maven → Annotation Processing and selecting the option Automatically configure JDT APT.

2.3.4 Apache NetBeans

Apache NetBeans can open Maven and Gradle projects out of the box.

Make sure that the Java Web and EE feature is enabled at Tools → Plugins → Installed, in order to have additional support for Micronaut, like code completion for configuration and data elements.

NetBeans Plugins

2.4 Creating a Client

As mentioned previously, the Micronaut framework includes both an HTTP server and an HTTP client. A low-level HTTP client is provided which you can use to test the HelloController created in the previous section.

Testing Hello World
import io.micronaut.http.HttpRequest;
import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.runtime.server.EmbeddedServer;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;

import jakarta.inject.Inject;

import static org.junit.jupiter.api.Assertions.assertEquals;

@MicronautTest
public class HelloControllerSpec {

    @Inject
    EmbeddedServer server; // (1)

    @Inject
    @Client("/")
    HttpClient client; // (2)

    @Test
    void testHelloWorldResponse() {
        String response = client.toBlocking() // (3)
                .retrieve(HttpRequest.GET("/hello"));
        assertEquals("Hello World", response); // (4)
    }
}
Testing Hello World
import io.micronaut.http.HttpRequest
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import spock.lang.Specification

import jakarta.inject.Inject

@MicronautTest
class HelloControllerSpec extends Specification {

    @Inject
    EmbeddedServer embeddedServer // (1)

    @Inject
    @Client("/")
    HttpClient client // (2)

    void "test hello world response"() {
        expect:
            client.toBlocking() // (3)
                    .retrieve(HttpRequest.GET('/hello')) == "Hello World" // (4)
    }
}
Testing Hello World
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import jakarta.inject.Inject

@MicronautTest
class HelloControllerSpec {

    @Inject
    lateinit var server: EmbeddedServer // (1)

    @Inject
    @field:Client("/")
    lateinit var client: HttpClient // (2)

    @Test
    fun testHelloWorldResponse() {
        val rsp: String = client.toBlocking() // (3)
                .retrieve("/hello")
        assertEquals("Hello World", rsp) // (4)
    }
}
1 The EmbeddedServer is configured as a shared test field
2 A HttpClient instance shared field is also defined
3 The test uses the toBlocking() method to make a blocking call
4 The retrieve method returns the controller response as a String

In addition to a low-level client, the Micronaut framework features a declarative, compile-time HTTP client, powered by the Client annotation.

To create a client, create an interface annotated with @Client, for example:

src/main/java/hello/world/HelloClient.java
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.client.annotation.Client;
import org.reactivestreams.Publisher;
import io.micronaut.core.async.annotation.SingleResult;

@Client("/hello") // (1)
public interface HelloClient {

    @Get(consumes = MediaType.TEXT_PLAIN) // (2)
    @SingleResult
    Publisher<String> hello(); // (3)
}
src/main/java/hello/world/HelloClient.java
import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import org.reactivestreams.Publisher
import io.micronaut.core.async.annotation.SingleResult

@Client("/hello") // (1)
interface HelloClient {

    @Get(consumes = MediaType.TEXT_PLAIN) // (2)
    @SingleResult
    Publisher<String> hello() // (3)
}
src/main/java/hello/world/HelloClient.java
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import io.micronaut.core.async.annotation.SingleResult
import org.reactivestreams.Publisher

@Client("/hello") // (1)
interface HelloClient {

    @Get(consumes = [MediaType.TEXT_PLAIN]) // (2)
    @SingleResult
    fun hello(): Publisher<String>  // (3)
}
1 The @Client annotation is used with a value that is a relative path to the current server
2 The same @Get annotation used on the server is used to define the client mapping
3 A Publisher annotated with SingleResult is returned with the value read from the server

To test the HelloClient, retrieve it from the ApplicationContext associated with the server:

Testing HelloClient
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;

import jakarta.inject.Inject;
import reactor.core.publisher.Mono;

import static org.junit.jupiter.api.Assertions.assertEquals;

@MicronautTest // (1)
public class HelloClientSpec {

    @Inject
    HelloClient client; // (2)

    @Test
    public void testHelloWorldResponse() {
        assertEquals("Hello World", Mono.from(client.hello()).block());// (3)
    }
}
Testing HelloClient
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import reactor.core.publisher.Mono
import spock.lang.Specification

import jakarta.inject.Inject

@MicronautTest // (1)
class HelloClientSpec extends Specification {

    @Inject HelloClient client // (2)

    void "test hello world response"() {
        expect:
        Mono.from(client.hello()).block() == "Hello World" // (3)
    }

}
Testing HelloClient
import io.micronaut.context.annotation.Property
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import jakarta.inject.Inject
import reactor.core.publisher.Mono


@MicronautTest // (1)
class HelloClientSpec {

    @Inject
    lateinit var client: HelloClient // (2)

    @Test
    fun testHelloWorldResponse() {
        assertEquals("Hello World", Mono.from(client.hello()).block())// (3)
    }
}
1 The @MicronautTest annotation defines the test
2 The HelloClient is injected from the ApplicationContext
3 The client is invoked using the Project Reactor Mono::block method

The Client annotation produces an implementation automatically for you at compile time without the using proxies or runtime reflection.

The Client annotation is very flexible. See the section on the Micronaut HTTP Client for more information.

2.5 Deploying the Application

To deploy a Micronaut application you create an executable JAR file by running ./gradlew assemble or ./mvnw package.

The constructed JAR file can then be executed with java -jar. For example:

$ java -jar build/libs/hello-world-0.1-all.jar

if building with Gradle, or

$ java -jar target/hello-world.jar

if building with Maven.

The executable JAR can be run locally, or deployed to a virtual machine or managed Cloud service that supports executable JARs.

To publish a layered application to a Docker container registry, configure your Docker image name in build.gradle for Gradle:

dockerBuild {
    images = ["[REPO_URL]/[NAMESPACE]/my-image:$project.version"]
}

Then use dockerPush to push a built image of the application:

$ ./gradlew dockerPush

For Maven, define the following plugin in your POM:

<plugin>
  <groupId>com.google.cloud.tools</groupId>
  <artifactId>jib-maven-plugin</artifactId>
  <configuration>
    <to>
      <image>docker.io/my-company/my-image:${project.version}</image>
    </to>
  </configuration>
</plugin>

Then invoke the deploy lifecycle phase specifying the packaging type as either docker or docker-native:

$ ./mvnw deploy -Dpackaging=docker

Deployment Guides

See the following guides to learn more about deploying Micronaut Applications:

3 Inversion of Control

Unlike other frameworks which rely on runtime reflection and proxies, the Micronaut framework uses compile time data to implement dependency injection.

This is a similar approach taken by tools such as Google Dagger, which is designed primarily with Android in mind. Micronaut, on the other hand, is designed for building server-side microservices and provides many of the same tools and utilities as other frameworks but without using reflection or caching excessive amounts of reflection metadata.

The goals of the Micronaut IoC container are summarized as:

  • Use reflection as a last resort

  • Avoid proxies

  • Optimize start-up time

  • Reduce memory footprint

  • Provide clear, understandable error handling

Note that the IoC part of the Micronaut framework can be used completely independently of Micronaut for whatever application type you wish to build.

To do so, configure your build to include the micronaut-inject-java dependency as an annotation processor.

The easiest way to do this is with Micronaut’s Gradle or Maven plugins. For example with Gradle:

Configuring Gradle
plugins {
    id 'io.micronaut.library' version '1.3.2' (1)
}

version "0.1"
group "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version = "4.7.6" (2)
}
1 Define the Micronaut Library plugin
2 Specify the Micronaut framework version to use

The entry point for IoC is then the ApplicationContext interface, which includes a run method. The following example demonstrates using it:

Running the ApplicationContext
try (ApplicationContext context = ApplicationContext.run()) { (1)
    MyBean myBean = context.getBean(MyBean.class); (2)
    // do something with your bean
}
1 Run the ApplicationContext
2 Retrieve a bean from the ApplicationContext
The example uses Java try-with-resources syntax to ensure the ApplicationContext is cleanly shutdown when the application exits.

3.1 Defining Beans

A bean is an object whose lifecycle is managed by the Micronaut IoC container. That lifecycle may include creation, execution, and destruction. Micronaut implements the JSR-330 (jakarta.inject) - Dependency Injection for Java specification, hence to use Micronaut you simply use the annotations provided by jakarta.inject.

The following is a simple example:

public interface Engine { // (1)
    int getCylinders();
    String start();
}

@Singleton// (2)
public class V8Engine implements Engine {
    private int cylinders = 8;

    @Override
    public String start() {
        return "Starting V8";
    }

    @Override
    public int getCylinders() {
        return cylinders;
    }

    public void setCylinders(int cylinders) {
        this.cylinders = cylinders;
    }
}

@Singleton
public class Vehicle {
    private final Engine engine;

    public Vehicle(Engine engine) {// (3)
        this.engine = engine;
    }

    public String start() {
        return engine.start();
    }
}
interface Engine { // (1)
    int getCylinders()
    String start()
}

@Singleton // (2)
class V8Engine implements Engine {
    int cylinders = 8

    @Override
    String start() {
        "Starting V8"
    }
}

@Singleton
class Vehicle {
    final Engine engine

    Vehicle(Engine engine) { // (3)
        this.engine = engine
    }

    String start() {
        engine.start()
    }
}
interface Engine {
    // (1)
    val cylinders: Int

    fun start(): String
}

@Singleton// (2)
class V8Engine : Engine {

    override var cylinders = 8

    override fun start(): String {
        return "Starting V8"
    }
}

@Singleton
class Vehicle(private val engine: Engine) { // (3)
    fun start(): String {
        return engine.start()
    }
}
1 A common Engine interface is defined
2 A V8Engine implementation is defined and marked with Singleton scope
3 The Engine is injected via constructor injection

To perform dependency injection, run the BeanContext using the run() method and lookup a bean using getBean(Class), as per the following example:

final ApplicationContext context = ApplicationContext.run();
Vehicle vehicle = context.getBean(Vehicle.class);
System.out.println(vehicle.start());
ApplicationContext context = ApplicationContext.run()
Vehicle vehicle = context.getBean(Vehicle)
println vehicle.start()
val context = ApplicationContext.run()
val vehicle = context.getBean(Vehicle::class.java)
println(vehicle.start())

The Micronaut framework automatically discovers dependency injection metadata on the classpath and wires the beans together according to injection points you define.

3.2 How Does it Work?

At this point, you may be wondering how Micronaut framework performs the above dependency injection without requiring reflection.

The key is a set of AST transformations (for Groovy) and annotation processors (for Java) that generate classes that implement the BeanDefinition interface.

Micronaut framework uses the ASM bytecode library to generate classes, and because Micronaut knows ahead of time the injection points, there is no need to scan all methods, fields, constructors, etc. at runtime like other frameworks such as Spring do.

Also, since reflection is not used when constructing the bean, the JVM can inline and optimize the code far better, resulting in better runtime performance and reduced memory consumption. This is particularly important for non-singleton scopes where application performance depends on bean creation performance.

In addition, with Micronaut framework your application startup time and memory consumption are not affected by the size of your codebase in the same way as with a framework that uses reflection. Reflection-based IoC frameworks load and cache reflection data for every single field, method, and constructor in your code. Thus, as your code grows in size so do your memory requirements, whilst with Micronaut this is not the case.

3.3 The BeanContext

The BeanContext is a container object for all your bean definitions (it also implements BeanDefinitionRegistry).

It is also the point of initialization for Micronaut. Generally speaking however, you don’t interact directly with the BeanContext API and can simply use jakarta.inject annotations and the annotations in the io.micronaut.context.annotation package for your dependency injection needs.

3.4 Dependency Injection Types

The Micronaut framework supports the following types of dependency injection:

  • Constructor injection (with an access level of public, protected, or default - and in case of multiple constructors, you can specify the one to be chosen, annotating it with @Inject)

  • Field injection

  • JavaBean property injection

  • Method parameter injection

Classes or particular fields, methods can be excluded by adding an annotation @Vetoed
See the guide for Micronaut Dependency Injection Types to learn more.

3.4.1 Constructor Injection

Constructor injection is when dependencies are injected into the constructor for a type.

Constructor injection is the preferred and recommended injection type because Constructor injection:

  • Allows immutable types

  • Doesn’t require an additional annotation

  • Is less likely to result in a NullPointerException

  • More clearly expresses the dependencies of a particular type in one place.

The example in the next section uses constructor injection. Note that if you have multiple constructors you can disambiguate which constructor to invoke with the jakarta.inject.Inject annotation or the @Creator annotation:

Example of Constructor Injection
package io.micronaut.docs.ioc.injection.ctor;

import io.micronaut.core.annotation.Creator;
import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
class Vehicle {
    private final Engine engine;

    @Inject // (1)
    Vehicle(Engine engine) {
        this.engine = engine;
    }

    Vehicle() {
        this.engine = Engine.create(6);
    }

    void start() {
        engine.start();
    }
}

@Singleton
record Engine(int cylinders) {

    @Creator // (2)
    static Engine getDefault() {
        return new Engine(8);
    }

    static Engine create(int cylinders) {
        return new Engine(cylinders);
    }

    void start() {
        System.out.println("Vrooom! " + cylinders);
    }
}
Example of Constructor Injection
package io.micronaut.docs.ioc.injection.ctor

import io.micronaut.core.annotation.Creator
import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class Vehicle {
    private final Engine engine

    @Inject // (1)
    Vehicle(Engine engine) {
        this.engine = engine
    }

    Vehicle() {
        this.engine = Engine.create(6)
    }

    void start() {
        engine.start()
    }
}

@Singleton
record Engine(int cylinders) {

    @Creator // (2)
    static Engine getDefault() {
        return new Engine(8)
    }

    static Engine create(int cylinders) {
        return new Engine(cylinders)
    }

    void start() {
        println("Vrooom! $cylinders")
    }
}
Example of Constructor Injection
package io.micronaut.docs.ioc.injection.ctor

import io.micronaut.core.annotation.Creator
import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class Vehicle {
    private val engine: Engine

    @Inject // (1)
    constructor(engine: Engine) {
        this.engine = engine
    }

    constructor() {
        this.engine = Engine.create(6)
    }

    fun start() {
        engine.start()
    }
}

@Singleton
data class Engine(val cylinders: Int) {
    fun start() {
        println("Vrooom! $cylinders")
    }

    companion object {
        @Creator // (2)
        fun getDefault() : Engine
             = Engine(8)

        fun create(cylinders: Int): Engine {
            return Engine(cylinders)
        }
    }
}
1 Multiple constructors are defined so @Inject is used to select the correct constructor.
2 The @Creator annotation can also be used to select a static factory method to use as a constructor.

In the above example retrieving the Vehicle type from the BeanContext will result in calling the Vehicle(Engine engine) constructor which will in turn resolve the Engine using the getDefault() method since it is annotated with @Creator.

If no @Inject or @Creator is specified Micronaut will try to locate the first public constructor in the class otherwise a compilation error will occur.

If there are multiple possible candidates for a particular constructor argument a qualifier can be specified (such as jakarta.inject.Named) to disambiguate the injection. If it is not possible to disambiguate then the result will be a NonUniqueBeanException. See the Qualifiers section for more information.

If you use @Inject on a private constructor then the type will be instantiated via the Java reflection API which is not recommended.

3.4.2 Field Injection

You can inject non-final Java fields by annotating the field with jakarta.inject.Inject, for example:

Example of Field Injection
package io.micronaut.docs.ioc.injection.field;

import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
class Vehicle {
    @Inject Engine engine; // (1)

    void start() {
        engine.start();
    }
}

@Singleton
class Engine {
    void start() {
        System.out.println("Vrooom!" );
    }
}
Example of Field Injection
package io.micronaut.docs.ioc.injection.field

import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class Vehicle {
    @Inject Engine engine // (1)

    void start() {
        engine.start()
    }
}

@Singleton
class Engine {
    void start() {
        println("Vrooom!" )
    }
}
Example of Field Injection
package io.micronaut.docs.ioc.injection.field

import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
internal class Vehicle {
    @Inject
    var engine: Engine? = null // (1)

    fun start() {
        engine!!.start()
    }
}

@Singleton
class Engine {
    fun start() {
        println("Vrooom!")
    }
}
1 The field is annotated with jakarta.inject.Inject.
2 Note that for Kotlin instead of an optional type (a type ending with ?) you can use lateinit var

Trying to inject a field that is declared final will result in a compilation error. Field injection should be seen as inferior to constructor injection as explained in the previous sections since it can result in the code being less-structured, harder to read and harder to test.

If the field is private scope or inaccessible then the field will be injected using the Java reflection API which is not recommended.

3.4.3 Method Injection

You can inject methods by annotating the method with jakarta.inject.Inject. For each argument of the method Micronaut will attempt to resolve the method argument as a bean. If any of the methods are not resolvable a NoSuchBeanException will be thrown.

Example of Method Injection
package io.micronaut.docs.ioc.injection.method;

import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
class Vehicle {
    private Engine engine;

    @Inject // (1)
    void initialize(Engine engine) {
        this.engine = engine;
    }

    void start() {
        engine.start();
    }
}

@Singleton
class Engine {
    void start() {
        System.out.println("Vrooom!" );
    }
}
Example of Method Injection
package io.micronaut.docs.ioc.injection.method

import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class Vehicle {
    private Engine engine

    @Inject // (1)
    void initialize(Engine engine) {
        this.engine = engine
    }

    void start() {
        engine.start()
    }
}

@Singleton
class Engine {
    void start() {
        println("Vrooom!")
    }
}
Example of Method Injection
package io.micronaut.docs.ioc.injection.method

import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
internal class Vehicle {
    private var engine: Engine? = null

    @Inject
    fun initialize(engine: Engine) {
        this.engine = engine
    }

    fun start() {
        engine!!.start()
    }
}

@Singleton
internal class Engine {
    fun start() {
        println("Vrooom!")
    }
}
1 The method is annotated with jakarta.inject.Inject and will therefore be injected
If the method is private scope or inaccessible then the method will be injected using the Java reflection API which is not recommended.

Method injection can be useful if you need post construction initializers, however in general should be avoided in favour of constructor injection where possible.

3.4.4 Nullable Injection

Occasionally it is desirable for injection to be optional (ie. not fail with a NoSuchBeanException if there is no candidate bean available).

For example if you are trying to build an extensible system where a default implementation is provided by the type but consumers of your API can provide a bean that, if available, will be injected.

One way to make injection optional is to annotate the injected type with @Nullable which will result in null being injected by the framework if the bean is unavailable:

Example of Injecting null
package io.micronaut.docs.ioc.injection.nullable;

import io.micronaut.core.annotation.Nullable;
import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
class Vehicle {
    private final Engine engine;

    Vehicle(@Nullable Engine engine) { // (1)
        this.engine = engine != null ? engine : Engine.create(6); // (2)
    }
    void start() {
        engine.start();
    }

    public Engine getEngine() {
        return engine;
    }
}

record Engine(int cylinders) {

    static Engine create(int cylinders) {
        return new Engine(cylinders);
    }

    void start() {
        System.out.println("Vrooom! " + cylinders);
    }
}
Example of Injecting null
package io.micronaut.docs.ioc.injection.nullable

import io.micronaut.core.annotation.Nullable
import jakarta.inject.Singleton

@Singleton
class Vehicle {
    private final Engine engine

    Vehicle(@Nullable Engine engine) { // (1)
        this.engine = engine ?: Engine.create(6) // (2)
    }
    void start() {
        engine.start()
    }
}

record Engine(int cylinders) {

    static Engine create(int cylinders) {
        return new Engine(cylinders)
    }

    void start() {
        println("Vrooom! $cylinders")
    }
}
Example of Injecting null
package io.micronaut.docs.ioc.injection.nullable

import jakarta.inject.Singleton

@Singleton
class Vehicle(engine: Engine?) { // (1)
    private val engine = engine ?: Engine.create(6) // (2)
    fun start() {
        engine.start()
    }
}

data class Engine(val cylinders: Int) {
    fun start() {
        println("Vrooom! $cylinders")
    }

    companion object {
        fun create(cylinders: Int): Engine {
            return Engine(cylinders)
        }
    }
}
1 Here the constructor argument is annotated with @Nullable
2 Since there is no bean available null is injected and the code has to handle the possibility that the argument could be `null.

Using @Nullable has the following considerations:

  • Can be used with any of the injection types (constructor, method or field injection)

  • Somewhere the code has to handle what happens if null is injected, for constructors this is easy since the constructor can handle the null, but for fields and methods a @PostConstruct method would need to be implemented to handle null if null is not desirable.

  • Finally, @Nullable cannot be used on primitive types like int, long etc. when using configuration injection. To handle primitives you need to specify @Bindable(defaultValue="..") and provide a default value.

3.5 Injectable Container Types

In addition to being able to inject beans, Micronaut framework natively supports injecting the following types:

Table 1. Injectable Container Types
Type Description Example

java.util.Optional

An Optional of a bean. empty() is injected if the bean doesn’t exist

Optional<Engine>

java.util.Collection

An Collection or subtype of Collection (e.g. List, Set, etc.)

Collection<Engine>

java.util.Map

An Map or subtype of Map (e.g. LinkedHashMap, TreeMap, etc.) where the key is the qualifier

Map<String, Engine>

java.util.stream.Stream

A lazy Stream of beans

Stream<Engine>

Array

A native array of beans of a given type

Engine[]

Provider

A jakarta.inject.Provider if a circular dependency requires it, or to instantiate a prototype for each get call.

Provider<Engine>

BeanProvider

A io.micronaut.context.BeanProvider if a circular dependency requires it or to instantiate a prototype for each get call.

BeanProvider<Engine>

There are 3 different provider types supported, however the BeanProvider is the one we suggest to use.

When injecting a java.lang.Collection, or java.util.stream.Stream, Array of beans into a bean matching the injection type, then the owning bean will not be a member of the injected collection. A common pattern demonstrating this is aggregation. For example:

@Singleton
class AggregateEngine implements Engine {
  @Inject
  List<Engine> engines;

  @Override
  public void start() {
    engines.forEach(Engine::start);
  }

  ...
}

In this example, the injected member variable engines will not contain an instance of AggregateEngine

A prototype bean will have one instance created per place the bean is injected. When a prototype bean is injected as a provider, each call to get() creates a new instance.

Collection Ordering

When injecting a collection of beans, they are not ordered by default. Implement the Ordered interface to inject an ordered collection. If the requested bean type does not implement Ordered, Micronaut framework searches for the @Order annotation on beans.

The @Order annotation is especially useful for ordering beans created by factories where the bean type is a class in a third-party library. In this example, both LowRateLimit and HighRateLimit implement the RateLimit interface.

Factory with @Order
import io.micronaut.context.annotation.Factory;
import io.micronaut.core.annotation.Order;

import jakarta.inject.Singleton;
import java.time.Duration;

@Factory
public class RateLimitsFactory {

    @Singleton
    @Order(20)
    LowRateLimit rateLimit2() {
        return new LowRateLimit(Duration.ofMinutes(50), 100);
    }

    @Singleton
    @Order(10)
    HighRateLimit rateLimit1() {
        return new HighRateLimit(Duration.ofMinutes(50), 1000);
    }
}
Factory with @Order
import io.micronaut.context.annotation.Factory
import io.micronaut.core.annotation.Order

import jakarta.inject.Singleton
import java.time.Duration

@Factory
class RateLimitsFactory {

    @Singleton
    @Order(20)
    LowRateLimit rateLimit2() {
        new LowRateLimit(Duration.ofMinutes(50), 100);
    }

    @Singleton
    @Order(10)
    HighRateLimit rateLimit1() {
        new HighRateLimit(Duration.ofMinutes(50), 1000);
    }
}
Factory with @Order
import io.micronaut.context.annotation.Factory
import io.micronaut.core.annotation.Order
import java.time.Duration
import jakarta.inject.Singleton

@Factory
class RateLimitsFactory {

    @Singleton
    @Order(20)
    fun rateLimit2(): LowRateLimit {
        return LowRateLimit(Duration.ofMinutes(50), 100)
    }

    @Singleton
    @Order(10)
    fun rateLimit1(): HighRateLimit {
        return HighRateLimit(Duration.ofMinutes(50), 1000)
    }
}

When a collection of RateLimit beans are requested from the context, they are returned in ascending order based on the value in the annotation.

Injecting a Bean by Order

When injecting a single instance of a bean the @Order annotation can also be used to define which bean has the highest precedence and hence should be injected.

The Ordered interface is not taken into account when selecting a single instance as this would require instantiating the bean to resolve the order.

3.6 Bean Qualifiers

If you have multiple possible implementations for a given interface to inject, you need to use a qualifier.

Once again Micronaut framework leverages JSR-330 and the Qualifier and Named annotations to support this use case.

Qualifying By Name

To qualify by name, use the Named annotation. For example, consider the following classes:

public interface Engine { // (1)
    int getCylinders();
    String start();
}

@Singleton
public class V6Engine implements Engine {  // (2)
    @Override
    public String start() {
        return "Starting V6";
    }

    @Override
    public int getCylinders() {
        return 6;
    }
}

@Singleton
public class V8Engine implements Engine {  // (3)
    @Override
    public String start() {
        return "Starting V8";
    }

    @Override
    public int getCylinders() {
        return 8;
    }

}

@Singleton
public class Vehicle {
    private final Engine engine;

    @Inject
    public Vehicle(@Named("v8") Engine engine) {// (4)
        this.engine = engine;
    }

    public String start() {
        return engine.start();// (5)
    }
}
interface Engine { // (1)
    int getCylinders()
    String start()
}

@Singleton
class V6Engine implements Engine { // (2)
    int cylinders = 6

    @Override
    String start() {
        "Starting V6"
    }
}

@Singleton
class V8Engine implements Engine { // (3)
    int cylinders = 8

    @Override
    String start() {
        "Starting V8"
    }
}

@Singleton
class Vehicle {
    final Engine engine

    @Inject Vehicle(@Named('v8') Engine engine) { // (4)
        this.engine = engine
    }

    String start() {
        engine.start() // (5)
    }
}
interface Engine { // (1)
    val cylinders: Int
    fun start(): String
}

@Singleton
class V6Engine : Engine {  // (2)

    override var cylinders: Int = 6

    override fun start(): String {
        return "Starting V6"
    }
}

@Singleton
class V8Engine : Engine {

    override var cylinders: Int = 8

    override fun start(): String {
        return "Starting V8"
    }

}

@Singleton
class Vehicle @Inject
constructor(@param:Named("v8") private val engine: Engine) { // (4)

    fun start(): String {
        return engine.start() // (5)
    }
}
1 The Engine interface defines the common contract
2 The V6Engine class is the first implementation
3 The V8Engine class is the second implementation
4 The jakarta.inject.Named annotation indicates that the V8Engine implementation is required
5 Calling the start method prints: "Starting V8"

Micronaut framework is capable of injecting V8Engine in the previous example, because:

@Named qualifier value (v8) + type being injected simple name (Engine) == (case-insensitive) == The simple name of a bean of type Engine (V8Engine)

You can also declare @Named at the class level of a bean to explicitly define the name of the bean.

Qualifying By Annotation

In addition to being able to qualify by name, you can build your own qualifiers using the Qualifier annotation. For example, consider the following annotation:

import jakarta.inject.Qualifier;
import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Qualifier
@Retention(RUNTIME)
public @interface V8 {
}
import jakarta.inject.Qualifier
import java.lang.annotation.Retention

import static java.lang.annotation.RetentionPolicy.RUNTIME

@Qualifier
@Retention(RUNTIME)
@interface V8 {
}
import jakarta.inject.Qualifier
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy.RUNTIME

@Qualifier
@Retention(RUNTIME)
annotation class V8

The above annotation is itself annotated with the @Qualifier annotation to designate it as a qualifier. You can then use the annotation at any injection point in your code. For example:

@Inject Vehicle(@V8 Engine engine) {
    this.engine = engine;
}
@Inject Vehicle(@V8 Engine engine) {
    this.engine = engine
}
@Inject constructor(@V8 val engine: Engine) {

Qualifying By Annotation Members

Since Micronaut framework 3.0, annotation qualifiers can also use annotation members to resolve the correct bean to inject. For example, consider the following annotation:

import io.micronaut.context.annotation.NonBinding;
import jakarta.inject.Qualifier;
import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Qualifier // (1)
@Retention(RUNTIME)
public @interface Cylinders {
    int value();

    @NonBinding // (2)
    String description() default "";
}
import io.micronaut.context.annotation.NonBinding
import jakarta.inject.Qualifier
import java.lang.annotation.Retention

import static java.lang.annotation.RetentionPolicy.RUNTIME

@Qualifier // (1)
@Retention(RUNTIME)
@interface Cylinders {
    int value();

    @NonBinding // (2)
    String description() default "";
}
import io.micronaut.context.annotation.NonBinding
import jakarta.inject.Qualifier
import kotlin.annotation.Retention

@Qualifier // (1)
@Retention(AnnotationRetention.RUNTIME)
annotation class Cylinders(
    val value: Int,
    @get:NonBinding // (2)
    val description: String = ""
)
1 The @Cylinders annotation is meta-annotated with @Qualifier
2 The annotation has two members. The @NonBinding annotation is used to exclude the description member from being considered during dependency resolution.

You can then use the @Cylinders annotation on any bean and the members that are not annotated with @NonBinding are considered during dependency resolution:

@Singleton
@Cylinders(value = 6, description = "6-cylinder V6 engine")  // (1)
public class V6Engine implements Engine { // (2)

    @Override
    public int getCylinders() {
        return 6;
    }

    @Override
    public String start() {
        return "Starting V6";
    }
}
@Singleton
@Cylinders(value = 6, description = "6-cylinder V6 engine")  // (1)
class V6Engine implements Engine { // (2)

    @Override
    int getCylinders() {
        return 6
    }

    @Override
    String start() {
        return "Starting V6"
    }
}
@Singleton
@Cylinders(value = 6, description = "6-cylinder V6 engine") // (1)
class V6Engine : Engine { // (2)
    // (2)
    override val cylinders: Int
        get() = 6

    override fun start(): String {
        return "Starting V6"
    }
}
1 Here the value member is set to 6 for the V6Engine type
2 The class implements an Engine interface
@Singleton
@Cylinders(value = 8, description = "8-cylinder V8 engine") // (1)
public class V8Engine implements Engine { // (2)
    @Override
    public int getCylinders() {
        return 8;
    }

    @Override
    public String start() {
        return "Starting V8";
    }
}
@Singleton
@Cylinders(value = 8, description = "8-cylinder V8 engine") // (1)
class V8Engine implements Engine { // (2)
    @Override
    int getCylinders() {
        return 8
    }

    @Override
    String start() {
        return "Starting V8"
    }
}
@Singleton
@Cylinders(value = 8, description = "8-cylinder V8 engine") // (1)
class V8Engine : Engine { // (2)
    override val cylinders: Int
        get() = 8

    override fun start(): String {
        return "Starting V8"
    }
}
1 Here the value member is set to 8 for the V8Engine type
2 The class implements an Engine interface

You can then use the @Cylinders qualifier on any injection point to select the correct bean to inject. For example:

@Inject Vehicle(@Cylinders(8) Engine engine) {
    this.engine = engine;
}
@Inject Vehicle(@Cylinders(8) Engine engine) {
    this.engine = engine
}
@Singleton
class Vehicle(@param:Cylinders(8) val engine: Engine) {
    fun start(): String {
        return engine.start()
    }
}

Qualifying by Generic Type Arguments

Since Micronaut framework 3.0, it is possible to select which bean to inject based on the generic type arguments of the class or interface. Consider the following example:

public interface CylinderProvider {
    int getCylinders();
}
interface CylinderProvider {
    int getCylinders()
}
interface CylinderProvider {
    val cylinders: Int
}

The CylinderProvider interface provides the number of cylinders.

public interface Engine<T extends CylinderProvider> { // (1)
    default int getCylinders() {
        return getCylinderProvider().getCylinders();
    }

    default String start() {
        return "Starting " + getCylinderProvider().getClass().getSimpleName();
    }

    T getCylinderProvider();
}
interface Engine<T extends CylinderProvider> { // (1)
    default int getCylinders() { cylinderProvider.cylinders }

    default String start() { "Starting ${cylinderProvider.class.simpleName}" }

    T getCylinderProvider()
}
interface Engine<T : CylinderProvider> { // (1)
    val cylinders: Int
        get() = cylinderProvider.cylinders

    fun start(): String {
        return "Starting ${cylinderProvider.javaClass.simpleName}"
    }

    val cylinderProvider: T
}
1 The engine class defines a generic type argument <T> that must be an instance of CylinderProvider

You can define implementations of the Engine interface with different generic type arguments. For example for a V6 engine:

public class V6 implements CylinderProvider {
    @Override
    public int getCylinders() {
        return 6;
    }
}
class V6 implements CylinderProvider {
    @Override
    int getCylinders() { 6 }
}
class V6 : CylinderProvider {
    override val cylinders: Int = 6
}

The above defines a V6 class that implements the CylinderProvider interface.

@Singleton
public class V6Engine implements Engine<V6> {  // (1)
    @Override
    public V6 getCylinderProvider() {
        return new V6();
    }
}
@Singleton
class V6Engine implements Engine<V6> {  // (1)
    @Override
    V6 getCylinderProvider() { new V6() }
}
@Singleton
class V6Engine : Engine<V6> { // (1)
    override val cylinderProvider: V6
        get() = V6()
}
1 The V6Engine implements Engine providing V6 as a generic type parameter

And a V8 engine:

public class V8 implements CylinderProvider {
    @Override
    public int getCylinders() {
        return 8;
    }
}
class V8 implements CylinderProvider {
    @Override
    int getCylinders() { 8 }
}
class V8 : CylinderProvider {
    override val cylinders: Int = 8
}

The above defines a V8 class that implements the CylinderProvider interface.

@Singleton
public class V8Engine implements Engine<V8> {  // (1)
    @Override
    public V8 getCylinderProvider() {
        return new V8();
    }
}
@Singleton
class V8Engine implements Engine<V8> {  // (1)
    @Override
    V8 getCylinderProvider() { new V8() }
}
@Singleton
class V8Engine : Engine<V8> { // (1)
    override val cylinderProvider: V8
        get() = V8()
}
1 The V8Engine implements Engine providing V8 as a generic type parameter

You can then use the generic arguments when defining the injection point and Micronaut framework will pick the correct bean to inject based on the specific generic type arguments:

@Inject
public Vehicle(Engine<V8> engine) {
    this.engine = engine;
}
@Inject
Vehicle(Engine<V8> engine) {
    this.engine = engine
}
@Singleton
class Vehicle(val engine: Engine<V8>) {

In the above example the V8Engine bean is injected.

Primary and Secondary Beans

Primary is a qualifier that indicates that a bean is the primary bean to be selected in the case of multiple interface implementations.

Consider the following example:

public interface ColorPicker {
    String color();
}
interface ColorPicker {
    String color()
}
interface ColorPicker {
    fun color(): String
}

ColorPicker is implemented by these classes:

The Primary Bean
import io.micronaut.context.annotation.Primary;
import jakarta.inject.Singleton;

@Primary
@Singleton
class Green implements ColorPicker {

    @Override
    public String color() {
        return "green";
    }
}
The Primary Bean
import io.micronaut.context.annotation.Primary
import jakarta.inject.Singleton

@Primary
@Singleton
class Green implements ColorPicker {

    @Override
    String color() {
        return "green"
    }
}
The Primary Bean
import io.micronaut.context.annotation.Primary
import jakarta.inject.Singleton

@Primary
@Singleton
class Green: ColorPicker {
    override fun color(): String {
        return "green"
    }
}

The Green bean class implements ColorPicker and is annotated with @Primary.

Another Bean of the Same Type
import jakarta.inject.Singleton;

@Singleton
public class Blue implements ColorPicker {

    @Override
    public String color() {
        return "blue";
    }
}
Another Bean of the Same Type
import jakarta.inject.Singleton

@Singleton
class Blue implements ColorPicker {

    @Override
    String color() {
        return "blue"
    }
}
Another Bean of the Same Type
import jakarta.inject.Singleton

@Singleton
class Blue: ColorPicker {
    override fun color(): String {
        return "blue"
    }
}

The Blue bean class also implements ColorPicker and hence you have two possible candidates when injecting the ColorPicker interface. Since Green is the primary, it will always be favoured.

@Controller("/testPrimary")
public class TestController {

    protected final ColorPicker colorPicker;

    public TestController(ColorPicker colorPicker) { // (1)
        this.colorPicker = colorPicker;
    }

    @Get
    public String index() {
        return colorPicker.color();
    }
}
@Controller("/test")
class TestController {

    protected final ColorPicker colorPicker

    TestController(ColorPicker colorPicker) { // (1)
        this.colorPicker = colorPicker
    }

    @Get
    String index() {
        colorPicker.color()
    }
}
@Controller("/test")
class TestController(val colorPicker: ColorPicker) { // (1)

    @Get
    fun index(): String {
        return colorPicker.color()
    }
}
1 Although there are two ColorPicker beans, Green gets injected due to the @Primary annotation.

If multiple possible candidates are present and no @Primary is defined a NonUniqueBeanException is thrown.

In addition to @Primary, there is also a Secondary annotation which causes the opposite effect and allows de-prioritizing a bean.

See the guide for Micronaut Patterns - Composite to learn more.

Injecting Any Bean

If you are not particular about which bean gets injected then you can use the @Any qualifier which will inject the first available bean, for example:

Injecting Any Instance
@Inject @Any
Engine engine;
Injecting Any Instance
@Inject @Any
Engine engine
Injecting Any Instance
@Inject
@field:Any
lateinit var engine: Engine

The @Any qualifier is typically used in conjunction with the BeanProvider interface to allow more dynamic use cases. For example the following Vehicle implementation will start the Engine if the bean is present:

Using BeanProvider with Any
import io.micronaut.context.BeanProvider;
import io.micronaut.context.annotation.Any;
import jakarta.inject.Singleton;

@Singleton
public class Vehicle {
    final BeanProvider<Engine> engineProvider;

    public Vehicle(@Any BeanProvider<Engine> engineProvider) { // (1)
        this.engineProvider = engineProvider;
    }
    void start() {
        engineProvider.ifPresent(Engine::start); // (2)
    }
}
Using BeanProvider with Any
import io.micronaut.context.BeanProvider
import io.micronaut.context.annotation.Any
import jakarta.inject.Singleton

@Singleton
class Vehicle {
    final BeanProvider<Engine> engineProvider

    Vehicle(@Any BeanProvider<Engine> engineProvider) { // (1)
        this.engineProvider = engineProvider
    }
    void start() {
        engineProvider.ifPresent(Engine::start) // (2)
    }
}
Using BeanProvider with Any
import io.micronaut.context.BeanProvider
import io.micronaut.context.annotation.Any
import jakarta.inject.Singleton

@Singleton
class Vehicle(@param:Any val engineProvider: BeanProvider<Engine>) { // (1)
    fun start() {
        engineProvider.ifPresent { it.start() } // (2)
    }
    fun startAll() {
        if (engineProvider.isPresent) { // (1)
            engineProvider.forEach { it.start() } // (2)
        }
}
1 Use @Any to inject the BeanProvider
2 Call the start method if the underlying bean is present using the ifPresent method

If there are multiple beans you can also adapt the behaviour. The following example starts all the engines installed in the Vehicle if any are present:

Using BeanProvider with Any
void startAll() {
    if (engineProvider.isPresent()) { // (1)
        engineProvider.stream().forEach(Engine::start); // (2)
    }
}
Using BeanProvider with Any
void startAll() {
    if (engineProvider.isPresent()) { // (1)
        engineProvider.each {it.start() } // (2)
    }
}
Using BeanProvider with Any
fun startAll() {
    if (engineProvider.isPresent) { // (1)
        engineProvider.forEach { it.start() } // (2)
    }
1 Check if any beans present
2 If so iterate over each one via the stream().forEach(..) method, starting the engines

3.7 Limiting Injectable Types

By default, when you annotate a bean with a scope such as @Singleton the bean class and all interfaces it implements and super classes it extends from become injectable via @Inject.

Consider the following example from the previous section on defining beans:

@Singleton
public class V8Engine implements Engine {  // (3)
    @Override
    public String start() {
        return "Starting V8";
    }

    @Override
    public int getCylinders() {
        return 8;
    }

}
@Singleton
class V8Engine implements Engine { // (3)
    int cylinders = 8

    @Override
    String start() {
        "Starting V8"
    }
}
@Singleton
class V8Engine : Engine {

    override var cylinders: Int = 8

    override fun start(): String {
        return "Starting V8"
    }

}

In the above case other classes in your application can choose to either inject the interface Engine or the concrete implementation V8Engine.

If this is undesirable you can use the typed member of the @Bean annotation to limit the exposed types. For example:

@Singleton
@Bean(typed = Engine.class) // (1)
public class V8Engine implements Engine {  // (2)
    @Override
    public String start() {
        return "Starting V8";
    }

    @Override
    public int getCylinders() {
        return 8;
    }
}
@Singleton
@Bean(typed = Engine) // (1)
class V8Engine implements Engine {  // (2)
    @Override
    String start() { "Starting V8" }

    @Override
    int getCylinders() { 8 }
}
@Singleton
@Bean(typed = [Engine::class]) // (1)
class V8Engine : Engine { // (2)
    override fun start(): String {
        return "Starting V8"
    }

    override val cylinders: Int = 8
}
1 @Bean(typed=..) is used to only allow injection the interface Engine and not the concrete type
2 The class must implement the class or interface defined by typed otherwise a compilation error will occur

The following test demonstrates the behaviour of typed using programmatic lookup and the BeanContext API:

@MicronautTest
public class EngineSpec {
    @Inject
    BeanContext beanContext;

    @Test
    public void testEngine() {
        assertThrows(NoSuchBeanException.class, () ->
                beanContext.getBean(V8Engine.class) // (1)
        );
        final Engine engine = beanContext.getBean(Engine.class); // (2)
        assertTrue(engine instanceof V8Engine);
    }
}
class EngineSpec extends Specification {
    @Shared @AutoCleanup
    ApplicationContext beanContext = ApplicationContext.run()

    void 'test engine'() {
        when:'the class is looked up'
        beanContext.getBean(V8Engine) // (1)

        then:'a no such bean exception is thrown'
        thrown(NoSuchBeanException)

        and:'it is possible to lookup by the typed interface'
        beanContext.getBean(Engine) instanceof V8Engine // (2)
    }
}
@MicronautTest
class EngineSpec {
    @Inject
    lateinit var beanContext: BeanContext

    @Test
    fun testEngine() {
        assertThrows(NoSuchBeanException::class.java) {
            beanContext.getBean(V8Engine::class.java) // (1)
        }

        val engine = beanContext.getBean(Engine::class.java) // (2)
        assertTrue(engine is V8Engine)
    }
}
1 Trying to lookup V8Engine throws a NoSuchBeanException
2 Whilst looking up the Engine interface succeeds

3.8 Scopes

Micronaut framework features an extensible bean scoping mechanism based on JSR-330. The following default scopes are supported:

3.8.1 Built-In Scopes

Table 1. Micronaut Built-in Scopes
Type Description

@Singleton

Singleton scope indicates only one instance of the bean will exist

@Context

Context scope indicates that the bean will be created at the same time as the ApplicationContext (eager initialization)

@Prototype

Prototype scope indicates that a new instance of the bean is created each time it is injected

@Infrastructure

Infrastructure scope represents a bean that cannot be overridden or replaced using @Replaces because it is critical to the functioning of the system.

@ThreadLocal

@ThreadLocal scope is a custom scope that associates a bean per thread via a ThreadLocal

@Refreshable

@Refreshable scope is a custom scope that allows a bean’s state to be refreshed via the /refresh endpoint.

@RequestScope

@RequestScope scope is a custom scope that indicates a new instance of the bean is created and associated with each HTTP request

The @Prototype annotation is a synonym for @Bean because the default scope is prototype.

Additional scopes can be added by defining a @Singleton bean that implements the CustomScope interface.

Note that when starting an ApplicationContext, by default @Singleton-scoped beans are created lazily and on-demand. This is by design to optimize startup time.

If this presents a problem for your use case you have the option of using the @Context annotation which binds the lifecycle of your object to the lifecycle of the ApplicationContext. In other words when the ApplicationContext is started your bean will be created.

Alternatively, annotate any @Singleton-scoped bean with @Parallel which allows parallel initialization of your bean without impacting overall startup time.

If your bean fails to initialize in parallel, the application will be automatically shut down.

3.8.1.1 Eager Initialization of Singletons

Eager initialization of @Singleton beans maybe desirable in certain scenarios, such as on AWS Lambda where more CPU resources are assigned to Lambda construction than execution.

You can specify whether to eagerly initialize @Singleton-scoped beans using the ApplicationContextBuilder interface:

Enabling Eager Initialization of Singletons
public class Application {

    public static void main(String[] args) {
        Micronaut.build(args)
            .eagerInitSingletons(true) (1)
            .mainClass(Application.class)
            .start();
    }
}
1 Setting eager init to true initializes all singletons

When you use Micronaut framework in environments such as Serverless Functions, you will not have an Application class, and instead you extend a Micronaut-provided class. In those cases, Micronaut provides methods which you can override to enhance the ApplicationContextBuilder

Override of newApplicationContextBuilder()
public class MyFunctionHandler extends MicronautRequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {
...
    @Nonnull
    @Override
    protected ApplicationContextBuilder newApplicationContextBuilder() {
        ApplicationContextBuilder builder = super.newApplicationContextBuilder();
        builder.eagerInitSingletons(true);
        return builder;
    }
    ...
}

@ConfigurationReader beans such as @EachProperty or @ConfigurationProperties are singleton beans. To eagerly init configuration but keep other @Singleton-scoped bean creation lazy, use eagerInitConfiguration:

Enabling Eager Initialization of Configuration
public class Application {

    public static void main(String[] args) {
        Micronaut.build(args)
            .eagerInitConfiguration(true) (1)
            .mainClass(Application.class)
            .start();
    }
}
1 Setting eager init to true initializes all configuration reader beans.

3.8.2 Refreshable Scope

The Refreshable scope is a custom scope that allows a bean’s state to be refreshed via:

The following example illustrates @Refreshable scope behavior.

@Refreshable // (1)
static class WeatherService {
    private String forecast;

    @PostConstruct
    public void init() {
        forecast = "Scattered Clouds " + new SimpleDateFormat("dd/MMM/yy HH:mm:ss.SSS").format(new Date());// (2)
    }

    public String latestForecast() {
        return forecast;
    }
}
@Refreshable // (1)
static class WeatherService {

    String forecast

    @PostConstruct
    void init() {
        forecast = "Scattered Clouds ${new SimpleDateFormat("dd/MMM/yy HH:mm:ss.SSS").format(new Date())}" // (2)
    }

    String latestForecast() {
        return forecast
    }
}
@Refreshable // (1)
open class WeatherService {
    private var forecast: String? = null

    @PostConstruct
    open fun init() {
        forecast = "Scattered Clouds " + SimpleDateFormat("dd/MMM/yy HH:mm:ss.SSS").format(Date())// (2)
    }

    open fun latestForecast(): String? {
        return forecast
    }
}
1 The WeatherService is annotated with @Refreshable scope which stores an instance until a refresh event is triggered
2 The value of the forecast property is set to a fixed value when the bean is created and won’t change until the bean is refreshed

If you invoke latestForecast() twice, you will see identical responses such as "Scattered Clouds 01/Feb/18 10:29.199".

When the /refresh endpoint is invoked or a RefreshEvent is published, the instance is invalidated and a new instance is created the next time the object is requested. For example:

applicationContext.publishEvent(new RefreshEvent());
applicationContext.publishEvent(new RefreshEvent())
applicationContext.publishEvent(RefreshEvent())

3.8.3 Scopes on Meta Annotations

Scopes can be defined on meta annotations that you can then apply to your classes. Consider the following example meta annotation:

Driver.java Annotation
import io.micronaut.context.annotation.Requires;

import jakarta.inject.Singleton;
import java.lang.annotation.Documented;
import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Requires(classes = Car.class) // (1)
@Singleton // (2)
@Documented
@Retention(RUNTIME)
public @interface Driver {
}
Driver.java Annotation
import io.micronaut.context.annotation.Requires

import jakarta.inject.Singleton
import java.lang.annotation.Documented
import java.lang.annotation.Retention

import static java.lang.annotation.RetentionPolicy.RUNTIME

@Requires(classes = Car.class) // (1)
@Singleton // (2)
@Documented
@Retention(RUNTIME)
@interface Driver {
}
Driver.java Annotation
import io.micronaut.context.annotation.Requires
import jakarta.inject.Singleton
import kotlin.annotation.AnnotationRetention.RUNTIME

@Requires(classes = [Car::class]) // (1)
@Singleton // (2)
@MustBeDocumented
@Retention(RUNTIME)
annotation class Driver
1 The scope declares a requirement on a Car class using Requires
2 The annotation is declared as @Singleton

In the example above the @Singleton annotation is applied to the @Driver annotation which results in every class that is annotated with @Driver being regarded as singleton.

Note that in this case it is not possible to alter the scope when the annotation is applied. For example, the following will not override the scope declared by @Driver and is invalid:

Declaring Another Scope
@Driver
@Prototype
class Foo {}

For the scope to be overridable, instead use the DefaultScope annotation on @Driver which allows a default scope to be specified if none other is present:

Using @DefaultScope
@Requires(classes = Car.class)
@DefaultScope(Singleton.class) (1)
@Documented
@Retention(RUNTIME)
public @interface Driver {
}
@Requires(classes = Car.class)
@DefaultScope(Singleton.class) (1)
@Documented
@Retention(RUNTIME)
@interface Driver {
}
@Requires(classes = [Car::class])
@DefaultScope(Singleton::class) (1)
@Documented
@Retention(RUNTIME)
annotation class Driver
1 DefaultScope declares the scope to use if none is specified

3.9 Bean Factories

In many cases, you may want to make available as a bean a class that is not part of your codebase such as those provided by third-party libraries. In this case, you cannot annotate the compiled class. Instead, implement a @Factory.

A factory is a class annotated with the Factory annotation that provides one or more methods annotated with a bean scope annotation. Which annotation you use depends on what scope you want the bean to be in. See the section on bean scopes for more information.

The factory has the default scope singleton and will be destroyed with the context. If you want to dispose the factory after it produces a bean, use @Prototype scope.

The return types of methods annotated with a bean scope annotation are the bean types. This is best illustrated by an example:

@Singleton
class CrankShaft {
}

class V8Engine implements Engine {
    private final int cylinders = 8;
    private final CrankShaft crankShaft;

    public V8Engine(CrankShaft crankShaft) {
        this.crankShaft = crankShaft;
    }

    @Override
    public String start() {
        return "Starting V8";
    }
}

@Factory
class EngineFactory {

    @Singleton
    Engine v8Engine(CrankShaft crankShaft) {
        return new V8Engine(crankShaft);
    }
}
@Singleton
class CrankShaft {
}

class V8Engine implements Engine {
    final int cylinders = 8
    final CrankShaft crankShaft

    V8Engine(CrankShaft crankShaft) {
        this.crankShaft = crankShaft
    }

    @Override
    String start() {
        "Starting V8"
    }
}

@Factory
class EngineFactory {

    @Singleton
    Engine v8Engine(CrankShaft crankShaft) {
        new V8Engine(crankShaft)
    }
}
@Singleton
internal class CrankShaft

internal class V8Engine(private val crankShaft: CrankShaft) : Engine {
    private val cylinders = 8

    override fun start(): String {
        return "Starting V8"
    }
}

@Factory
internal class EngineFactory {

    @Singleton
    fun v8Engine(crankShaft: CrankShaft): Engine {
        return V8Engine(crankShaft)
    }
}

In this case, a V8Engine is created by the EngineFactory class' v8Engine method. Note that you can inject parameters into the method, and they will be resolved as beans. The resulting V8Engine bean will be a singleton.

A factory can have multiple methods annotated with bean scope annotations, each one returning a distinct bean type.

If you take this approach you should not invoke other bean methods internally within the class. Instead, inject the types via parameters.
To allow the resulting bean to participate in the application context shutdown process, annotate the method with @Bean and set the preDestroy argument to the name of the method to be called to close the bean.

Beans from Fields

With Micronaut framework 3.0 or above it is also possible to produce beans from fields by declaring the @Bean annotation on a field.

Whilst generally this approach should be discouraged in favour for factory methods, which provide more flexibility it does simplify testing code. For example with bean fields you can easily produce mocks in your test code:

import io.micronaut.context.annotation.*;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;
import jakarta.inject.Inject;

import static org.junit.jupiter.api.Assertions.assertEquals;

@MicronautTest
public class VehicleMockSpec {
    @Requires(beans = VehicleMockSpec.class)
    @Bean @Replaces(Engine.class)
    Engine mockEngine = () -> "Mock Started"; // (1)

    @Inject Vehicle vehicle; // (2)

    @Test
    void testStartEngine() {
        final String result = vehicle.start();
        assertEquals("Mock Started", result); // (3)
    }
}
import io.micronaut.context.annotation.*
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import spock.lang.Specification
import jakarta.inject.Inject

@MicronautTest
class VehicleMockSpec extends Specification {
    @Requires(beans=VehicleMockSpec.class)
    @Bean @Replaces(Engine.class)
    Engine mockEngine = {-> "Mock Started" } as Engine  // (1)

    @Inject Vehicle vehicle // (2)

    void "test start engine"() {
        given:
        final String result = vehicle.start()

        expect:
        result == "Mock Started" // (3)
    }
}
import io.micronaut.context.annotation.Bean
import io.micronaut.context.annotation.Replaces
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions
import org.junit.jupiter.api.Test
import jakarta.inject.Inject

@MicronautTest
class VehicleMockSpec {
    @get:Bean
    @get:Replaces(Engine::class)
    val mockEngine: Engine = object : Engine { // (1)
        override fun start(): String {
            return "Mock Started"
        }
    }

    @Inject
    lateinit var vehicle : Vehicle // (2)

    @Test
    fun testStartEngine() {
        val result = vehicle.start()
        Assertions.assertEquals("Mock Started", result) // (3)
    }
}
1 A bean is defined from a field that replaces the existing Engine.
2 The Vehicle is injected.
3 The code asserts that the mock implementation is called.

Note that only public or package protected fields are supported on non-primitive types. If the field is static, private, or protected a compilation error will occur.

If the bean method/field includes a scope or a qualifier any scope or qualifiers from the type will be omitted.
Qualifiers from the factory instance aren’t inherited to the beans.

Primitive Beans and Arrays

Since Micronaut framework 3.1 it is possible to define and inject primitive types and array types from factories.

For example:

import io.micronaut.context.annotation.Bean;
import io.micronaut.context.annotation.Factory;
import jakarta.inject.Named;

@Factory
class CylinderFactory {
    @Bean
    @Named("V8") // (1)
    final int v8 = 8;

    @Bean
    @Named("V6") // (1)
    final int v6 = 6;
}
import io.micronaut.context.annotation.Bean
import io.micronaut.context.annotation.Factory
import jakarta.inject.Named

@Factory
class CylinderFactory {
    @Bean
    @Named("V8") // (1)
    final int v8 = 8

    @Bean
    @Named("V6") // (1)
    final int v6 = 6
}
import io.micronaut.context.annotation.Bean
import io.micronaut.context.annotation.Factory
import jakarta.inject.Named

@Factory
class CylinderFactory {
    @get:Bean
    @get:Named("V8") // (1)
    val v8 = 8

    @get:Bean
    @get:Named("V6") // (1)
    val v6 = 6
}
1 Two primitive integer beans are defined with different names

Primitive beans can be injected like any other bean:

import jakarta.inject.Named;
import jakarta.inject.Singleton;

@Singleton
public class V8Engine {
    private final int cylinders;

    public V8Engine(@Named("V8") int cylinders) { // (1)
        this.cylinders = cylinders;
    }

    public int getCylinders() {
        return cylinders;
    }
}
import jakarta.inject.Named
import jakarta.inject.Singleton

@Singleton
class V8Engine {
    private final int cylinders

    V8Engine(@Named("V8") int cylinders) { // (1)
        this.cylinders = cylinders
    }

    int getCylinders() {
        return cylinders
    }
}
import jakarta.inject.Named
import jakarta.inject.Singleton

@Singleton
class V8Engine(
    @param:Named("V8") val cylinders: Int // (1)
)

Note that primitive beans and primitive array beans have the following limitations:

  • AOP advice cannot be applied to primitives or wrapper types

  • Due to the above custom scopes that proxy are not supported

  • The @Bean(preDestroy=..) member is not supported

Programmatically Disabling Beans

Factory methods can throw DisabledBeanException to conditionally disable beans. Using @Requires should always be the preferred method to conditionally create beans; throwing an exception in a factory method should only be done if using @Requires is not possible.

For example:

public interface Engine {
    Integer getCylinders();
}

@EachProperty("engines")
public class EngineConfiguration implements Toggleable {

    private boolean enabled = true;
    private Integer cylinders;

    @NotNull
    public Integer getCylinders() {
        return cylinders;
    }

    public void setCylinders(Integer cylinders) {
        this.cylinders = cylinders;
    }

    @Override
    public boolean isEnabled() {
        return enabled;
    }

    public void setEnabled(boolean enabled) {
        this.enabled = enabled;
    }

}

@Factory
public class EngineFactory {

    @EachBean(EngineConfiguration.class)
    public Engine buildEngine(EngineConfiguration engineConfiguration) {
        if (engineConfiguration.isEnabled()) {
            return engineConfiguration::getCylinders;
        } else {
            throw new DisabledBeanException("Engine configuration disabled");
        }
    }
}
interface Engine {
    Integer getCylinders()
}

@EachProperty("engines")
class EngineConfiguration implements Toggleable {
    boolean enabled = true
    @NotNull
    Integer cylinders
}

@Factory
class EngineFactory {

    @EachBean(EngineConfiguration)
    Engine buildEngine(EngineConfiguration engineConfiguration) {
        if (engineConfiguration.enabled) {
            (Engine) { -> engineConfiguration.cylinders }
        } else {
            throw new DisabledBeanException("Engine configuration disabled")
        }
    }
}
interface Engine {
    fun getCylinders(): Int
}

@EachProperty("engines")
class EngineConfiguration : Toggleable {

    var enabled = true

    @NotNull
    val cylinders: Int? = null

    override fun isEnabled(): Boolean {
        return enabled
    }
}

@Factory
class EngineFactory {

    @EachBean(EngineConfiguration::class)
    fun buildEngine(engineConfiguration: EngineConfiguration): Engine? {
        return if (engineConfiguration.isEnabled) {
            object : Engine {
                override fun getCylinders(): Int {
                    return engineConfiguration.cylinders!!
                }
            }
        } else {
            throw DisabledBeanException("Engine configuration disabled")
        }
    }
}

Injection Point

A common use case with factories is to take advantage of annotation metadata from the point at which an object is injected such that behaviour can be modified based on said metadata.

Consider an annotation such as the following:

@Documented
@Retention(RUNTIME)
@Target(ElementType.PARAMETER)
public @interface Cylinders {
    int value() default 8;
}
@Documented
@Retention(RUNTIME)
@Target(ElementType.PARAMETER)
@interface Cylinders {
    int value() default 8
}
@MustBeDocumented
@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.VALUE_PARAMETER)
annotation class Cylinders(val value: Int = 8)

The above annotation could be used to customize the type of engine we want to inject into a vehicle at the point at which the injection point is defined:

@Singleton
class Vehicle {

    private final Engine engine;

    Vehicle(@Cylinders(6) Engine engine) {
        this.engine = engine;
    }

    String start() {
        return engine.start();
    }
}
@Singleton
class Vehicle {

    private final Engine engine

    Vehicle(@Cylinders(6) Engine engine) {
        this.engine = engine
    }

    String start() {
        return engine.start()
    }
}
@Singleton
internal class Vehicle(@param:Cylinders(6) private val engine: Engine) {
    fun start(): String {
        return engine.start()
    }
}

The above Vehicle class specifies an annotation value of @Cylinders(6) indicating an Engine of six cylinders is required.

To implement this use case, define a factory that accepts the InjectionPoint instance to analyze the defined annotation values:

@Factory
class EngineFactory {

    @Prototype
    Engine v8Engine(InjectionPoint<?> injectionPoint, CrankShaft crankShaft) { // (1)
        final int cylinders = injectionPoint
                .getAnnotationMetadata()
                .intValue(Cylinders.class).orElse(8); // (2)
        switch (cylinders) { // (3)
            case 6:
                return new V6Engine(crankShaft);
            case 8:
                return new V8Engine(crankShaft);
            default:
                throw new IllegalArgumentException("Unsupported number of cylinders specified: " + cylinders);
        }
    }
}
@Factory
class EngineFactory {

    @Prototype
    Engine v8Engine(InjectionPoint<?> injectionPoint, CrankShaft crankShaft) { // (1)
        final int cylinders = injectionPoint
                .getAnnotationMetadata()
                .intValue(Cylinders.class).orElse(8) // (2)
        switch (cylinders) { // (3)
            case 6:
                return new V6Engine(crankShaft)
            case 8:
                return new V8Engine(crankShaft)
            default:
                throw new IllegalArgumentException("Unsupported number of cylinders specified: $cylinders")
        }
    }
}
@Factory
internal class EngineFactory {

    @Prototype
    fun v8Engine(injectionPoint: InjectionPoint<*>, crankShaft: CrankShaft): Engine { // (1)
        val cylinders = injectionPoint
                .annotationMetadata
                .intValue(Cylinders::class.java).orElse(8) // (2)
        return when (cylinders) { // (3)
            6 -> V6Engine(crankShaft)
            8 -> V8Engine(crankShaft)
            else -> throw IllegalArgumentException("Unsupported number of cylinders specified: $cylinders")
        }
    }
}
1 The factory method defines a parameter of type InjectionPoint.
2 The annotation metadata is used to obtain the value of the @Cylinder annotation
3 The value is used to construct an engine, throwing an exception if an engine cannot be constructed.
It is important to note that the factory is declared as @Prototype scope so the method is invoked for each injection point. If the V8Engine and V6Engine types are required to be singletons, the factory should use a Map to ensure the objects are only constructed once.

3.10 Conditional Beans

At times, you may want a bean to load conditionally based on various potential factors including the classpath, the configuration, the presence of other beans, etc.

The Requires annotation provides the ability to define one or many conditions on a bean.

Consider the following example:

Using @Requires
@Singleton
@Requires(beans = DataSource.class)
@Requires(property = "datasource.url")
public class JdbcBookService implements BookService {

    DataSource dataSource;

    public JdbcBookService(DataSource dataSource) {
        this.dataSource = dataSource;
    }
Using @Requires
@Singleton
@Requires(beans = DataSource)
@Requires(property = "datasource.url")
class JdbcBookService implements BookService {

    DataSource dataSource
Using @Requires
@Singleton
@Requirements(Requires(beans = [DataSource::class]), Requires(property = "datasource.url"))
class JdbcBookService(internal var dataSource: DataSource) : BookService {

The above bean defines two requirements. The first indicates that a DataSource bean must be present for the bean to load. The second requirement ensures that the datasource.url property is set before loading the JdbcBookService bean.

If multiple beans require the same combination of requirements, you can define a meta-annotation with the requirements:

Using a @Requires meta-annotation
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.PACKAGE, ElementType.TYPE})
@Requires(beans = DataSource.class)
@Requires(property = "datasource.url")
public @interface RequiresJdbc {
}
Using a @Requires meta-annotation
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target([ElementType.PACKAGE, ElementType.TYPE])
@Requires(beans = DataSource)
@Requires(property = "datasource.url")
@interface RequiresJdbc {
}
Using a @Requires meta-annotation
@Documented
@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.CLASS, AnnotationTarget.FILE)
@Requirements(Requires(beans = [DataSource::class]), Requires(property = "datasource.url"))
annotation class RequiresJdbc

In the above example the RequiresJdbc annotation can be used on the JdbcBookService instead:

Using a meta-annotation
@RequiresJdbc
public class JdbcBookService implements BookService {
    ...
}

If you have multiple beans that need to fulfill a given requirement before loading, you may want to consider a bean configuration group, as explained in the next section.

Configuration Requirements

The @Requires annotation is very flexible and can be used for a variety of use cases. The following table summarizes some possibilities:

Table 1. Using @Requires
Requirement Example

Require the presence of one or more classes

@Requires(classes=javax.servlet.Servlet)

Require the absence of one or more classes

@Requires(missing=javax.servlet.Servlet)

Require the presence one or more beans

@Requires(beans=javax.sql.DataSource)

Require the absence of one or more beans

@Requires(missingBeans=javax.sql.DataSource)

Require the environment to be applied

@Requires(env="test")

Require the environment to not be applied

@Requires(notEnv="test")

Require the presence of another configuration package

@Requires(configuration="foo.bar")

Require the absence of another configuration package

@Requires(missingConfigurations="foo.bar")

Require particular SDK version

@Requires(sdk=Sdk.JAVA, value="1.8")

Requires classes annotated with the given annotations to be available to the application via package scanning

@Requires(entities=javax.persistence.Entity)

Require a property with an optional value

@Requires(property="data-source.url")

Require a property to not be part of the configuration

@Requires(missingProperty="data-source.url")

Require the presence of one or more files in the file system

@Requires(resources="file:/path/to/file")

Require the presence of one or more classpath resources

@Requires(resources="classpath:myFile.properties")

Require the current operating system to be in the list

@Requires(os={Requires.Family.WINDOWS})

Require the current operating system to not be in the list

@Requires(notOs={Requires.Family.WINDOWS})

Requires bean to be present in case no beanProperty specified

@Requires(bean=Config.class)

Requires the specified property of bean to be present

@Requires(bean=Config.class, beanProperty="enabled")

Additional Notes on Property Requirements.

Adding a requirement on a property has some additional functionality. You can require the property to be a certain value, not be a certain value, and use a default in those checks if it is not set.

@Requires(property="foo") (1)
@Requires(property="foo", value="John") (2)
@Requires(property="foo", value="John", defaultValue="John") (3)
@Requires(property="foo", notEquals="Sally") (4)
1 Requires the property to be set
2 Requires the property to be "John"
3 Requires the property to be "John" or not set
4 Requires the property to not be "Sally" or not set

Referencing bean properties in @Requires.

You can also reference other beans properties in @Requires to conditionally load beans. Similar to property requirements, you can specify required value or set the value bean property should not be equal to using notEquals annotation member. For the bean property to be checked, the bean of type specified in bean annotation member should be present within context, otherwise conditional bean will not be loaded.

@Requires(bean=Config.class, beanProperty="foo") (1)
@Requires(bean=Config.class, beanProperty="foo", value="John") (2)
@Requires(bean=Config.class, beanProperty="foo", notEquals="Sally") (3)
1 Requires the "foo" property on Config bean to be set
2 Requires the "foo" property on Config bean to be "John"
3 Requires the "foo" property on Config bean to not be "Sally" or not set

Specified bean property is accessed through respective getter method whose presence and availability will be checked at compilation time.

Note that bean property is considered to be present in case it’s value is not null. Keep in mind that primitive properties are initialized with default values such as false for boolean and 0 for int, so they are considered to be set even if no value is explicitly specified for them.

Debugging Conditional Beans

If you have multiple conditions and complex requirements it may become difficult to understand why a particular bean has not been loaded.

To help resolve issues with conditional beans you can enable debug logging for the io.micronaut.context.condition package which will log the reasons why beans were not loaded.

logback.xml
<logger name="io.micronaut.context.condition" level="DEBUG"/>

Consult the logging chapter for details howto setup logging.

3.11 Bean Replacement

One significant difference between Micronaut’s Dependency Injection system and Spring’s is the way beans are replaced.

In a Spring application, beans have names and are overridden by creating a bean with the same name, regardless of the type of the bean. Spring also has the notion of bean registration order, hence in Spring Boot you have @AutoConfigureBefore and @AutoConfigureAfter annotations that control how beans override each other.

This strategy leads to problems that are difficult to debug, for example:

  • Bean loading order changes, leading to unexpected results

  • A bean with the same name overrides another bean with a different type

To avoid these problems, Micronaut’s DI has no concept of bean names or load order. Beans have a type and a Qualifier. You cannot override a bean of a completely different type with another.

A useful benefit of Spring’s approach is that it allows overriding existing beans to customize behaviour. To support the same ability, Micronaut’s DI provides an explicit @Replaces annotation, which integrates nicely with support for Conditional Beans and clearly documents and expresses the intention of the developer.

Any existing bean can be replaced by another bean that declares @Replaces. For example, consider the following class:

JdbcBookService
@Singleton
@Requires(beans = DataSource.class)
@Requires(property = "datasource.url")
public class JdbcBookService implements BookService {

    DataSource dataSource;

    public JdbcBookService(DataSource dataSource) {
        this.dataSource = dataSource;
    }
JdbcBookService
@Singleton
@Requires(beans = DataSource)
@Requires(property = "datasource.url")
class JdbcBookService implements BookService {

    DataSource dataSource
JdbcBookService
@Singleton
@Requirements(Requires(beans = [DataSource::class]), Requires(property = "datasource.url"))
class JdbcBookService(internal var dataSource: DataSource) : BookService {

You can define a class in src/test/java that replaces this class just for your tests:

Using @Replaces
@Replaces(JdbcBookService.class) // (1)
@Singleton
public class MockBookService implements BookService {

    Map<String, Book> bookMap = new LinkedHashMap<>();

    @Override
    public Book findBook(String title) {
        return bookMap.get(title);
    }
}
Using @Replaces
@Replaces(JdbcBookService.class) // (1)
@Singleton
class MockBookService implements BookService {

    Map<String, Book> bookMap = [:]

    @Override
    Book findBook(String title) {
        bookMap.get(title)
    }
}
Using @Replaces
@Replaces(JdbcBookService::class) // (1)
@Singleton
class MockBookService : BookService {

    var bookMap: Map<String, Book> = LinkedHashMap()

    override fun findBook(title: String): Book? {
        return bookMap[title]
    }
}
1 The MockBookService declares that it replaces JdbcBookService

Factory Replacement

The @Replaces annotation also supports a factory argument. That argument allows the replacement of factory beans in their entirety or specific types created by the factory.

For example, it may be desired to replace all or part of the given factory class:

BookFactory
@Factory
public class BookFactory {

    @Singleton
    Book novel() {
        return new Book("A Great Novel");
    }

    @Singleton
    TextBook textBook() {
        return new TextBook("Learning 101");
    }
}
BookFactory
@Factory
class BookFactory {

    @Singleton
    Book novel() {
        new Book('A Great Novel')
    }

    @Singleton
    TextBook textBook() {
        new TextBook('Learning 101')
    }
}
BookFactory
@Factory
class BookFactory {

    @Singleton
    internal fun novel(): Book {
        return Book("A Great Novel")
    }

    @Singleton
    internal fun textBook(): TextBook {
        return TextBook("Learning 101")
    }
}
To replace a factory entirely, your factory methods must match the return types of all methods in the replaced factory.

In this example, BookFactory#textBook() is not replaced because this factory does not have a factory method that returns a TextBook.

CustomBookFactory
@Factory
@Replaces(factory = BookFactory.class)
public class CustomBookFactory {

    @Singleton
    Book otherNovel() {
        return new Book("An OK Novel");
    }
}
CustomBookFactory
@Factory
@Replaces(factory = BookFactory)
class CustomBookFactory {

    @Singleton
    Book otherNovel() {
        new Book('An OK Novel')
    }
}
CustomBookFactory
@Factory
@Replaces(factory = BookFactory::class)
class CustomBookFactory {

    @Singleton
    internal fun otherNovel(): Book {
        return Book("An OK Novel")
    }
}

To replace one or more factory methods but retain the rest, apply the @Replaces annotation on the method(s) and denote the factory to apply to.

TextBookFactory
@Factory
public class TextBookFactory {

    @Singleton
    @Replaces(value = TextBook.class, factory = BookFactory.class)
    TextBook textBook() {
        return new TextBook("Learning 305");
    }
}
TextBookFactory
@Factory
class TextBookFactory {

    @Singleton
    @Replaces(value = TextBook, factory = BookFactory)
    TextBook textBook() {
        new TextBook('Learning 305')
    }
}
TextBookFactory
@Factory
class TextBookFactory {

    @Singleton
    @Replaces(value = TextBook::class, factory = BookFactory::class)
    internal fun textBook(): TextBook {
        return TextBook("Learning 305")
    }
}

The BookFactory#novel() method will not be replaced because the TextBook class is defined in the annotation.

Default Implementation

When exposing an API, you may want to define an implementation of the interface that is used as the default when injecting a particular interface. For this you can use the @DefaultImplementation annotation.

It may also be desirable to not expose the default implementation of an interface as part of the public API by making it package private in Java.

Doing so prevents users from being able to replace the implementation because they will not be able to reference the class.

The @DefaultImplementation annotation allows the framework to establish the implementation to replace if a user creates a bean that declares @Replaces(YourInterface.class).

For example consider:

A public API contract

import io.micronaut.context.annotation.DefaultImplementation;

@DefaultImplementation(DefaultResponseStrategy.class)
public interface ResponseStrategy {
}
import io.micronaut.context.annotation.DefaultImplementation

@DefaultImplementation(DefaultResponseStrategy)
interface ResponseStrategy {
}
import io.micronaut.context.annotation.DefaultImplementation

@DefaultImplementation(DefaultResponseStrategy::class)
interface ResponseStrategy

The default implementation

import jakarta.inject.Singleton;

@Singleton
class DefaultResponseStrategy implements ResponseStrategy {

}
import jakarta.inject.Singleton

@Singleton
class DefaultResponseStrategy implements ResponseStrategy {

}
import jakarta.inject.Singleton

@Singleton
internal class DefaultResponseStrategy : ResponseStrategy

The custom implementation

import io.micronaut.context.annotation.Replaces;
import jakarta.inject.Singleton;

@Singleton
@Replaces(ResponseStrategy.class)
public class CustomResponseStrategy implements ResponseStrategy {

}
import io.micronaut.context.annotation.Replaces
import jakarta.inject.Singleton

@Singleton
@Replaces(ResponseStrategy)
class CustomResponseStrategy implements ResponseStrategy {

}
import io.micronaut.context.annotation.Replaces
import jakarta.inject.Singleton

@Singleton
@Replaces(ResponseStrategy::class)
class CustomResponseStrategy : ResponseStrategy

In the above example, the CustomResponseStrategy replaces the DefaultResponseStrategy because the DefaultImplementation annotation points to the DefaultResponseStrategy.

3.12 Bean Configurations

A bean @Configuration is a grouping of multiple bean definitions within a package.

The @Configuration annotation is applied at the package level and informs the Micronaut framework that the beans defined with the package form a logical grouping.

The @Configuration annotation is typically applied to package-info classes. For example:

package-info.groovy
@Configuration
package my.package

import io.micronaut.context.annotation.Configuration

Where this grouping becomes useful is when the bean configuration is made conditional via the @Requires annotation. For example:

package-info.groovy
@Configuration
@Requires(beans = javax.sql.DataSource)
package my.package

In the above example, all bean definitions within the annotated package are only loaded and made available if a javax.sql.DataSource bean is present. This lets you implement conditional autoconfiguration of bean definitions.

Java and Kotlin also support this functionality via package-info.java. Kotlin does not support a package-info.kt as of version 1.3.

3.13 Life-Cycle Methods

When The Bean Is Constructed

To invoke a method when the bean is constructed, use the jakarta.annotation.PostConstruct annotation:

import jakarta.annotation.PostConstruct; // (1)
import jakarta.inject.Singleton;

@Singleton
public class V8Engine implements Engine {

    private int cylinders = 8;
    private boolean initialized = false; // (2)

    @Override
    public String start() {
        if (!initialized) {
            throw new IllegalStateException("Engine not initialized!");
        }

        return "Starting V8";
    }

    @Override
    public int getCylinders() {
        return cylinders;
    }

    public boolean isInitialized() {
        return initialized;
    }

    @PostConstruct // (3)
    public void initialize() {
        initialized = true;
    }
}
import jakarta.annotation.PostConstruct // (1)
import jakarta.inject.Singleton

@Singleton
class V8Engine implements Engine {

    int cylinders = 8
    boolean initialized = false // (2)

    @Override
    String start() {
        if (!initialized) {
            throw new IllegalStateException("Engine not initialized!")
        }

        return "Starting V8"
    }

    @PostConstruct // (3)
    void initialize() {
        initialized = true
    }
}
import jakarta.annotation.PostConstruct
import jakarta.inject.Singleton

@Singleton
class V8Engine : Engine {

    override val cylinders = 8

    var initialized = false
        private set // (2)

    override fun start(): String {
        check(initialized) { "Engine not initialized!" }

        return "Starting V8"
    }

    @PostConstruct // (3)
    fun initialize() {
        initialized = true
    }
}
1 The PostConstruct annotation is imported
2 A field is defined that requires initialization
3 A method is annotated with @PostConstruct and will be invoked once the object is constructed and fully injected.

To manage when a bean is constructed, see the section on bean scopes.

When The Bean Is Destroyed

To invoke a method when the bean is destroyed, use the jakarta.annotation.PreDestroy annotation:

import jakarta.annotation.PreDestroy; // (1)
import jakarta.inject.Singleton;
import java.util.concurrent.atomic.AtomicBoolean;

@Singleton
public class PreDestroyBean implements AutoCloseable {

    AtomicBoolean stopped = new AtomicBoolean(false);

    @PreDestroy // (2)
    @Override
    public void close() throws Exception {
        stopped.compareAndSet(false, true);
    }
}
import jakarta.annotation.PreDestroy // (1)
import jakarta.inject.Singleton
import java.util.concurrent.atomic.AtomicBoolean

@Singleton
class PreDestroyBean implements AutoCloseable {

    AtomicBoolean stopped = new AtomicBoolean(false)

    @PreDestroy // (2)
    @Override
    void close() throws Exception {
        stopped.compareAndSet(false, true)
    }
}
import jakarta.annotation.PreDestroy // (1)
import jakarta.inject.Singleton
import java.util.concurrent.atomic.AtomicBoolean

@Singleton
class PreDestroyBean : AutoCloseable {

    internal var stopped = AtomicBoolean(false)

    @PreDestroy // (2)
    @Throws(Exception::class)
    override fun close() {
        stopped.compareAndSet(false, true)
    }
}
1 The PreDestroy annotation is imported
2 A method is annotated with @PreDestroy and will be invoked when the context is closed.

For factory beans, the preDestroy value in the Bean annotation tells Micronaut framework which method to invoke.

import io.micronaut.context.annotation.Bean;
import io.micronaut.context.annotation.Factory;

import jakarta.inject.Singleton;

@Factory
public class ConnectionFactory {

    @Bean(preDestroy = "stop") // (1)
    @Singleton
    public Connection connection() {
        return new Connection();
    }
}
import io.micronaut.context.annotation.Bean
import io.micronaut.context.annotation.Factory

import jakarta.inject.Singleton

@Factory
class ConnectionFactory {

    @Bean(preDestroy = "stop") // (1)
    @Singleton
    Connection connection() {
        new Connection()
    }
}
import io.micronaut.context.annotation.Bean
import io.micronaut.context.annotation.Factory

import jakarta.inject.Singleton

@Factory
class ConnectionFactory {

    @Bean(preDestroy = "stop") // (1)
    @Singleton
    fun connection(): Connection {
        return Connection()
    }
}

import java.util.concurrent.atomic.AtomicBoolean;

public class Connection {

    AtomicBoolean stopped = new AtomicBoolean(false);

    public void stop() { // (2)
        stopped.compareAndSet(false, true);
    }

}
import java.util.concurrent.atomic.AtomicBoolean

class Connection {

    AtomicBoolean stopped = new AtomicBoolean(false)

    void stop() { // (2)
        stopped.compareAndSet(false, true)
    }

}
import java.util.concurrent.atomic.AtomicBoolean

class Connection {

    internal var stopped = AtomicBoolean(false)

    fun stop() { // (2)
        stopped.compareAndSet(false, true)
    }

}
1 The preDestroy value is set on the annotation
2 The annotation value matches the method name
Simply implementing the Closeable or AutoCloseable interface is not enough for a bean to be closed with the context. One of the above methods must be used.

Dependent Beans

Dependent beans are the beans used in the construction of your bean. If the dependent bean’s scope is @Prototype or unknown, it will be destroyed along with your instance.

3.14 Context Events

The Micronaut framework supports a general event system through the context. The ApplicationEventPublisher API publishes events and the ApplicationEventListener API is used to listen to events. The event system is not limited to events that Micronaut publishes and supports custom events created by users. Context Events require Micronaut Context dependency:

implementation("io.micronaut:micronaut-context")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-context</artifactId>
</dependency>

micronaut-context is a transitive dependency of micronaut-http. If you use a Micronaut HTTP runtime, your project already includes the Micronaut-context dependency.

Publishing Events

The ApplicationEventPublisher API supports events of any type, although all events that the Micronaut framework publishes extend ApplicationEvent.

To publish an event, use dependency injection to obtain an instance of ApplicationEventPublisher where the generic type is the type of event and invoke the publishEvent method with your event object.

"Publishing an Event
public class SampleEvent {
    private String message = "Something happened";

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

import io.micronaut.context.event.ApplicationEventPublisher;
import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
public class SampleEventEmitterBean {

    @Inject
    ApplicationEventPublisher<SampleEvent> eventPublisher;

    public void publishSampleEvent() {
        eventPublisher.publishEvent(new SampleEvent());
    }

}
"Publishing an Event
class SampleEvent {
    String message = "Something happened"
}

import io.micronaut.context.event.ApplicationEventPublisher
import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class SampleEventEmitterBean {

    @Inject
    ApplicationEventPublisher<SampleEvent> eventPublisher

    void publishSampleEvent() {
        eventPublisher.publishEvent(new SampleEvent())
    }

}
"Publishing an Event
data class SampleEvent(val message: String = "Something happened")

import io.micronaut.context.event.ApplicationEventPublisher
import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class SampleEventEmitterBean {

    @Inject
    internal var eventPublisher: ApplicationEventPublisher<SampleEvent>? = null

    fun publishSampleEvent() {
        eventPublisher!!.publishEvent(SampleEvent())
    }

}
Publishing an event is synchronous by default! The publishEvent method will not return until all listeners have been executed. Move this work off to a thread pool if it is time-intensive.

Listening for Events

To listen to an event, register a bean that implements ApplicationEventListener where the generic type is the type of event.

Listening for Events with ApplicationEventListener
import io.micronaut.context.event.ApplicationEventListener;
import io.micronaut.docs.context.events.SampleEvent;
import jakarta.inject.Singleton;

@Singleton
public class SampleEventListener implements ApplicationEventListener<SampleEvent> {
    private int invocationCounter = 0;

    @Override
    public void onApplicationEvent(SampleEvent event) {
        invocationCounter++;
    }

    public int getInvocationCounter() {
        return invocationCounter;
    }
}


import io.micronaut.context.ApplicationContext;
import io.micronaut.docs.context.events.SampleEventEmitterBean;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

class SampleEventListenerSpec {

    @Test
    void testEventListenerIsNotified() {
        try (ApplicationContext context = ApplicationContext.run()) {
            SampleEventEmitterBean emitter = context.getBean(SampleEventEmitterBean.class);
            SampleEventListener listener = context.getBean(SampleEventListener.class);
            assertEquals(0, listener.getInvocationCounter());
            emitter.publishSampleEvent();
            assertEquals(1, listener.getInvocationCounter());
        }
    }
}
Listening for Events with ApplicationEventListener
import io.micronaut.context.event.ApplicationEventListener
import io.micronaut.docs.context.events.SampleEvent
import jakarta.inject.Singleton

@Singleton
class SampleEventListener implements ApplicationEventListener<SampleEvent> {
    int invocationCounter = 0

    @Override
    void onApplicationEvent(SampleEvent event) {
        invocationCounter++
    }
}

import io.micronaut.context.ApplicationContext
import io.micronaut.docs.context.events.SampleEventEmitterBean
import spock.lang.Specification

class SampleEventListenerSpec extends Specification {

    void "test event listener is notified"() {
        given:
        ApplicationContext context = ApplicationContext.run()
        SampleEventEmitterBean emitter = context.getBean(SampleEventEmitterBean)
        SampleEventListener listener = context.getBean(SampleEventListener)

        expect:
        listener.invocationCounter == 0

        when:
        emitter.publishSampleEvent()

        then:
        listener.invocationCounter == 1

        cleanup:
        context.close()
    }
}
Listening for Events with ApplicationEventListener
import io.micronaut.context.event.ApplicationEventListener
import io.micronaut.docs.context.events.SampleEvent
import jakarta.inject.Singleton

@Singleton
class SampleEventListener : ApplicationEventListener<SampleEvent> {
    var invocationCounter = 0

    override fun onApplicationEvent(event: SampleEvent) {
        invocationCounter++
    }
}

import io.kotest.matchers.shouldBe
import io.kotest.core.spec.style.AnnotationSpec
import io.micronaut.context.ApplicationContext
import io.micronaut.docs.context.events.SampleEventEmitterBean

class SampleEventListenerSpec : AnnotationSpec() {

    @Test
    fun testEventListenerWasNotified() {
        val context = ApplicationContext.run()
        val emitter = context.getBean(SampleEventEmitterBean::class.java)
        val listener = context.getBean(SampleEventListener::class.java)
        listener.invocationCounter.shouldBe(0)
        emitter.publishSampleEvent()
        listener.invocationCounter.shouldBe(1)

        context.close()
    }
}
The supports method can be overridden to further clarify events to be processed.

Alternatively, use the @EventListener annotation if you do not wish to implement an interface or utilize one of the built-in events like StartupEvent and ShutdownEvent:

Listening for Events with @EventListener
import io.micronaut.docs.context.events.SampleEvent;
import io.micronaut.context.event.StartupEvent;
import io.micronaut.context.event.ShutdownEvent;
import io.micronaut.runtime.event.annotation.EventListener;

@Singleton
public class SampleEventListener {
    private int invocationCounter = 0;

    @EventListener
    public void onSampleEvent(SampleEvent event) {
        invocationCounter++;
    }

    @EventListener
    public void onStartupEvent(StartupEvent event) {
        // startup logic here
    }

    @EventListener
    public void onShutdownEvent(ShutdownEvent event) {
        // shutdown logic here
    }

    public int getInvocationCounter() {
        return invocationCounter;
    }
}
Listening for Events with @EventListener
import io.micronaut.docs.context.events.SampleEvent
import io.micronaut.context.event.StartupEvent
import io.micronaut.context.event.ShutdownEvent
import io.micronaut.runtime.event.annotation.EventListener

@Singleton
class SampleEventListener {
    int invocationCounter = 0

    @EventListener
    void onSampleEvent(SampleEvent event) {
        invocationCounter++
    }

    @EventListener
    void onStartupEvent(StartupEvent event) {
        // startup logic here
    }

    @EventListener
    void onShutdownEvent(ShutdownEvent event) {
        // shutdown logic here
    }
}
Listening for Events with @EventListener
import io.micronaut.docs.context.events.SampleEvent
import io.micronaut.context.event.StartupEvent
import io.micronaut.context.event.ShutdownEvent
import io.micronaut.runtime.event.annotation.EventListener

@Singleton
class SampleEventListener {
    var invocationCounter = 0

    @EventListener
    internal fun onSampleEvent(event: SampleEvent) {
        invocationCounter++
    }

    @EventListener
    internal fun onStartupEvent(event: StartupEvent) {
        // startup logic here
    }

    @EventListener
    internal fun onShutdownEvent(event: ShutdownEvent) {
        // shutdown logic here
    }
}

If your listener performs work that might take a while, use the @Async annotation to run the operation on a separate thread:

Asynchronously listening for Events with @EventListener
import io.micronaut.docs.context.events.SampleEvent;
import io.micronaut.runtime.event.annotation.EventListener;
import io.micronaut.scheduling.annotation.Async;

@Singleton
public class SampleEventListener {
    private AtomicInteger invocationCounter = new AtomicInteger(0);

    @EventListener
    @Async
    public void onSampleEvent(SampleEvent event) {
        invocationCounter.getAndIncrement();
    }

    public int getInvocationCounter() {
        return invocationCounter.get();
    }
}


import io.micronaut.context.ApplicationContext;
import io.micronaut.docs.context.events.SampleEventEmitterBean;
import org.junit.jupiter.api.Test;

import static java.util.concurrent.TimeUnit.SECONDS;
import static org.awaitility.Awaitility.await;
import static org.hamcrest.Matchers.equalTo;
import static org.junit.jupiter.api.Assertions.assertEquals;

class SampleEventListenerSpec {

    @Test
    void testEventListenerIsNotified() {
        try (ApplicationContext context = ApplicationContext.run()) {
            SampleEventEmitterBean emitter = context.getBean(SampleEventEmitterBean.class);
            SampleEventListener listener = context.getBean(SampleEventListener.class);
            assertEquals(0, listener.getInvocationCounter());
            emitter.publishSampleEvent();
            await().atMost(5, SECONDS).until(listener::getInvocationCounter, equalTo(1));
        }
    }
}
Asynchronously listening for Events with @EventListener
import io.micronaut.docs.context.events.SampleEvent
import io.micronaut.runtime.event.annotation.EventListener
import io.micronaut.scheduling.annotation.Async

@Singleton
class SampleEventListener {
    AtomicInteger invocationCounter = new AtomicInteger(0)

    @EventListener
    @Async
    void onSampleEvent(SampleEvent event) {
        invocationCounter.getAndIncrement()
    }
}

import io.micronaut.context.ApplicationContext
import io.micronaut.docs.context.events.SampleEventEmitterBean
import spock.lang.Specification
import spock.util.concurrent.PollingConditions

class SampleEventListenerSpec extends Specification {

    void "test event listener is notified"() {
        given:
        def context = ApplicationContext.run()
        def emitter = context.getBean(SampleEventEmitterBean)
        def listener = context.getBean(SampleEventListener)

        expect:
        listener.invocationCounter.get() == 0

        when:
        emitter.publishSampleEvent()

        then:
        new PollingConditions(timeout: 5).eventually {
            listener.invocationCounter.get() == 1
        }

        cleanup:
        context.close()
    }
}
Asynchronously listening for Events with @EventListener
import io.micronaut.docs.context.events.SampleEvent
import io.micronaut.runtime.event.annotation.EventListener
import io.micronaut.scheduling.annotation.Async
import java.util.concurrent.atomic.AtomicInteger

@Singleton
open class SampleEventListener {

    var invocationCounter = AtomicInteger(0)

    @EventListener
    @Async
    open fun onSampleEvent(event: SampleEvent) {
        println("Incrementing invocation counter...")
        invocationCounter.getAndIncrement()
    }
}

import io.kotest.assertions.timing.eventually
import io.kotest.matchers.shouldBe
import io.kotest.core.spec.style.AnnotationSpec
import io.micronaut.context.ApplicationContext
import io.micronaut.docs.context.events.SampleEventEmitterBean
import org.opentest4j.AssertionFailedError
import kotlin.time.DurationUnit
import kotlin.time.ExperimentalTime
import kotlin.time.toDuration

@ExperimentalTime
class SampleEventListenerSpec : AnnotationSpec() {

    @Test
    suspend fun testEventListenerWasNotified() {
        val context = ApplicationContext.run()
        val emitter = context.getBean(SampleEventEmitterBean::class.java)
        val listener = context.getBean(SampleEventListener::class.java)
        listener.invocationCounter.get().shouldBe(0)
        emitter.publishSampleEvent()

        eventually(5.toDuration(DurationUnit.SECONDS), AssertionFailedError::class) {
            println("Current value of counter: " + listener.invocationCounter.get())
            listener.invocationCounter.get().shouldBe(1)
        }

        context.close()
    }
}

The event listener by default runs on the scheduled executor. You can configure this thread pool as required in your configuration file (e.g application.yml):

Configuring Scheduled Task Thread Pool
micronaut.executors.scheduled.type=scheduled
micronaut.executors.scheduled.core-pool-size=30
micronaut:
  executors:
    scheduled:
      type: scheduled
      core-pool-size: 30
[micronaut]
  [micronaut.executors]
    [micronaut.executors.scheduled]
      type="scheduled"
      core-pool-size=30
micronaut {
  executors {
    scheduled {
      type = "scheduled"
      corePoolSize = 30
    }
  }
}
{
  micronaut {
    executors {
      scheduled {
        type = "scheduled"
        core-pool-size = 30
      }
    }
  }
}
{
  "micronaut": {
    "executors": {
      "scheduled": {
        "type": "scheduled",
        "core-pool-size": 30
      }
    }
  }
}

3.15 Bean Events

You can hook into the creation of beans using one of the following interfaces:

  • BeanInitializedEventListener - allows modifying or replacing a bean after properties have been set but prior to @PostConstruct event hooks.

  • BeanCreatedEventListener - allows modifying or replacing a bean after the bean is fully initialized and all @PostConstruct hooks called.

The BeanInitializedEventListener interface is commonly used in combination with Factory beans. Consider the following example:

public class V8Engine implements Engine {
    private final int cylinders = 8;
    private double rodLength; // (1)

    public V8Engine(double rodLength) {
        this.rodLength = rodLength;
    }

    @Override
    public String start() {
        return "Starting V" + getCylinders() + " [rodLength=" + getRodLength() + ']';
    }

    @Override
    public final int getCylinders() {
        return cylinders;
    }

    public double getRodLength() {
        return rodLength;
    }

    public void setRodLength(double rodLength) {
        this.rodLength = rodLength;
    }
}

@Factory
public class EngineFactory {

    private V8Engine engine;
    private double rodLength = 5.7;

    @PostConstruct
    public void initialize() {
        engine = new V8Engine(rodLength); // (2)
    }

    @Singleton
    public Engine v8Engine() {
        return engine;// (3)
    }

    public void setRodLength(double rodLength) {
        this.rodLength = rodLength;
    }
}

@Singleton
public class EngineInitializer implements BeanInitializedEventListener<EngineFactory> { // (4)
    @Override
    public EngineFactory onInitialized(BeanInitializingEvent<EngineFactory> event) {
        EngineFactory engineFactory = event.getBean();
        engineFactory.setRodLength(6.6);// (5)
        return engineFactory;
    }
}
class V8Engine implements Engine {
    final int cylinders = 8
    double rodLength // (1)

    @Override
    String start() {
        return "Starting V$cylinders [rodLength=$rodLength]"
    }
}

@Factory
class EngineFactory {
    private V8Engine engine
    double rodLength = 5.7

    @PostConstruct
    void initialize() {
        engine = new V8Engine(rodLength: rodLength) // (2)
    }

    @Singleton
    Engine v8Engine() {
        return engine // (3)
    }
}

@Singleton
class EngineInitializer implements BeanInitializedEventListener<EngineFactory> { // (4)
    @Override
    EngineFactory onInitialized(BeanInitializingEvent<EngineFactory> event) {
        EngineFactory engineFactory = event.bean
        engineFactory.rodLength = 6.6 // (5)
        return engineFactory
    }
}
class V8Engine(var rodLength: Double) : Engine {  // (1)

    override val cylinders = 8

    override fun start(): String {
        return "Starting V$cylinders [rodLength=$rodLength]"
    }
}

@Factory
class EngineFactory {

    private var engine: V8Engine? = null
    private var rodLength = 5.7

    @PostConstruct
    fun initialize() {
        engine = V8Engine(rodLength) // (2)
    }

    @Singleton
    fun v8Engine(): Engine? {
        return engine// (3)
    }

    fun setRodLength(rodLength: Double) {
        this.rodLength = rodLength
    }
}

@Singleton
class EngineInitializer : BeanInitializedEventListener<EngineFactory> { // (4)
    override fun onInitialized(event: BeanInitializingEvent<EngineFactory>): EngineFactory {
        val engineFactory = event.bean
        engineFactory.setRodLength(6.6) // (5)
        return engineFactory
    }
}
1 The V8Engine class defines a rodLength property
2 The EngineFactory initializes the value of rodLength and creates the instance
3 The created instance is returned as a Bean
4 The BeanInitializedEventListener interface is implemented to listen for the initialization of the factory
5 Within the onInitialized method the rodLength is overridden prior to the engine being created by the factory bean.

The BeanCreatedEventListener interface is more typically used to decorate or enhance a fully initialized bean, for example by creating a proxy.

Bean event listeners are initialized before type converters. If your event listener relies on type conversion either by relying on a configuration properties bean or by any other mechanism, you may see errors related to type conversion.

3.16 Bean Introspection

Since Micronaut framework 1.1, a compile-time replacement for the JDK’s Introspector class has been included.

The BeanIntrospector and BeanIntrospection interfaces allow looking up bean introspections to instantiate and read/write bean properties without using reflection or caching reflective metadata, which consume excessive memory for large beans.

3.16.1 Making a Bean Available for Introspection

Unlike the JDK’s Introspector, every class is not automatically available for introspection. To make a class available for introspection you must at a minimum enable Micronaut’s annotation processor (micronaut-inject-java for Java and Kotlin and micronaut-inject-groovy for Groovy) in your build and ensure you have a runtime time dependency on micronaut-core.

annotationProcessor("io.micronaut:micronaut-inject-java")
<annotationProcessorPaths>
    <path>
        <groupId>io.micronaut</groupId>
        <artifactId>micronaut-inject-java</artifactId>
    </path>
</annotationProcessorPaths>

For Kotlin, add the micronaut-inject-java dependency in kapt scope, and for Groovy add micronaut-inject-groovy in compileOnly scope.

runtimeOnly("io.micronaut:micronaut-core")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-core</artifactId>
    <scope>runtime</scope>
</dependency>

Once your build is configured you have a few ways to generate introspection data.

3.16.2 Use the @Introspected Annotation

The @Introspected annotation can be used on any class to make it available for introspection. Simply annotate the class with @Introspected:

import io.micronaut.core.annotation.Introspected;

@Introspected
public class Person {

    private String name;
    private int age = 18;

    public Person(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}
import groovy.transform.Canonical
import io.micronaut.core.annotation.Introspected

@Introspected
@Canonical
class Person {

    String name
    int age = 18

    Person(String name) {
        this.name = name
    }
}
import io.micronaut.core.annotation.Introspected

@Introspected
data class Person(var name : String) {
    var age : Int = 18
}

Once introspection data has been produced at compile time, retrieve it via the BeanIntrospection API:

final BeanIntrospection<Person> introspection = BeanIntrospection.getIntrospection(Person.class); // (1)
Person person = introspection.instantiate("John"); // (2)
System.out.println("Hello " + person.getName());

final BeanProperty<Person, String> property = introspection.getRequiredProperty("name", String.class); // (3)
property.set(person, "Fred"); // (4)
String name = property.get(person); // (5)
System.out.println("Hello " + person.getName());
def introspection = BeanIntrospection.getIntrospection(Person) // (1)
Person person = introspection.instantiate("John") // (2)
println("Hello $person.name")

BeanProperty<Person, String> property = introspection.getRequiredProperty("name", String) // (3)
property.set(person, "Fred") // (4)
String name = property.get(person) // (5)
println("Hello $person.name")
val introspection = BeanIntrospection.getIntrospection(Person::class.java) // (1)
val person : Person = introspection.instantiate("John") // (2)
print("Hello ${person.name}")

val property : BeanProperty<Person, String> = introspection.getRequiredProperty("name", String::class.java) // (3)
property.set(person, "Fred") // (4)
val name = property.get(person) // (5)
print("Hello ${person.name}")
1 You can retrieve a BeanIntrospection with the static getIntrospection method
2 Once you have a BeanIntrospection you can instantiate a bean with the instantiate method.
3 A BeanProperty can be retrieved from the introspection
4 Use the set method to set the property value
5 Use the get method to retrieve the property value

3.16.3 Use @Introspected with @AccessorsStyle

It is possible to use the @AccessorsStyle annotation with @Introspected:

import io.micronaut.core.annotation.AccessorsStyle;
import io.micronaut.core.annotation.Introspected;

@Introspected
@AccessorsStyle(readPrefixes = "", writePrefixes = "") // (1)
public class Person {

    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String name() { // (2)
        return name;
    }

    public void name(String name) { // (2)
        this.name = name;
    }

    public int age() { // (2)
        return age;
    }

    public void age(int age) { // (2)
        this.age = age;
    }
}
import io.micronaut.core.annotation.AccessorsStyle
import io.micronaut.core.annotation.Introspected

@Introspected
@AccessorsStyle(readPrefixes = "", writePrefixes = "") // (1)
class Person {

    private String name
    private int age

    Person(String name, int age) {
        this.name = name
        this.age = age
    }

    String name() { // (2)
        return name
    }

    void name(String name) { // (2)
        this.name = name
    }

    int age() { // (2)
        return age
    }

    void age(int age) { // (2)
        this.age = age
    }
}
import io.micronaut.core.annotation.AccessorsStyle
import io.micronaut.core.annotation.Introspected

@Introspected
@AccessorsStyle(readPrefixes = [""], writePrefixes = [""]) // (1)
class Person(private var name: String, private var age: Int) {
    fun name(): String { // (2)
        return name
    }

    fun name(name: String) { // (2)
        this.name = name
    }

    fun age(): Int { // (2)
        return age
    }

    fun age(age: Int) { // (2)
        this.age = age
    }
}
1 Annotate the class with @AccessorsStyle to define empty read and write prefixes for getters and setters.
2 Define the getters and setters without a prefix.

Now it is possible to retrieve the compile time generated introspection using the BeanIntrospection API:

        BeanIntrospection<Person> introspection = BeanIntrospection.getIntrospection(Person)
        Person person = introspection.instantiate('John', 42)

        person.name() == 'John'
        person.age() == 42
        val introspection = BeanIntrospection.getIntrospection(Person::class.java)
        val person = introspection.instantiate("John", 42)

        Assertions.assertEquals("John", person.name())
        Assertions.assertEquals(42, person.age())

3.16.4 Bean Fields

By default, Java introspections treat only JavaBean getters/setters or Java 16 record components as bean properties. You can however define classes with public or package protected fields in Java using the accessKind member of the @Introspected annotation:

import io.micronaut.core.annotation.Introspected;

@Introspected(accessKind = Introspected.AccessKind.FIELD)
public class User {
    public final String name; // (1)
    public int age = 18; // (2)

    public User(String name) {
        this.name = name;
    }
}
import io.micronaut.core.annotation.Introspected

@Introspected(accessKind = Introspected.AccessKind.FIELD)
class User {
    public final String name // (1)
    public int age = 18 // (2)

    User(String name) {
        this.name = name
    }
}
@Introspected(accessKind = [Introspected.AccessKind.FIELD])
class User(
    val name: String // (1)
) {
    var age = 18 // (2)
}
1 Final fields are treated like read-only properties
2 Mutable fields are treated like read-write properties
The accessKind accepts an array, so it is possible to allow for both types of accessors but prefer one or the other depending on the order they appear in the annotation. The first one in the list has priority.
Introspections on fields are not possible in Kotlin because it is not possible to declare fields directly.

3.16.5 Constructor Methods

For classes with multiple constructors, apply the @Creator annotation to the constructor to use.

import io.micronaut.core.annotation.Creator;
import io.micronaut.core.annotation.Introspected;

import javax.annotation.concurrent.Immutable;

@Introspected
@Immutable
public class Vehicle {

    private final String make;
    private final String model;
    private final int axles;

    public Vehicle(String make, String model) {
        this(make, model, 2);
    }

    @Creator // (1)
    public Vehicle(String make, String model, int axles) {
        this.make = make;
        this.model = model;
        this.axles = axles;
    }

    public String getMake() {
        return make;
    }

    public String getModel() {
        return model;
    }

    public int getAxles() {
        return axles;
    }
}
import io.micronaut.core.annotation.Creator
import io.micronaut.core.annotation.Introspected

import javax.annotation.concurrent.Immutable

@Introspected
@Immutable
class Vehicle {

    final String make
    final String model
    final int axles

    Vehicle(String make, String model) {
        this(make, model, 2)
    }

    @Creator // (1)
    Vehicle(String make, String model, int axles) {
        this.make = make
        this.model = model
        this.axles = axles
    }
}
import io.micronaut.core.annotation.Creator
import io.micronaut.core.annotation.Introspected

import javax.annotation.concurrent.Immutable

@Introspected
@Immutable
class Vehicle @Creator constructor(val make: String, val model: String, val axles: Int) { // (1)

    constructor(make: String, model: String) : this(make, model, 2) {}
}
1 The @Creator annotation denotes which constructor to use
This class has no default constructor, so calls to instantiate without arguments throw an InstantiationException.

3.16.6 Static Creator Methods

The @Creator annotation can be applied to static methods that create class instances.

import io.micronaut.core.annotation.Creator;
import io.micronaut.core.annotation.Introspected;

import javax.annotation.concurrent.Immutable;

@Introspected
@Immutable
public class Business {

    private final String name;

    private Business(String name) {
        this.name = name;
    }

    @Creator // (1)
    public static Business forName(String name) {
        return new Business(name);
    }

    public String getName() {
        return name;
    }
}
import io.micronaut.core.annotation.Creator
import io.micronaut.core.annotation.Introspected

import javax.annotation.concurrent.Immutable

@Introspected
@Immutable
class Business {

    final String name

    private Business(String name) {
        this.name = name
    }

    @Creator // (1)
    static Business forName(String name) {
        new Business(name)
    }
}
import io.micronaut.core.annotation.Creator
import io.micronaut.core.annotation.Introspected

import javax.annotation.concurrent.Immutable

@Introspected
@Immutable
class Business private constructor(val name: String) {
    companion object {

        @Creator // (1)
        fun forName(name: String): Business {
            return Business(name)
        }
    }

}
1 The @Creator annotation is applied to the static method which instantiates the class
There can be multiple "creator" methods annotated. If there is one without arguments, it will be the default construction method. The first method with arguments will be used as the primary construction method.

3.16.7 Builders

If a type can only be constructed via the builder pattern then you can use the builder member of the @Introspected annotation to generate a dynamic builder. For example given this class:

@Introspected(builder = @Introspected.IntrospectionBuilder(
    builderClass = Person.Builder.class
))
public class Person {
    private final String name;
    private final int age;
    private Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public static Builder builder() {
        return new Builder();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Person person = (Person) o;

        if (age != person.age) return false;
        return Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }

    public static final class Builder {
        private String name;
        private int age;

        public Builder name(String name) {
            this.name = name;
            return this;
        }

        public Builder age(int age) {
            this.age = age;
            return this;
        }

        public Person build() {
            Objects.requireNonNull(name);
            if (age < 1) {
                throw new IllegalArgumentException("Age must be a positive number");
            }
            return new Person(name, age);
        }
    }
}
import io.micronaut.core.annotation.Introspected
@CompileStatic
@Introspected(builder = @Introspected.IntrospectionBuilder(
        builderClass = Person.Builder.class
))
@EqualsAndHashCode
class Person {
    final String name
    final int age

    private Person(String name, int age) {
        this.name = name
        this.age = age
    }

    static Builder builder() {
        new Builder()
    }

    static final class Builder {
        private String name
        private int age

        Builder name(String name) {
            this.name = name
            this
        }

        Builder age(int age) {
            this.age = age
            this
        }

        Person build() {
            Objects.requireNonNull(name)
            if (age < 1) {
                throw new IllegalArgumentException("Age must be a positive number")
            }
            new Person(name, age)
        }
    }
}
@Introspected(builder = Introspected.IntrospectionBuilder(builderClass = Person.Builder::class))
data class Person private constructor(val name: String, val age: Int) {
    data class Builder(
        var name: String? = null,
        var age: Int = 0
    ) {
        fun name(name: String) = apply { this.name = name }
        fun age(age: Int) = apply { this.age = age }

        fun build(): Person {
            requireNotNull(name) { "Name must be specified" }
            require(age >= 1) { "Age must be a positive number" }
            return Person(name!!, age)
        }
    }
}

You can use the builder() method of the BeanIntrospection API to construct the instance:

BeanIntrospection<Person> introspection = BeanIntrospection.getIntrospection(Person.class);
BeanIntrospection.Builder<Person> builder = introspection.builder();
Person person = builder
    .with("age", 25)
    .with("name", "Fred")
    .build();
BeanIntrospection<Person> introspection = BeanIntrospection.getIntrospection(Person.class);
BeanIntrospection.Builder<Person> builder = introspection.builder()
Person person = builder
        .with("age", 25)
        .with("name", "Fred")
        .build()
val introspection = BeanIntrospection.getIntrospection(
    Person::class.java
)
val builder = introspection.builder()
val person = builder
    .with("age", 25)
    .with("name", "Fred")
    .build()
The builder() method also works regardless if the type uses a builder and can be used as a general abstraction for object construction. Note however that there is a slight performance overhead vs direct instantiation via the instantiate() method, hence the hasBuilder() method can be checked if optimized code paths are needed.
Introspection Builder does not work with Groovy @Builder AST.

3.16.8 Introspect Enums

It is possible to introspect enums as well. Add the annotation to the enum, and it can be constructed through the standard valueOf method.

3.16.9 Use the @Introspected Annotation on a Configuration Class

If the class to introspect is already compiled and not under your control, an alternative option is to define a configuration class with the classes member of the @Introspected annotation set.

import io.micronaut.core.annotation.Introspected;

@Introspected(classes = Person.class)
public class PersonConfiguration {
}
import io.micronaut.core.annotation.Introspected

@Introspected(classes = Person)
class PersonConfiguration {
}
import io.micronaut.core.annotation.Introspected

@Introspected(classes = [Person::class])
class PersonConfiguration

In the above example the PersonConfiguration class generates introspections for the Person class.

You can also use the packages member of the @Introspected which package scans at compile time and generates introspections for all classes within a package. Note however this feature is currently regarded as experimental.

3.16.10 Write an AnnotationMapper to Introspect Existing Annotations

If there is an existing annotation that you wish to introspect by default you can write an AnnotationMapper.

An example of this is EntityIntrospectedAnnotationMapper which ensures all beans annotated with javax.persistence.Entity are introspectable by default.

The AnnotationMapper must be on the annotation processor classpath.

3.16.11 The BeanWrapper API

A BeanProperty provides raw access to read and write a property value for a given class and does not provide any automatic type conversion.

It is expected that the values you pass to the set and get methods match the underlying property type, otherwise an exception will occur.

To provide additional type conversion smarts the BeanWrapper interface allows wrapping an existing bean instance and setting and getting properties from the bean, plus performing type conversion as necessary.

final BeanWrapper<Person> wrapper = BeanWrapper.getWrapper(new Person("Fred")); // (1)

wrapper.setProperty("age", "20"); // (2)
int newAge = wrapper.getRequiredProperty("age", int.class); // (3)

System.out.println("Person's age now " + newAge);
final BeanWrapper<Person> wrapper = BeanWrapper.getWrapper(new Person("Fred")) // (1)

wrapper.setProperty("age", "20") // (2)
int newAge = wrapper.getRequiredProperty("age", Integer) // (3)

println("Person's age now $newAge")
val wrapper = BeanWrapper.getWrapper(Person("Fred")) // (1)

wrapper.setProperty("age", "20") // (2)
val newAge = wrapper.getRequiredProperty("age", Int::class.java) // (3)

println("Person's age now $newAge")
1 Use the static getWrapper method to obtain a BeanWrapper for a bean instance.
2 You can set properties, and the BeanWrapper will perform type conversion, or throw ConversionErrorException if conversion is not possible.
3 You can retrieve a property using getRequiredProperty and request the appropriate type. If the property doesn’t exist a IntrospectionException is thrown, and if it cannot be converted a ConversionErrorException is thrown.

3.16.12 Jackson and Bean Introspection

Jackson is configured to use the BeanIntrospection API to read and write property values and construct objects, resulting in reflection-free serialization/deserialization. This is beneficial from a performance perspective and requires less configuration to operate correctly with runtimes such as GraalVM native.

This feature is enabled by default; disable it by setting the jackson.bean-introspection-module configuration to false.

Currently only bean properties (private field with public getter/setter) are supported and usage of public fields is not supported.

3.16.13 Kotlin and Bean Introspection

You can annotate a Kotlin Data Class with @Introspected:

Kotlin Data Class annotated with @Introspected
@Introspected
data class UserDataClass(val name: String)

and instantiate it with the BeanIntrospection API:

Kotlin Data Class instantiated via BeanIntrospection API
val introspection: BeanIntrospection<UserDataClass> = BeanIntrospection.getIntrospection(UserDataClass::class.java)
val user: UserDataClass = introspection.instantiate("John")
Kotlin Inline Value Classes are not supported yet by the BeanIntrospection API.

3.17 Bean Mappers

Since 4.1.x the @Mapper annotation can be used on any abstract method to automatically create a mapping between one type and another.

Inspired by similar functionality in libraries like Map Struct, a Mapper uses the Bean Introspection and Expressions features, built into the Micronaut Framework, which are already reflection free.

For Mapping, base and target types need to be introspected.

@Mapper Example

Given the following types:

@Introspected
public record ContactForm(String firstName, String lastName) {
}
@Introspected
class ContactForm {
    String firstName
    String lastName
}
@Introspected
data class ContactForm(val firstName: String, val lastName: String)
@Introspected
public record ContactEntity(Long id, String firstName, String lastName) {
}
@Introspected
class ContactEntity {
    Long id
    String firstName
    String lastName
}
@Introspected
data class ContactEntity(@Nullable var id: Long? = null, val firstName: String, val lastName: String)

You can write an interface to define a mapping between both types by simply annotating a method with @Mapper.

import io.micronaut.context.annotation.Mapper;

public interface ContactMappers {
    @Mapper
    ContactEntity toEntity(ContactForm contactForm);
}
import io.micronaut.context.annotation.Mapper

interface ContactMappers {
    @Mapper
    ContactEntity toEntity(ContactForm contactForm)
}
import io.micronaut.context.annotation.Mapper

interface ContactMappers {
    @Mapper
    fun toEntity(contactForm: ContactForm) : ContactEntity
}

The Micronaut compiler generates an implementation the previous an interface at compilation-time.

You can then inject a bean of type ContactMappers and easily map from one type to another.

ContactMappers contactMappers = context.getBean(ContactMappers.class);
ContactEntity contactEntity = contactMappers.toEntity(new ContactForm("John", "Snow"));
assertEquals("John", contactEntity.firstName());
assertEquals("Snow", contactEntity.lastName());
ContactMappers contactMappers = context.getBean(ContactMappers)
ContactEntity contactEntity = contactMappers.toEntity(new ContactForm(firstName: "John", lastName: "Snow"))
assertEquals("John", contactEntity.firstName)
assertEquals("Snow", contactEntity.lastName)
val contactMappers = context.getBean(ContactMappers::class.java)
val entity : ContactEntity = contactMappers.toEntity(ContactForm("John", "Snow"))
Assertions.assertEquals("John", entity.firstName)
Assertions.assertEquals("Snow", entity.lastName)

@Mapping Example

Each abstract method can define a single @Mapper annotation or one or many @Mapping annotations to define how properties map onto the target type.

For example, given the following type:

import io.micronaut.core.annotation.Introspected;

@Introspected
public record Product(
    String name,
    double price,
    String manufacturer) {
}
import groovy.transform.Canonical
import io.micronaut.core.annotation.Introspected

@Canonical
@Introspected
class Product {
    String name
    double price
    String manufacturer
}
@Introspected
data class Product(val name: String, val price: Double, val manufacturer: String)

It is common to want to alter this type’s representation in HTTP responses. For example, consider this response type:

import io.micronaut.core.annotation.Introspected;

@Introspected
public record ProductDTO(String name, String price, String distributor) {
}
import io.micronaut.core.annotation.Introspected;

@Introspected
@Canonical
class ProductDTO {
    String name
    String price
    String distributor
}
@Introspected
data class ProductDTO(val name: String, val price: String, val distributor: String)

Here the price property is of a different type and an extra property exists called distributor. You could write manual logic to deal the mapping and these differences, or you could define a mapping:

import io.micronaut.context.annotation.Mapper.Mapping;
import jakarta.inject.Singleton;

@Singleton
public interface ProductMappers {
    @Mapping(
        to = "price",
        from = "#{product.price * 2}",
        format = "$#.00"
    )
    @Mapping(
        to = "distributor",
        from = "#{this.getDistributor()}"
    )
    ProductDTO toProductDTO(Product product);

    default String getDistributor() {
        return "Great Product Company";
    }
}
import io.micronaut.context.annotation.Mapper.Mapping
import jakarta.inject.Singleton

@Singleton
interface ProductMappers {
    @Mapping(
        to = "price",
        from = "#{product.price * 2}",
        format = '$#.00'
    )
    @Mapping(
        to = "distributor",
        from = "#{this.getDistributor()}"
    )
    ProductDTO toProductDTO(Product product);

    default String getDistributor() {
        return "Great Product Company"
    }
}
import io.micronaut.context.annotation.Mapper.Mapping
import jakarta.inject.Singleton

@Singleton
abstract class ProductMappers {
    @Mapping(to = "price", from = "#{product.price * 2}", format = "$#.00")
    @Mapping(to = "distributor", from = "#{this.getDistributor()}")
    abstract fun toProductDTO(product: Product): ProductDTO
    fun getDistributor() : String = "Great Product Company"
}

The from member can be used to define either a property name on the source type or an expression that reads values from the method argument and transforms them in whatever way you choose, including invoking other methods of the instance.

A @Mapping definition is only needed if you need to apply a transformation for the mapping to be successful. Other properties will be automatically mapped and converted.

You can retrieve from the context or inject a bean of type ProductMappers. Then, you can use the toProductDTO method to map from the Product type to the ProductDTO type:

ProductMappers productMappers = context.getBean(ProductMappers.class);

ProductDTO productDTO = productMappers.toProductDTO(new Product(
    "MacBook",
    910.50,
    "Apple"
));

assertEquals("MacBook", productDTO.name());
assertEquals("$1821.00", productDTO.price());
assertEquals("Great Product Company", productDTO.distributor());
given:
ProductMappers productMappers = context.getBean(ProductMappers.class)

when:
ProductDTO productDTO = productMappers.toProductDTO(new Product(
        "MacBook",
        910.50,
        "Apple"
))

then:
productDTO.name == 'MacBook'
productDTO.price == '$1821.00'
productDTO.distributor == "Great Product Company"
val productMappers = context.getBean(ProductMappers::class.java)
val (name, price, distributor) = productMappers.toProductDTO(
    Product(
        "MacBook",
        910.50,
        "Apple"
    )
)
Assertions.assertEquals("MacBook", name)
Assertions.assertEquals("$1821.00", price)
Assertions.assertEquals("Great Product Company", distributor)

3.18 Bean Validation

3.19 Bean Annotation Metadata

The methods provided by Java’s AnnotatedElement API in general don’t provide the ability to introspect annotations without loading the annotations themselves. Nor do they provide any ability to introspect annotation stereotypes (often called meta-annotations; an annotation stereotype is where an annotation is annotated with another annotation, essentially inheriting its behaviour).

To solve this problem many frameworks produce runtime metadata or perform expensive reflection to analyze the annotations of a class.

The Micronaut framework instead produces this annotation metadata at compile time, avoiding expensive reflection and saving memory.

The BeanContext API can be used to obtain a reference to a BeanDefinition which implements the AnnotationMetadata interface.

For example the following code obtains all bean definitions annotated with a particular stereotype:

Lookup Bean Definitions by Stereotype
BeanContext beanContext = ... // obtain the bean context
Collection<BeanDefinition> definitions =
    beanContext.getBeanDefinitions(Qualifiers.byStereotype(Controller.class))

for (BeanDefinition definition : definitions) {
    AnnotationValue<Controller> controllerAnn = definition.getAnnotation(Controller.class);
    // do something with the annotation
}

The above example finds all BeanDefinition instances annotated with @Controller whether @Controller is used directly or inherited via an annotation stereotype.

Note that the getAnnotation method and the variations of the method return an AnnotationValue type and not a Java annotation. This is by design, and you should generally try to work with this API when reading annotation values, since synthesizing a proxy implementation is worse from a performance and memory consumption perspective.

If you require a reference to an annotation instance you can use the synthesize method, which creates a runtime proxy that implements the annotation interface:

Synthesizing Annotation Instances
Controller controllerAnn = definition.synthesize(Controller.class);

This approach is not recommended however, as it requires reflection and increases memory consumption due to the use of runtime generated proxies, and should be used as a last resort, for example if you need an instance of the annotation to integrate with a third-party library.

Annotation Inheritance

The Micronaut framework will respect the rules defined in Java’s AnnotatedElement API with regard to annotation inheritance:

  • Annotations meta-annotated with Inherited will be available via the getAnnotation* methods of the AnnotationMetadata API whilst those directly declared are available via the getDeclaredAnnotation* methods.

  • Annotations not meta-annotated with Inherited will not be included in the metadata

The Micronaut framework differs from the AnnotatedElement API in that it extends these rules to methods and method parameters such that:

In general behaviour which you may wish to override is not inherited by default including Bean Scopes, Bean Qualifiers, Bean Conditions, Validation Rules and so on.

If you wish a particular scope, qualifier, or set of requirements to be inherited when subclassing then you can define a meta-annotation that is annotated with @Inherited. For example:

Defining Inherited Meta Annotations
import io.micronaut.context.annotation.AliasFor;
import io.micronaut.context.annotation.Requires;
import io.micronaut.core.annotation.AnnotationMetadata;
import jakarta.inject.Named;
import jakarta.inject.Singleton;

import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Inherited // (1)
@Retention(RetentionPolicy.RUNTIME)
@Requires(property = "datasource.url") // (2)
@Named // (3)
@Singleton // (4)
public @interface SqlRepository {
    @AliasFor(annotation = Named.class, member = AnnotationMetadata.VALUE_MEMBER) // (5)
    String value() default "";
}
Defining Inherited Meta Annotations
import io.micronaut.context.annotation.AliasFor
import io.micronaut.context.annotation.Requires
import io.micronaut.core.annotation.AnnotationMetadata
import jakarta.inject.Named
import jakarta.inject.Singleton

import java.lang.annotation.Inherited
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy

@Inherited // (1)
@Retention(RetentionPolicy.RUNTIME)
@Requires(property = "datasource.url") // (2)
@Named // (3)
@Singleton // (4)
@interface SqlRepository {
    @AliasFor(annotation = Named.class, member = AnnotationMetadata.VALUE_MEMBER) // (5)
    String value() default "";
}
Defining Inherited Meta Annotations
import io.micronaut.context.annotation.Requires
import jakarta.inject.Named
import jakarta.inject.Singleton
import java.lang.annotation.Inherited

@Inherited // (1)
@kotlin.annotation.Retention(AnnotationRetention.RUNTIME)
@Requires(property = "datasource.url") // (2)
@Named // (3)
@Singleton // (4)
annotation class SqlRepository(
    val value: String = ""
)
1 The annotation is declared as @Inherited
2 Bean Conditions will be inherited by child classes
3 Bean Qualifiers will be inherited by child classes
4 Bean Scopes will be inherited by child classes
5 You can also alias annotations and they will be inherited

With this meta-annotation in place you can add the annotation to a super class:

Using Inherited Meta Annotations on a Super Class
@SqlRepository
public abstract class BaseSqlRepository {
}
Using Inherited Meta Annotations on a Super Class
@SqlRepository
abstract class BaseSqlRepository {
}
Using Inherited Meta Annotations on a Super Class
@SqlRepository
abstract class BaseSqlRepository

And then a subclass will inherit all the annotations:

Inherting Annotations in a Child Class
import jakarta.inject.Named;
import javax.sql.DataSource;

@Named("bookRepository")
public class BookRepository extends BaseSqlRepository {
    private final DataSource dataSource;

    public BookRepository(DataSource dataSource) {
        this.dataSource = dataSource;
    }
}
Inherting Annotations in a Child Class
import jakarta.inject.Named
import javax.sql.DataSource

@Named("bookRepository")
class BookRepository extends BaseSqlRepository {
    private final DataSource dataSource

    BookRepository(DataSource dataSource) {
        this.dataSource = dataSource
    }
}
Inherting Annotations in a Child Class
import jakarta.inject.Named
import javax.sql.DataSource

@Named("bookRepository")
class BookRepository(private val dataSource: DataSource) : BaseSqlRepository()
A child class must at least have one bean definition annotation such as a scope or qualifier.

Aliasing / Mapping Annotations

There are times when you may want to alias the value of an annotation member to the value of another annotation member. To do this, use the @AliasFor annotation.

A common use case is for example when an annotation defines the value() member, but also supports other members. for example the @Client annotation:

The @Client Annotation
public @interface Client {

    /**
     * @return The URL or service ID of the remote service
     */
    @AliasFor(member = "id") (1)
    String value() default "";

    /**
     * @return The ID of the client
     */
    @AliasFor(member = "value") (2)
    String id() default "";
}
1 The value member also sets the id member
2 The id member also sets the value member

With these aliases in place, whether you define @Client("foo") or @Client(id="foo"), both the value and id members will be set, making it easier to parse and work with the annotation.

If you do not have control over the annotation, another approach is to use an AnnotationMapper. To create an AnnotationMapper, do the following:

  • Implement the AnnotationMapper interface

  • Define a META-INF/services/io.micronaut.inject.annotation.AnnotationMapper file referencing the implementation class

  • Add the JAR file containing the implementation to the annotationProcessor classpath (kapt for Kotlin)

Because AnnotationMapper implementations must be on the annotation processor classpath, they should generally be in a project that includes few external dependencies to avoid polluting the annotation processor classpath.

The following is an example AnnotationMapper that improves the introspection capabilities of JPA entities.

EntityIntrospectedAnnotationMapper Mapper Example
public class EntityIntrospectedAnnotationMapper implements NamedAnnotationMapper {
    @NonNull
    @Override
    public String getName() {
        return "javax.persistence.Entity";
    }

    @Override
    public List<AnnotationValue<?>> map(AnnotationValue<Annotation> annotation, VisitorContext visitorContext) { (1)
        (2)
        return Arrays.asList(
                AnnotationValue.builder(Introspected.class).build(),
                AnnotationValue.builder(ReflectiveAccess.class).build()
        );
    }
}
1 The map method receives a AnnotationValue with the values for the annotation.
2 One or more annotations can be returned.
The example above implements the NamedAnnotationMapper interface which allows for annotations to be mixed with runtime code. To operate against a concrete annotation type, use TypedAnnotationMapper instead, although note it requires the annotation class itself to be on the annotation processor classpath.

3.20 Importing Beans from Libraries

You can use the @Import annotation to import beans from external, already compiled libraries that use JSR-330 annotations.

Bean import is currently only supported in the Java language as other languages have limitations on classpath scanning during source code processing.

For example, to import the JSR-330 TCK into an application, add a dependency on the TCK:

implementation("io.micronaut:jakarta.inject-tck:2.0.1")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>jakarta.inject-tck</artifactId>
    <version>2.0.1</version>
</dependency>

Then define the @Import annotation on your Application class:

package example;

import io.micronaut.context.annotation.Import;

@Import( (1)
        packages = { (2)
                "org.atinject.tck.auto",
                "org.atinject.tck.auto.accessories"},
        annotated = "*") (3)
public class Application {
}
1 The @Import is defined
2 The packages to import are defined. Note that the Micronaut framework will not recurse through sub-packages so sub-packages need to be listed explicitly
3 By default, Micronaut framework will only import classes that feature a scope or a qualifier. By using * you can make every type a bean.
In general @Import should be used in applications rather than libraries since if two libraries import the same beans the result will likely be a NonUniqueBeanException

3.21 Nullability Annotations

In Java, you can use annotations showing whether a variable can or cannot be null. Such annotations aren’t part of the standard library, but you can add them separately.

Micronaut framework comes with its own set of annotations to declare nullability; @Nullable and @NonNull.

Why does the Micronaut framework add its own set of nullability annotations instead of using one of the existing nullability annotations libraries?

Throughout the history of the framework, we used other nullability annotation libraries. However, licensing issues made us change nullability annotations several times. To avoid having to change nullability annotations in the future, we added our own set of nullability annotations in Micronaut framework 2.4

Are Micronaut Nullability annotations recognized by Kotlin?

Yes, Micronaut framework’s nullability annotations are mapped at compilation time to jakarta.annotation.Nullable and jakarta.annotation.Nonnull.

Why should you use nullability annotations in your code?

Moreover, you can use @Nullable annotation to mark:

  • A Controller method parameter as optional.

  • An injection point as optional. For example, when using constructor injection you can annotate one a constructor parameter as optional by adding the @Nullable annotation.

3.22 Micronaut Beans And Spring

Micronaut framework has integrations with Spring in several forms. See the Micronaut Spring Documentation for more information.

3.23 Android Support

Since Micronaut dependency injection is based on annotation processors and doesn’t rely on reflection, it can be used on Android when using the Android plugin 3.0.0 or higher.

This lets you use the same application framework for both your Android client and server implementation.

Configuring Your Android Build

To get started, add the Micronaut annotation processors to the processor classpath using the annotationProcessor dependency configuration.

Include the Micronaut micronaut-inject-java dependency in both the annotationProcessor and compileOnly scopes of your Android build configuration:

Example Android build.gradle
dependencies {
    ...
    annotationProcessor "io.micronaut:micronaut-inject-java:4.7.6"
    compileOnly "io.micronaut:micronaut-inject-java:4.7.6"
    ...
}

If you use lint as part of your build you may also need to disable the invalid packages check since Android includes a hard-coded check that regards the jakarta.inject package as invalid unless you use Dagger:

Configure lint within build.gradle
android {
    ...
    lintOptions {
        lintOptions { warning 'InvalidPackage' }
    }
}

You can find more information on configuring annotations processors in the Android documentation.

Micronaut inject-java dependency uses Android Java 8 support features.

Enabling Dependency Injection

Once you have configured the classpath correctly, the next step is start the ApplicationContext.

The following example demonstrates creating a subclass of android.app.Application for that purpose:

Example Android Application Class
import android.app.Activity;
import android.app.Application;
import android.os.Bundle;

import io.micronaut.context.ApplicationContext;
import io.micronaut.context.env.Environment;

public class BaseApplication extends Application { (1)

    private ApplicationContext ctx;

    @Override
    public void onCreate() {
        super.onCreate();
        ctx = ApplicationContext.run(MainActivity.class, Environment.ANDROID); (2)
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() { (3)
            @Override
            public void onActivityCreated(Activity activity, Bundle bundle) {
                ctx.inject(activity);
            }
            ... // shortened for brevity; it is not necessary to implement other methods
        });
    }
}
1 Extend the android.app.Application class
2 Run the ApplicationContext with the ANDROID environment
3 Register an ActivityLifecycleCallbacks instance to allow dependency injection of Android Activity instances

4 Application Configuration

Micronaut features a flexible configuration mechanism that allows reading configuration from a variety of sources into a unified model that can be bound to Java types annotated with @ConfigurationProperties.

Configuration can by default be provided in Java properties files or JSON with the ability to add support for more formats (such as YAML or Groovy configuration) by adding addition third-party libraries to your classpath. The convention is to search for a file named application.properties or application.json with support for other formats requiring additional dependencies as described by the following table:

Table 1. Supported Configuration Formats
Format File Dependency Required

YAML

application.yml

org.yaml:snakeyaml

Groovy Config

application.groovy

io.micronaut.groovy:micronaut-runtime-groovy

HOCON

application.conf

io.micronaut.kotlin:micronaut-kotlin-runtime

TOML

application.toml

io.micronaut.toml:micronaut-toml

In addition, Micronaut framework allows overriding any property via system properties or environment variables.

Each source of configuration is modeled with the PropertySource interface and the mechanism is extensible, allowing the implementation of additional PropertySourceLoader implementations.

4.1 The Environment

The application environment is modelled by the Environment interface, which allows specifying one or many unique environment names when creating an ApplicationContext.

Initializing the Environment
ApplicationContext applicationContext = ApplicationContext.run("test", "android");
Environment environment = applicationContext.getEnvironment();

assertTrue(environment.getActiveNames().contains("test"));
assertTrue(environment.getActiveNames().contains("android"));
Initializing the Environment
when:
ApplicationContext applicationContext = ApplicationContext.run("test", "android")
Environment environment = applicationContext.getEnvironment()

then:
environment.activeNames.contains("test")
environment.activeNames.contains("android")
Initializing the Environment
val applicationContext = ApplicationContext.run("test", "android")
val environment = applicationContext.environment

environment.activeNames shouldContain "test"
environment.activeNames shouldContain "android"

The active environment names allow loading different configuration files depending on the environment, and also using the @Requires annotation to conditionally load beans or bean @Configuration packages.

In addition, the Micronaut framework attempts to detect the current environments. For example within a Spock or JUnit test the TEST environment is automatically active.

Additional active environments can be specified using the micronaut.environments system property or the MICRONAUT_ENVIRONMENTS environment variable. These are specified as a comma-separated list. For example:

Specifying environments
$ java -Dmicronaut.environments=foo,bar -jar myapp.jar

The above activates environments called foo and bar.

It is also possible to enable the detection of the Cloud environment the application is deployed to (this feature is disabled by default since Micronaut framework 4). See the section on Cloud Configuration for more information.

Environment Priority

The Micronaut framework loads property sources based on the environments specified, and if the same property key exists in multiple property sources specific to an environment, the environment order determines which value to use.

The Micronaut framework uses the following hierarchy for environment processing (lowest to highest priority):

  • Deduced environments

  • Environments from the micronaut.environments system property

  • Environments from the MICRONAUT_ENVIRONMENTS environment variable

  • Environments specified explicitly through the application context builder

    This also applies to @MicronautTest(environments = …​)

Disabling Environment Detection

Automatic detection of environments can be disabled by setting the micronaut.env.deduction system property or the MICRONAUT_ENV_DEDUCTION environment variable to false. This prevents the Micronaut framework from detecting current environments, while still using any environments that are specifically provided as shown above.

Disabling environment detection via system property
$  java -Dmicronaut.env.deduction=false -jar myapp.jar

Alternatively, you can disable environment deduction using the ApplicationContextBuilder deduceEnvironment method when setting up your application.

Using ApplicationContextBuilder to disable environment deduction
@Test
void testDisableEnvironmentDeductionViaBuilder() {
    ApplicationContext ctx = ApplicationContext.builder()
            .deduceEnvironment(false)
            .properties(Collections.singletonMap("micronaut.server.port", -1))
            .start();
    assertFalse(ctx.getEnvironment().getActiveNames().contains(Environment.TEST));
    ctx.close();
}
Using ApplicationContextBuilder to disable environment deduction
void "test disable environment deduction via builder"() {
    when:
    ApplicationContext ctx = ApplicationContext.builder().deduceEnvironment(false).start()

    then:
    !ctx.environment.activeNames.contains(Environment.TEST)

    cleanup:
    ctx.close()
}
Using ApplicationContextBuilder to disable environment deduction
"test disable environment deduction via builder"() {
    val ctx = ApplicationContext.builder().deduceEnvironment(false).start()
    ctx.environment.activeNames.shouldNotContain(Environment.TEST)
    ctx.close()
}

Default Environment

The Micronaut framework supports the concept of one or many default environments. A default environment is one that is only applied if no other environments are explicitly specified or deduced. Environments can be explicitly specified either through the application context builder Micronaut.build().environments(…​), through the micronaut.environments system property, or the MICRONAUT_ENVIRONMENTS environment variable. Environments can be deduced to automatically apply the environment appropriate for cloud deployments. If an environment is found through any of the above means, the default environment will not be applied.

To set the default environments, include a public static class that implements ApplicationContextConfigurer and is annotated with ContextConfigurer:

public class Application {

    @ContextConfigurer
    public static class DefaultEnvironmentConfigurer implements ApplicationContextConfigurer {
        @Override
        public void configure(@NonNull ApplicationContextBuilder builder) {
            builder.defaultEnvironments(defaultEnvironment);
        }
    }

    public static void main(String[] args) {
        Micronaut.run(Application.class, args);
    }
}
Previously, we recommended using Micronaut.defaultEnvironments("dev") however this does not allow the Ahead of Time (AOT) compiler to detect the default environments.

Micronaut Banner

Since Micronaut framework 2.3 a banner is shown when the application starts. It is enabled by default, and it also shows the Micronaut version.

$ ./gradlew run
 __  __ _                                  _
|  \/  (_) ___ _ __ ___  _ __   __ _ _   _| |_
| |\/| | |/ __| '__/ _ \| '_ \ / _` | | | | __|
| |  | | | (__| | | (_) | | | | (_| | |_| | |_
|_|  |_|_|\___|_|  \___/|_| |_|\__,_|\__,_|\__|
  Micronaut (4.7.6)

17:07:22.997 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 611ms. Server Running: http://localhost:8080

To customize the banner with your own ASCII Art (just plain ASCII at this moment), create the file src/main/resources/micronaut-banner.txt and it will be used instead.

To disable it, modify your Application class:

public class Application {

    public static void main(String[] args) {
        Micronaut.build(args)
                 .banner(false) (1)
                 .start();
    }
}
1 Disable the banner

4.2 Externalized Configuration with PropertySources

Additional PropertySource instances can be added to the environment prior to initializing the ApplicationContext.

Initializing the Environment
ApplicationContext applicationContext = ApplicationContext.run(
        PropertySource.of(
                "test",
                CollectionUtils.mapOf(
                    "micronaut.server.host", "foo",
                    "micronaut.server.port", 8080
                )
        ),
        "test", "android");
Environment environment = applicationContext.getEnvironment();

assertEquals("foo", environment.getProperty("micronaut.server.host", String.class).orElse("localhost"));
Initializing the Environment
when:
ApplicationContext applicationContext = ApplicationContext.run(
        PropertySource.of(
                "test",
                [
                    "micronaut.server.host": "foo",
                    "micronaut.server.port": 8080
                ]
        ),
        "test", "android")
Environment environment = applicationContext.getEnvironment()

then:
"foo" == environment.getProperty("micronaut.server.host", String.class).orElse("localhost")
Initializing the Environment
val applicationContext = ApplicationContext.run(
    PropertySource.of(
        "test",
        mapOf(
            "micronaut.server.host" to "foo",
            "micronaut.server.port" to 8080
        )
    ),
    "test", "android"
)
val environment = applicationContext.environment

environment.getProperty("micronaut.server.host", String::class.java).orElse("localhost") shouldBe "foo"

The PropertySource.of method can be used to create a PropertySource from a map of values.

Alternatively one can register a PropertySourceLoader by creating a META-INF/services/io.micronaut.context.env.PropertySourceLoader file containing a reference to the class name of the PropertySourceLoader.

Included PropertySource Loaders

Micronaut framework by default contains PropertySourceLoader implementations that load properties from the given locations and priority:

  1. Command line arguments

  2. Properties from SPRING_APPLICATION_JSON (for Spring compatibility)

  3. Properties from MICRONAUT_APPLICATION_JSON

  4. Java System Properties

  5. OS environment variables

  6. Configuration files loaded in order from the system property 'micronaut.config.files' or the environment variable MICRONAUT_CONFIG_FILES. The value can be a comma-separated list of paths with the last file having precedence. The files can be referenced from:

    1. the file system as an absolute path (without any prefix),

    2. the classpath with a classpath: prefix.

  7. Environment-specific properties from application-{environment}.{extension}

  8. Application-specific properties from application.{extension}

'micronaut.config.files' will be ignored in bootstrap.yml or application.yml. Loading additional configuration files from a configuration file is not supported
.properties, .json, .yml are supported out of the box. For Groovy users .groovy is supported as well.

Note that if you want full control of where your application loads configuration from you can disable the default PropertySourceLoader implementations listed above by calling the enableDefaultPropertySources(false) method of the ApplicationContextBuilder interface when starting your application.

In this case only explicit PropertySource instances that you add via the propertySources(..) method of the ApplicationContextBuilder interface will be used.

Supplying Configuration via Command Line

Configuration can be supplied at the command line using Gradle or our Maven plugin. For example:

Gradle
$ ./gradlew run --args="-endpoints.health.enabled=true -config.property=test"
Maven
$ ./mvnw mn:run -Dmn.appArgs="-endpoints.health.enabled=true -config.property=test"

For the configuration to be a part of the context, the args from the main method must be passed to the context builder. For example:

import io.micronaut.runtime.Micronaut;

public class Application {

    public static void main(String[] args) {
        Micronaut.run(Application.class, args); // passing args
    }
}

Secrets and Sensitive Configuration

It is important to note that it is not recommended to store sensitive configuration such as passwords and tokens within configuration files that can potentially be checked into source control systems.

It is good practise to instead externalize sensitive configuration completely from the application code using preferably an external secret manager system (there are many options here, many provided by Cloud providers) or environment variables that are set during the deployment of the application. You can also use property placeholders (see the following section), to customize names of the environment variables to use and supply default values:

Using Property Value Placeholders to Define Secure Configuration
datasources.default.url=${JDBC_URL:`jdbc:mysql://localhost:3306/db`}
datasources.default.username=${JDBC_USER:root}
datasources.default.password=${JDBC_PASSWORD:}
datasources.default.dialect=MYSQL
datasources.default.driverClassName=${JDBC_DRIVER:com.mysql.cj.jdbc.Driver}
datasources:
  default:
    url: ${JDBC_URL:`jdbc:mysql://localhost:3306/db`}
    username: ${JDBC_USER:root}
    password: ${JDBC_PASSWORD:}
    dialect: MYSQL
    driverClassName: ${JDBC_DRIVER:com.mysql.cj.jdbc.Driver}
[datasources]
  [datasources.default]
    url="${JDBC_URL:`jdbc:mysql://localhost:3306/db`}"
    username="${JDBC_USER:root}"
    password="${JDBC_PASSWORD:}"
    dialect="MYSQL"
    driverClassName="${JDBC_DRIVER:com.mysql.cj.jdbc.Driver}"
datasources {
  'default' {
    url = "${JDBC_URL:`jdbc:mysql://localhost:3306/db`}"
    username = "${JDBC_USER:root}"
    password = "${JDBC_PASSWORD:}"
    dialect = "MYSQL"
    driverClassName = "${JDBC_DRIVER:com.mysql.cj.jdbc.Driver}"
  }
}
{
  datasources {
    default {
      url = "${JDBC_URL:`jdbc:mysql://localhost:3306/db`}"
      username = "${JDBC_USER:root}"
      password = "${JDBC_PASSWORD:}"
      dialect = "MYSQL"
      driverClassName = "${JDBC_DRIVER:com.mysql.cj.jdbc.Driver}"
    }
  }
}
{
  "datasources": {
    "default": {
      "url": "${JDBC_URL:`jdbc:mysql://localhost:3306/db`}",
      "username": "${JDBC_USER:root}",
      "password": "${JDBC_PASSWORD:}",
      "dialect": "MYSQL",
      "driverClassName": "${JDBC_DRIVER:com.mysql.cj.jdbc.Driver}"
    }
  }
}

To securely externalize configuration consider using a secrets manager system supported by the Micronaut framework such as:

Property Value Placeholders

As mentioned in the previous section, the Micronaut framework includes a property placeholder syntax to reference configuration properties both within configuration values and with any Micronaut annotation. See @Value and the section on Configuration Injection.

Programmatic usage is also possible via the PropertyPlaceholderResolver interface.

The basic syntax is to wrap a reference to a property in ${…​}. For example:

Defining Property Placeholders
myapp.endpoint=http://${micronaut.server.host}:${micronaut.server.port}/foo
myapp:
  endpoint: http://${micronaut.server.host}:${micronaut.server.port}/foo
[myapp]
  endpoint="http://${micronaut.server.host}:${micronaut.server.port}/foo"
myapp {
  endpoint = "http://${micronaut.server.host}:${micronaut.server.port}/foo"
}
{
  myapp {
    endpoint = "http://${micronaut.server.host}:${micronaut.server.port}/foo"
  }
}
{
  "myapp": {
    "endpoint": "http://${micronaut.server.host}:${micronaut.server.port}/foo"
  }
}

The above example embeds references to the micronaut.server.host and micronaut.server.port properties.

You can specify default values by defining a value after the : character. For example:

Using Default Values
myapp.endpoint=http://${micronaut.server.host:localhost}:${micronaut.server.port:8080}/foo
myapp:
  endpoint: http://${micronaut.server.host:localhost}:${micronaut.server.port:8080}/foo
[myapp]
  endpoint="http://${micronaut.server.host:localhost}:${micronaut.server.port:8080}/foo"
myapp {
  endpoint = "http://${micronaut.server.host:localhost}:${micronaut.server.port:8080}/foo"
}
{
  myapp {
    endpoint = "http://${micronaut.server.host:localhost}:${micronaut.server.port:8080}/foo"
  }
}
{
  "myapp": {
    "endpoint": "http://${micronaut.server.host:localhost}:${micronaut.server.port:8080}/foo"
  }
}

The above example defaults to localhost and port 8080 if no value is found (rather than throwing an exception). Note that if the default value contains a : character, you must escape it using backticks:

Using Backticks
myapp.endpoint=${server.address:`http://localhost:8080`}/foo
myapp:
  endpoint: ${server.address:`http://localhost:8080`}/foo
[myapp]
  endpoint="${server.address:`http://localhost:8080`}/foo"
myapp {
  endpoint = "${server.address:`http://localhost:8080`}/foo"
}
{
  myapp {
    endpoint = "${server.address:`http://localhost:8080`}/foo"
  }
}
{
  "myapp": {
    "endpoint": "${server.address:`http://localhost:8080`}/foo"
  }
}

The above example looks for a server.address property and defaults to http://localhost:8080. This default value is escaped with backticks since it has a : character.

Property Value Binding

Note that these property references should be in kebab case (lowercase and hyphen-separated) when placing references in code or in placeholder values. For example, use micronaut.server.default-charset and not micronaut.server.defaultCharset.

The Micronaut framework still allows specifying the latter in configuration, but normalizes the properties into kebab case form to optimize memory consumption and reduce complexity when resolving properties. The following table summarizes how properties are normalized from different sources:

Table 1. Property Value Normalization
Configuration Value Resulting Properties Property Source

myApp.myStuff

my-app.my-stuff

Properties, YAML etc.

my-app.myStuff

my-app.my-stuff

Properties, YAML etc.

myApp.my-stuff

my-app.my-stuff

Properties, YAML etc.

MYAPP_MYSTUFF

myapp.mystuff, myapp-mystuff

Environment Variable

MY_APP_MY_STUFF

my.app.my.stuff, my.app.my-stuff, my.app-my.stuff, my.app-my-stuff, my-app.my.stuff, my-app.my-stuff, my-app-my.stuff, my-app-my-stuff

Environment Variable

Environment variables are treated specially to allow more flexibility. Note that there is no way to reference an environment variable with camel-case.

Because the number of properties generated is exponential based on the number of _ characters in an environment variable, it is recommended to refine which, if any, environment variables are included in configuration if the number of environment variables with multiple underscores is high.

To control how environment properties participate in configuration, call the respective methods on the Micronaut builder.

Application class
import io.micronaut.runtime.Micronaut;

public class Application {

    public static void main(String[] args) {
        Micronaut.build(args)
                .mainClass(Application.class)
                .environmentPropertySource(false)
                //or
                .environmentVariableIncludes("THIS_ENV_ONLY")
                //or
                .environmentVariableExcludes("EXCLUDED_ENV")
                .start();
    }
}
Application class
import io.micronaut.runtime.Micronaut

class Application {

    static void main(String[] args) {
        Micronaut.build()
                .mainClass(Application)
                .environmentPropertySource(false)
                //or
                .environmentVariableIncludes("THIS_ENV_ONLY")
                //or
                .environmentVariableExcludes("EXCLUDED_ENV")
                .start()
    }
}
Application class
import io.micronaut.runtime.Micronaut

object Application {

    @JvmStatic
    fun main(args: Array<String>) {
        Micronaut.build(null)
                .mainClass(Application::class.java)
                .environmentPropertySource(false)
                //or
                .environmentVariableIncludes("THIS_ENV_ONLY")
                //or
                .environmentVariableExcludes("EXCLUDED_ENV")
                .start()
    }
}
The configuration above does not have any impact on property placeholders. It is still possible to reference an environment variable in a placeholder regardless of whether environment configuration is disabled, or even if the specific property is explicitly excluded.

Using Random Properties

You can use random values by using the following properties. These can be used in configuration files as variables like the following.

micronaut.application.name=myapplication
micronaut.application.instance.id=${random.shortuuid}
micronaut:
  application:
    name: myapplication
    instance:
      id: ${random.shortuuid}
[micronaut]
  [micronaut.application]
    name="myapplication"
    [micronaut.application.instance]
      id="${random.shortuuid}"
micronaut {
  application {
    name = "myapplication"
    instance {
      id = "${random.shortuuid}"
    }
  }
}
{
  micronaut {
    application {
      name = "myapplication"
      instance {
        id = "${random.shortuuid}"
      }
    }
  }
}
{
  "micronaut": {
    "application": {
      "name": "myapplication",
      "instance": {
        "id": "${random.shortuuid}"
      }
    }
  }
}
Table 2. Random Values
Property Value

random.port

An available random port number

random.int

Random int

random.integer

Random int

random.long

Random long

random.float

Random float

random.shortuuid

Random UUID of only 10 chars in length (Note: As this isn’t full UUID, collision COULD occur)

random.uuid

Random UUID with dashes

random.uuid2

Random UUID without dashes

The random.int, random.integer, random.long and random.float properties supports a range suffix whose syntax is one of as follows:

  • (max) where max is an exclusive value

  • [min,max] where min being inclusive and max being exclusive values.

instance.id=${random.int[5,10]}
instance.count=${random.int(5)}
instance:
  id: ${random.int[5,10]}
  count: ${random.int(5)}
[instance]
  id="${random.int[5,10]}"
  count="${random.int(5)}"
instance {
  id = "${random.int[5,10]}"
  count = "${random.int(5)}"
}
{
  instance {
    id = "${random.int[5,10]}"
    count = "${random.int(5)}"
  }
}
{
  "instance": {
    "id": "${random.int[5,10]}",
    "count": "${random.int(5)}"
  }
}
The range could vary from negative to positive as well.

Fail Fast Property Injection

For beans that inject required properties, the injection and potential failure will not occur until the bean is requested. To verify at startup that the properties exist and can be injected, the bean can be annotated with @Context. Context-scoped beans are injected at startup, and startup fails if any required properties are missing or cannot be converted to the required type.

It is recommended to use this feature sparingly to ensure fast startup.

4.3 Configuration Injection

You can inject configuration values into beans using the @Value annotation.

Using the @Value Annotation

Consider the following example:

@Value Example
import io.micronaut.context.annotation.Value;
import io.micronaut.core.annotation.Nullable;

import jakarta.inject.Singleton;

@Singleton
public class EngineImpl implements Engine {

    @Value("${my.engine.cylinders:6}") // (1)
    protected int cylinders;

    @Nullable
    @Value("${my.engine.description}")
    protected String description;

    @Override
    public int getCylinders() {
        return cylinders;
    }

    @Override
    public String start() {// (2)
        return "Starting V" + getCylinders() + " Engine";
    }

    @Override
    public String getDescription() {
        return description;
    }

}
@Value Example
import io.micronaut.context.annotation.Value

import jakarta.inject.Singleton

@Singleton
class EngineImpl implements Engine {

    @Value('${my.engine.cylinders:6}') // (1)
    protected int cylinders

    @Override
    int getCylinders() {
        cylinders
    }

    @Override
    String start() { // (2)
        "Starting V$cylinders Engine"
    }
}
@Value Example
import io.micronaut.context.annotation.Value

import jakarta.inject.Singleton

@Singleton
class EngineImpl : Engine {

    @Value("\${my.engine.cylinders:6}") // (1)
    override var cylinders: Int = 0
        protected set

    override fun start(): String { // (2)
        return "Starting V$cylinders Engine"
    }
}
1 The @Value annotation accepts a string that can have embedded placeholder values (the default value can be provided by specifying a value after the colon : character). Also try to avoid setting the member visibility to private, since this requires the Micronaut Framework to use reflection. Prefer to use protected.
2 The injected value can then be used within code.

Note that @Value can also be used to inject a static value. For example the following injects the number 10:

Static @Value Example
@Value("10")
int number;

This is even more useful when used to compose injected values combining static content and placeholders. For example to set up a URL:

Placeholders with @Value
@Value("http://${my.host}:${my.port}")
URL url;

In the above example the URL is constructed from two placeholder properties that must be present in configuration: my.host and my.port.

Remember that to specify a default value in a placeholder expression, you use the colon : character. However, if the default you specify includes a colon, you must escape the value with backticks. For example:

Placeholders with @Value
@Value("${my.url:`http://foo.com`}")
URL url;

Note that there is nothing special about @Value itself regarding the resolution of property value placeholders.

Due to Micronaut’s extensive support for annotation metadata you can use property placeholder expressions on any annotation. For example, to make the path of a @Controller configurable you can do:

@Controller("${hello.controller.path:/hello}")
class HelloController {
    ...
}

In the above case, if hello.controller.path is specified in configuration the controller will be mapped to the specified path, otherwise it will be mapped to /hello.

You can also make the target server for @Client configurable (although service discovery approaches are often better), for example:

@Client("${my.server.url:`http://localhost:8080`}")
interface HelloClient {
    ...
}

In the above example the property my.server.url can be used to configure the client, otherwise the client falls back to a localhost address.

Using the @Property Annotation

Recall that the @Value annotation receives a String value which can be a mix of static content and placeholder expressions. This can lead to confusion if you attempt to do the following:

Incorrect usage of @Value
@Value("my.url")
String url;

In the above case the literal string value my.url is injected and set to the url field and not the value of the my.url property from your application configuration. This is because @Value only resolves placeholders within the value specified to it.

To inject a specific property name, you may be better off using @Property:

Using @Property
import io.micronaut.context.annotation.Property;

import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
public class Engine {

    @Property(name = "my.engine.cylinders") // (1)
    protected int cylinders; // (2)

    private String manufacturer;

    public int getCylinders() {
        return cylinders;
    }

    public String getManufacturer() {
        return manufacturer;
    }

    @Inject
    public void setManufacturer(@Property(name = "my.engine.manufacturer") String manufacturer) { // (3)
        this.manufacturer = manufacturer;
    }

}
Using @Property
import io.micronaut.context.annotation.Property

import jakarta.inject.Singleton

@Singleton
class Engine {

    @Property(name = "my.engine.cylinders") // (1)
    protected int cylinders // (2)

    @Property(name = "my.engine.manufacturer") //(3)
    String manufacturer

    int getCylinders() {
        cylinders
    }
}
Using @Property
import io.micronaut.context.annotation.Property

import jakarta.inject.Inject
import jakarta.inject.Singleton


@Singleton
class Engine {

    @field:Property(name = "my.engine.cylinders") // (1)
    protected var cylinders: Int = 0 // (2)

    @set:Inject
    @setparam:Property(name = "my.engine.manufacturer") // (3)
    var manufacturer: String? = null

    fun cylinders(): Int {
        return cylinders
    }
}
1 The my.engine.cylinders property is resolved from configuration and injected into the field.
2 Fields subject to injection should not be private because expensive reflection must be used
3 The @Property annotation is used to inject through the setter
Because it is not possible to define a default value with @Property, if the value doesn’t exist or cannot be converted to the required type, bean instantiation will fail.

The above instead injects the value of the my.engine.cylinders property resolved from application configuration. If the property cannot be found in configuration, an exception is thrown. As with other types of injection, the injection point can also be annotated with @Nullable to make the injection optional.

You can also use this feature to resolve sub maps. For example, consider the following configuration:

datasources.default.name=mydb
jpa.default.properties.hibernate.hbm2ddl.auto=update
jpa.default.properties.hibernate.show_sql=true
datasources:
  default:
    name: 'mydb'
jpa:
  default:
    properties:
      hibernate:
        hbm2ddl:
          auto: update
        show_sql: true
[datasources]
  [datasources.default]
    name="mydb"
[jpa]
  [jpa.default]
    [jpa.default.properties]
      [jpa.default.properties.hibernate]
        show_sql=true
        [jpa.default.properties.hibernate.hbm2ddl]
          auto="update"
datasources {
  'default' {
    name = "mydb"
  }
}
jpa {
  'default' {
    properties {
      hibernate {
        hbm2ddl {
          auto = "update"
        }
        show_sql = true
      }
    }
  }
}
{
  datasources {
    default {
      name = "mydb"
    }
  }
  jpa {
    default {
      properties {
        hibernate {
          hbm2ddl {
            auto = "update"
          }
          show_sql = true
        }
      }
    }
  }
}
{
  "datasources": {
    "default": {
      "name": "mydb"
    }
  },
  "jpa": {
    "default": {
      "properties": {
        "hibernate": {
          "hbm2ddl": {
            "auto": "update"
          },
          "show_sql": true
        }
      }
    }
  }
}

To resolve a flattened map containing only the properties starting with hibernate, use @Property, for example:

Using @Property
@Property(name = "jpa.default.properties")
Map<String, String> jpaProperties;

The injected map will contain the keys hibernate.hbm2ddl.auto and hibernate.show_sql and their values.

The @MapFormat annotation can be used to customize the injected map depending on whether you want nested keys or flat keys, and it allows customization of the key style via the StringConvention enum.

4.4 Expression Language

Since 4.0, Micronaut framework supports embedding evaluated expressions in annotation values using #{…​} syntax which allows to achieve even more flexibility while configuring your application.

Evaluated Expression example
@Value("#{ T(Math).random() }")
double injectedValue;

Expressions can be defined whenever an annotation member accepts a string or an array of strings.

Expressions are currently not supported for "type use" annotations (that declare ElementType.TYPE_USE).
Evaluated Expression in array
@Singleton
@Requires(env = {"dev", "#{ 'test' }"})
public class EvaluatedExpressionInArray {}

You can also embed one or more expressions in a string template in a similar manner to embedding properties with the ${…​} syntax.

Evaluated Expression template
@Value("http://#{'hostname'}/#{'path'}")
String url;

Evaluated Expressions are validated and compiled at build time which guarantees type safety at runtime.

Once an application is running expressions are evaluated on demand as part of annotation metadata resolution. The usage of expressions does not impact performance as evaluation process is completely reflection free.

Note that, for security reasons expressions cannot be dynamically compiled at runtime from potentially untrusted input. All expressions are compiled and checked statically during the compilation process of the application with errors reported as compilation failures.

In general, expressions can be treated as statement written using a programming language with reduced set of available features. Even though the complexity of expression is only limited by the list of supported syntax constructs, it is in general not recommended to place complex logic inside an expression as there are usually better ways to achieve the same result.

Using Expressions in Micronaut framework

Expressions can be used anywhere throughout the Micronaut framework and associated modules, but as an example, you can use them to implement simple scheduled job control, for example:

Job Control with Expressions
import io.micronaut.scheduling.annotation.Scheduled;
import jakarta.inject.Singleton;

@Singleton
public class ExampleJob {
    private boolean jobRan = false;
    private boolean paused = true;


    @Scheduled(
        fixedRate = "1s",
        condition = "#{!this.paused}") // (1)
    void run() {
        System.out.println("Job Running");
        this.jobRan = true;
    }

    public boolean isPaused() {
        return paused;
    } // (2)

    public boolean hasJobRun() {
        return jobRan;
    }

    public void unpause() {
        paused = false;
    }

    public void pause() {
        paused = true;
    }

}
Job Control with Expressions
import io.micronaut.scheduling.annotation.Scheduled
import jakarta.inject.Singleton

@Singleton
class ExampleJob {
    boolean paused = true // (2)
    private boolean jobRan = false

    @Scheduled(
            fixedRate = "1s",
            condition = '#{!this.paused}') // (1)
    void run() {
        println("Job Running")
        this.jobRan = true
    }

    boolean hasJobRun() {
        return jobRan
    }

    void unpause() {
        paused = false
    }

    void pause() {
        paused = true
    }
}
Job Control with Expressions
import io.micronaut.scheduling.annotation.Scheduled
import jakarta.inject.Singleton

@Singleton
class ExampleJob {
    var paused = true
    private var jobRan = false
    @Scheduled(
        fixedRate = "1s",
        condition = "#{!this.paused}") // (1)
    fun run() {
        println("Job Running")
        jobRan = true
    }

    fun hasJobRun(): Boolean {
        return jobRan
    }

    fun unpause() {
        paused = false
    }

    fun pause() {
        paused = true
    }
}
1 Here the condition member of the @Scheduled annotation is used to only execute the job if a pre-condition is met.
2 The condition invokes a method of the type that checks if the job is paused. Other methods can be used to pause and resume execution of the job as desired.
You can also use expressions to perform conditional routing using the @RouteCondition annotation.

Evaluated Expression Language Reference

The Evaluated Expressions syntax supports the following functionality:

  • Literal Values

  • Math Operators

  • Comparison Operators

  • Logical Operators

  • Ternary Operator

  • Type References

  • Method Invocation

  • Property Access

  • Retrieving Beans from Bean Context

  • Retrieving Environment Properties

Literal Values

The following types of literal values are supported:

  • null

  • boolean values (true, false)

  • strings, which need to be surrounded with single quotation mark (')

  • numeric values (int, long, float, double)

Integer and Long values can also be specified in hexadecimal or octal notation. Float and Double values can also be specified in exponential notation. All numeric values can be negative as well.

Literal values examples
#{ null }
#{ true }
#{ 'string value' }
#{ 10 }
#{ 0xFFL }
#{ 10L }
#{ .123f }
#{ 1E+1d }
#{ 123D }

Math Operators

The supported mathematical operators are `, `-`, `*`, `/`, `%`, `^`. Math operators can only be applied to numeric values (except ` which can be used for string concatenation as well). Mathematical operations are performed in order enforced by standard operator precedence. You can also change evaluation order by using brackets ().

/ and % operators can be aliased by div and mod keywords respectively.

Math operators examples
#{ 1 + 2 }             // 3
#{ 'a' + 'b' + 'c' }   // 'abc'
#{ 7 - 3 }             // 4
#{ 7 * 3 }             // 21
#{ 7 * ( 3 + 1) }      // 28

#{ 15 / 3 }            // 5
#{ 15 div 3 }          // 5

#{ 15 % 3 }            // 0
#{ 15 mod 3 }          // 0

// Unlike in Java, ^ operator means exponentiation
#{ 3 ^ 2 }             // 9

Comparison Operators

The following comparison operators are supported: ==, !=, >, <, >=, <=, matches Comparison operations are performed in order enforced by standard operator precedence. You can also change evaluation order by using brackets ().

Equality check is supported for both primitive types and objects. It is performed using Object.equals() method.

>, <, >=, <= operations can be applied to numeric types or types that implement java.lang.Comparable interface.

matches keyword can be used to determine whether a string matches provided regular expression which has to be specified as string literal. The regular expression itself will be checked for validity at compilation time.

Comparison operators examples
#{ 1 + 2 == 3 }         // true
#{ 'abc' != 'abc' }     // false
#{ 7 > 3 }              // true
#{ 7 < 3 }              // false
#{ 7 >= 7 }             // true
#{ 7 <= 8 }             // false

#{ 'AbC' matches '[A-Za-z*'  }      // Compilation failure
#{ 'AbC' matches '[A-Za-z]*'  }     // true
#{ 'AbC' matches '[a-z]*'  }        // false

Logical Operators

The following logical operators are supported:

  • && (can be aliased with and)

  • || (can be aliased with or),

  • ! (can be aliaded with not)

  • empty / not empty (works with strings, collections, arrays, and maps)

Logical operations are performed in order enforced by standard operator precedence. You can also change evaluation order by using brackets ().

Logical operators examples
#{ true && false }         // false
#{ true and true }         // true

#{ true || false }         // true
#{ false or false }        // false

#{ !false }                // true
#{ !!true }                // true

#{ empty '' }              // true
#{ not empty '' }          // false

Ternary Operator

A standard ternary operator is supported to allow specifying if-then-else conditional logic in expression

condition ? thenBranch : elseBranch

where condition evaluation should provide boolean value, and the complexity of then and else branches is not limited.

Ternary operator examples
#{ 15 > 10 ? 'a' : 'b' }    // 'a'
#{ 15 >= 16 ? 'a' : 'b' }   // 'b'

Dot and Safe Navigation Operator

The dot operator can be used to access methods and properties of a value within an expression. For example:

Dot operator usage
#{ collection.size() > 0 }
#{ foo.bar.name == "Fred" }

You can also use the safe dereference operator ?. to navigate paths in a null safe way:

Safe dereference operator
#{ foo?.bar?.name == "Fred" }
When used, the safe dereference operator will also automatically unwrap Java’s Optional type.

Type References

A predefined syntax construct T(…​) can be used to reference a class. The value inside brackets should be fully qualified class name (including the package name). The only exception is java.lang.* classes which can be referenced directly by only specifying the simple class name. Primitive types can not be referenced.

Type References are evaluated in different ways depending on the context.

Simple type reference

A simple type reference is resolved as a Class<?> object.

Type reference example
#{ T(java.lang.String) }    // String.class

Same rule applies if type reference is specified as a method argument.

Type check with instanceof

A Type Reference can be used as the right-hand side part of the instanceof operator

Type check example
#{ 'abc' instanceof T(String) }  // true

which is equivalent to the following Java code and will be evaluated as a boolean value:

"abc" instanceof String

Static method invocation

Type Reference can be used to invoke a static method of a class

Static method invocation
#{ T(Math).random() }

Expression Evaluation Context

By default, the only methods you can invoke inside Evaluated Expressions are static methods using type references.

The available methods can be extended by extended the evaluation context. There are two ways to extend the evaluation context. The first involves registering new context class via a custom TypeElementVisitor.

The TypeElementVisitor has to be on the annotation processor classpath, therefore needs to be defined in a separate module that can be included on this classpath.

Once a class is registered within evaluation context the methods and properties of the class are available for referencing in evaluated expressions.

Consider the following example:

User-defined evaluated expression context
package io.micronaut.docs.expressions;

import jakarta.inject.Singleton;
import java.util.Random;

@Singleton
public class CustomEvaluationContext {
    public int generateRandom(int min, int max) {
        return new Random().nextInt(max - min) + min;
    }
}
The class should be resolvable as a bean can use jakarta.inject annotations to inject other types if necessary. In addition, for performance reasons all evaluation context classes are effectively singleton regardless of the defined scope.

Registering this class can be achieved with a custom implementation of ExpressionEvaluationContextRegistrar that is registered via service loader as a TypeElementVisitor (create a new META-INF/services/io.micronaut.inject.visitor.TypeElementVisitor file referencing the new class) and placed on the annotation processor classpath:

Defining a ExpressionEvaluationContextRegistrar
package io.micronaut.docs.expressions;

import io.micronaut.expressions.context.ExpressionEvaluationContextRegistrar;

public class ContextRegistrar implements ExpressionEvaluationContextRegistrar {
    @Override
    public String getContextClassName() {
        return "io.micronaut.docs.expressions.CustomEvaluationContext";
    }
}

Method generateRandom(int, int) can now be used within Evaluated Expression in the following way:

Usage of user-defined evaluated expression context
package io.micronaut.docs.expressions;

import io.micronaut.context.annotation.Value;
import jakarta.inject.Singleton;

@Singleton
public class ContextConsumer {

    @Value("#{ generateRandom(1, 10) }")
    public int randomField;

}

At runtime, the bean will be retrieved from application context and respective method will be invoked.

If a matching method is not found within evaluation context at compilation time, the compilation will fail. A compilation error will also occur if multiple suitable methods are found in the evaluation context, keep that in mind if you provide multiple ExpressionEvaluationContextRegistrar that a conflict can occur as these types are effectively global.

The methods will be considered ambiguous (leading to compilation failure) when their names are the same and list of provided arguments matches multiple methods parameters.

Using a ExpressionEvaluationContextRegistrar makes its methods and properties available for evaluated expressions within any annotation in a global manner.

However, you can also specify evaluation context scoped to concrete annotation or annotation member using @AnnotationExpressionContext.

Usage of annotation level evaluated expression context
package io.micronaut.docs.expressions;

import jakarta.inject.Singleton;
import io.micronaut.context.annotation.AnnotationExpressionContext;

@Singleton
@CustomAnnotation(value = "#{firstValue() + secondValue()}") // (1)
class Example {
}

@Singleton
class AnnotationContext { // (2)
    String firstValue() {
        return "first value";
    }
}

@Singleton
class AnnotationMemberContext { // (3)
    String secondValue() {
        return "second value";
    }
}

@AnnotationExpressionContext(AnnotationContext.class) // (4)
@interface CustomAnnotation {

    @AnnotationExpressionContext(AnnotationMemberContext.class) // (5)
    String value();
}
Usage of annotation level evaluated expression context
package io.micronaut.docs.expressions;

import jakarta.inject.Singleton;
import io.micronaut.context.annotation.AnnotationExpressionContext;

@Singleton
@CustomAnnotation(value = "#{firstValue() + secondValue()}") // (1)
class Example {
}

@Singleton
class AnnotationContext { // (2)
    String firstValue() {
        return "first value"
    }
}

@Singleton
class AnnotationMemberContext { // (3)
    String secondValue() {
        return "second value"
    }
}

@AnnotationExpressionContext(AnnotationContext.class) // (4)
@interface CustomAnnotation {

    @AnnotationExpressionContext(AnnotationMemberContext.class) // (5)
    String value();
}
Usage of annotation level evaluated expression context
package io.micronaut.docs.expressions

import io.micronaut.context.annotation.AnnotationExpressionContext
import jakarta.inject.Singleton

@Singleton
@CustomAnnotation(value = "#{firstValue() + secondValue()}") // (1)
class Example

@Singleton
class AnnotationContext { // (2)
    fun firstValue() = "first value"
}

@Singleton
class AnnotationMemberContext { // (3)
    fun secondValue() = "second value"
}

@AnnotationExpressionContext(AnnotationContext::class) // (4)
annotation class CustomAnnotation(
    @get:AnnotationExpressionContext(AnnotationMemberContext::class) // (5)
    val value: String
)
1 Here two new methods are introduced to the context called firstValue() and secondValue() only for the scope of the @CustomAnnotation
2 The firstValue() method is defined in a bean called AnnotationContext
3 The secondValue() method is defined in a bean called AnnotationMemberContext
4 On the @CustomAnnotation annotation the methods of the AnnotationContext type are exposed to all members of the annotation (type level context).
5 On the value() member of the @CustomAnnotation annotation the methods of the AnnotationContextExample are made available but scoped only to the value() member.

Again context classes need to be explicitly defined as beans to make them available for retrieval from application context at runtime.

Method Invocation

You can invoke both static methods using type references, methods from evaluation context and methods on objects, which means method chaining is supported.

Chaining methods in expression
import io.micronaut.context.annotation.Value;
import jakarta.inject.Singleton;

@Singleton
class CustomEvaluationContext {

    public String stringValue() {
        return "stringValue";
    }

}

@Singleton
class ContextConsumer {

    @Value("#{ #stringValue().length() }")
    public int stringLength;

}

Varargs methods invocation is supported as well. Note that if last parameter of a method is an array, you can still invoke it providing list of arguments separated by comma without explicitly wrapping it into array. So in this case it will be treated in same way as if last method argument was explicitly specified as varargs parameter.

Invoking varargs methods in expressions
import io.micronaut.context.annotation.Value;
import jakarta.inject.Singleton;

@Singleton
class CustomEvaluationContext {

    public int countIntegers(int... values) {
        return values.length;
    }

    public int countStrings(String[] values) {
        return values.length;
    }

}

@Singleton
class ContextConsumer {

    @Value("#{ #countIntegers(1, 2, 3) }")
    public int totalIntegers;

    @Value("#{ #countStrings('a', 'b', 'c') }")
    public int totalStrings;

}

Property Access

JavaBean properties can be accessed simply be referencing their names from evaluation context prefixed with #. Bean properties can also be chained with dot in the same way as methods.

Accessing bean properties in expressions
import io.micronaut.context.annotation.Value;
import jakarta.inject.Singleton;

@Singleton
class CustomEvaluationContext {

    public String getName() {
        return "Bob";
    }

    public int getAge() {
        return 25;
    }

}

@Singleton
class ContextConsumer {

    @Value("#{ 'Name is ' + #name + ', age is ' + #age }")
    public String value;

}

Retrieving Beans from Bean Context

A predefined syntax construct ctx[…​] can be used to retrieve beans from bean context. The argument inside square brackets has to be a fully qualified class name (note that T(…​) wrapper is optional and can be omitted for simplicity).

Retrieving beans from bean context
#{ ctx[T(io.micronaut.example.ContextBean)] }
#{ ctx[io.micronaut.example.ContextBean] }

Retrieving Environment Properties

A syntax construct env[…​] can be used to retrieve environment properties by name. The expression inside square brackets has to resolve to string value, otherwise compilation will fail. If property value will be absent at runtime, the expression will return null

Retrieving Environment Properties
#{ env['test.property'] }

4.5 Configuration Properties

You can create type-safe configuration by creating classes that are annotated with @ConfigurationProperties.

The Micronaut framework will produce a reflection-free @ConfigurationProperties bean and will also at compile time calculate the property paths to evaluate, greatly improving the speed and efficiency of loading @ConfigurationProperties.

For example:

@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationProperties;

import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import java.util.Optional;

@ConfigurationProperties("my.engine") // (1)
public class EngineConfig {

    public String getManufacturer() {
        return manufacturer;
    }

    public void setManufacturer(String manufacturer) {
        this.manufacturer = manufacturer;
    }

    public int getCylinders() {
        return cylinders;
    }

    public void setCylinders(int cylinders) {
        this.cylinders = cylinders;
    }

    public CrankShaft getCrankShaft() {
        return crankShaft;
    }

    public void setCrankShaft(CrankShaft crankShaft) {
        this.crankShaft = crankShaft;
    }

    @NotBlank // (2)
    private String manufacturer = "Ford"; // (3)

    @Min(1L)
    private int cylinders;

    private CrankShaft crankShaft = new CrankShaft();

    @ConfigurationProperties("crank-shaft")
    public static class CrankShaft { // (4)

        private Optional<Double> rodLength = Optional.empty(); // (5)

        public Optional<Double> getRodLength() {
            return rodLength;
        }

        public void setRodLength(Optional<Double> rodLength) {
            this.rodLength = rodLength;
        }
    }
}
@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationProperties

import jakarta.validation.constraints.Min
import jakarta.validation.constraints.NotBlank

@ConfigurationProperties('my.engine') // (1)
class EngineConfig {

    @NotBlank // (2)
    String manufacturer = "Ford" // (3)

    @Min(1L)
    int cylinders

    CrankShaft crankShaft = new CrankShaft()

    @ConfigurationProperties('crank-shaft')
    static class CrankShaft { // (4)
        Optional<Double> rodLength = Optional.empty() // (5)
    }
}
@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationProperties
import java.util.Optional
import jakarta.validation.constraints.Min
import jakarta.validation.constraints.NotBlank

@ConfigurationProperties("my.engine") // (1)
class EngineConfig {

    @NotBlank // (2)
    var manufacturer = "Ford" // (3)

    @Min(1L)
    var cylinders: Int = 0

    var crankShaft = CrankShaft()

    @ConfigurationProperties("crank-shaft")
    class CrankShaft { // (4)
        var rodLength: Optional<Double> = Optional.empty() // (5)
    }
}
1 The @ConfigurationProperties annotation takes the configuration prefix
2 You can use jakarta.validation annotations to validate the configuration
3 Default values can be assigned to the property
4 Static inner classes can provide nested configuration
5 Optional configuration values can be wrapped in java.util.Optional

Once you have prepared a type-safe configuration it can be injected into your beans like any other bean:

@ConfigurationProperties Dependency Injection
@Singleton
public class EngineImpl implements Engine {
    private final EngineConfig config;

    public EngineImpl(EngineConfig config) { // (1)
        this.config = config;
    }

    @Override
    public int getCylinders() {
        return config.getCylinders();
    }

    @Override
    public String start() {// (2)
        return getConfig().getManufacturer() + " Engine Starting V" + getConfig().getCylinders() +
                " [rodLength=" + getConfig().getCrankShaft().getRodLength().orElse(6d) + "]";
    }

    public final EngineConfig getConfig() {
        return config;
    }
}
@ConfigurationProperties Dependency Injection
@Singleton
class EngineImpl implements Engine {
    final EngineConfig config

    EngineImpl(EngineConfig config) { // (1)
        this.config = config
    }

    @Override
    int getCylinders() {
        config.cylinders
    }

    @Override
    String start() { // (2)
        "$config.manufacturer Engine Starting V$config.cylinders [rodLength=${config.crankShaft.rodLength.orElse(6.0d)}]"
    }
}
@ConfigurationProperties Dependency Injection
@Singleton
class EngineImpl(val config: EngineConfig) : Engine {// (1)

    override val cylinders: Int
        get() = config.cylinders

    override fun start(): String {// (2)
        return "${config.manufacturer} Engine Starting V${config.cylinders} [rodLength=${config.crankShaft.rodLength.orElse(6.0)}]"
    }
}
1 Inject the EngineConfig bean
2 Use the configuration properties

Configuration values can then be supplied from one of the PropertySource instances. For example:

Supply Configuration
Map<String, Object> map = new LinkedHashMap<>(1);
map.put("my.engine.cylinders", "8");
ApplicationContext applicationContext = ApplicationContext.run(map, "test");

Vehicle vehicle = applicationContext.getBean(Vehicle.class);
System.out.println(vehicle.start());
Supply Configuration
ApplicationContext applicationContext = ApplicationContext.run(
        ['my.engine.cylinders': '8'],
        "test"
)

def vehicle = applicationContext.getBean(Vehicle)
println(vehicle.start())
Supply Configuration
val map = mapOf( "my.engine.cylinders" to "8")
val applicationContext = ApplicationContext.run(map, "test")

val vehicle = applicationContext.getBean(Vehicle::class.java)
println(vehicle.start())

The above example prints: "Ford Engine Starting V8 [rodLength=6.0]"

You can directly reference configuration properties in @Requires annotation to conditionally load beans using the following syntax: @Requires(bean=Config.class, beanProperty="property", value="true")

Note for more complex configurations you can structure @ConfigurationProperties beans through inheritance.

For example creating a subclass of EngineConfig with @ConfigurationProperties('bar') will resolve all properties under the path my.engine.bar.

Includes / Excludes

For the cases where the configuration properties class inherits properties from a parent class, it may be desirable to exclude properties from the parent class. The includes and excludes members of the @ConfigurationProperties annotation allow for that functionality. The list applies to both local properties and inherited properties.

The names supplied to the includes/excludes list must be the "property" name. For example if a setter method is injected, the property name is the de-capitalized setter name (setConnectionTimeoutconnectionTimeout).

Change accessors style

Since 3.3, the Micronaut framework supports defining different accessors prefixes for getters and setter other than the default get and set defined for Java Beans. Annotate your POJO or @ConfigurationProperties class with the @AccessorsStyle annotation.

This is useful when you write the getters and setters in a fluent way. For example:

Using @AccessorsStyle
import io.micronaut.context.annotation.ConfigurationProperties;
import io.micronaut.core.annotation.AccessorsStyle;

@AccessorsStyle(readPrefixes = "", writePrefixes = "") (1)
@ConfigurationProperties("my.engine")
public class EngineConfig {

    private String manufacturer;
    private int cylinders;

    public EngineConfig(String manufacturer, int cylinders) {
        this.manufacturer = manufacturer;
        this.cylinders = cylinders;
    }

    public String manufacturer() { (2)
        return manufacturer;
    }

    public void manufacturer(String manufacturer) { (2)
        this.manufacturer = manufacturer;
    }

    public int cylinders() { (2)
        return cylinders;
    }

    public void cylinders(int cylinders) { (2)
        this.cylinders = cylinders;
    }

}
1 The Micronaut framework will use an empty prefix for getters and setters.
2 Define the getters and setters with an empty prefix.

Now you can inject EngineConfig and use it with engineConfig.manufacturer() and engineConfig.cylinders() to retrieve the values from configuration.

Property Type Conversion

The Micronaut framework uses the ConversionService bean to convert values when resolving properties. You can register additional converters for types not supported by Micronaut by defining beans that implement the TypeConverter interface.

The Micronaut framework features some built-in conversions that are useful, which are detailed below.

Duration Conversion

Durations can be specified by appending the unit with a number. Supported units are s, ms, m etc. The following table summarizes examples:

Table 1. Duration Conversion
Configuration Value Resulting Value

10ms

Duration of 10 milliseconds

10m

Duration of 10 minutes

10s

Duration of 10 seconds

10d

Duration of 10 days

10h

Duration of 10 hours

10ns

Duration of 10 nanoseconds

PT15M

Duration of 15 minutes using ISO-8601 format

For example to configure the default HTTP client read timeout:

Using Duration Values
micronaut.http.client.read-timeout=15s
micronaut:
  http:
    client:
      read-timeout: 15s
[micronaut]
  [micronaut.http]
    [micronaut.http.client]
      read-timeout="15s"
micronaut {
  http {
    client {
      readTimeout = "15s"
    }
  }
}
{
  micronaut {
    http {
      client {
        read-timeout = "15s"
      }
    }
  }
}
{
  "micronaut": {
    "http": {
      "client": {
        "read-timeout": "15s"
      }
    }
  }
}

List / Array Conversion

Lists and arrays can be specified in Java properties files as comma-separated values, or in YAML using native YAML lists. The generic types are used to convert the values. For example in YAML:

Specifying lists or arrays in YAML
my.app.integers[0]=1
my.app.integers[1]=2
my.app.urls[0]=http://foo.com
my.app.urls[1]=http://bar.com
my:
  app:
    integers:
      - 1
      - 2
    urls:
      - http://foo.com
      - http://bar.com
[my]
  [my.app]
    integers=[
      1,
      2
    ]
    urls=[
      "http://foo.com",
      "http://bar.com"
    ]
my {
  app {
    integers = [1, 2]
    urls = ["http://foo.com", "http://bar.com"]
  }
}
{
  my {
    app {
      integers = [1, 2]
      urls = ["http://foo.com", "http://bar.com"]
    }
  }
}
{
  "my": {
    "app": {
      "integers": [1, 2],
      "urls": ["http://foo.com", "http://bar.com"]
    }
  }
}

For the above example configurations you can define properties to bind to with the target type supplied via generics:

List<Integer> integers;
List<URL> urls;

Readable Bytes

You can annotate any setter parameter with @ReadableBytes to allow the value to be set using a shorthand syntax for specifying bytes, kilobytes etc. For example the following is taken from HttpClientConfiguration:

Using @ReadableBytes
public void setMaxContentLength(@ReadableBytes int maxContentLength) {
    this.maxContentLength = maxContentLength;
}

With the above in place you can set micronaut.http.client.max-content-length using the following values:

Table 2. @ReadableBytes Conversion
Configuration Value Resulting Value

10mb

10 megabytes

10kb

10 kilobytes

10gb

10 gigabytes

1024

A raw byte length

Formatting Dates

The @Format annotation can be used on setters to specify the date format to use when binding java.time date objects.

Using @Format for Dates
public void setMyDate(@Format("yyyy-MM-dd") LocalDate date) {
    this.myDate = date;
}

Configuration Builder

Many frameworks and tools already use builder-style classes to construct configuration.

You can use the @ConfigurationBuilder annotation to populate a builder-style class with configuration values. ConfigurationBuilder can be applied to fields or methods in a class annotated with @ConfigurationProperties.

Since there is no consistent way to define builders in the Java world, one or more method prefixes can be specified in the annotation to support builder methods like withXxx or setXxx. If the builder methods have no prefix, assign an empty string to the parameter.

A configuration prefix can also be specified to tell the Micronaut framework where to look for configuration values. By default, builder methods use the configuration prefix specified in a class-level @ConfigurationProperties annotation.

For example:

@ConfigurationBuilder Example
import io.micronaut.context.annotation.ConfigurationBuilder;
import io.micronaut.context.annotation.ConfigurationProperties;

@ConfigurationProperties("my.engine") // (1)
class EngineConfig {

    @ConfigurationBuilder(prefixes = "with") // (2)
    EngineImpl.Builder builder = EngineImpl.builder();

    @ConfigurationBuilder(prefixes = "with", configurationPrefix = "crank-shaft") // (3)
    CrankShaft.Builder crankShaft = CrankShaft.builder();

    private SparkPlug.Builder sparkPlug = SparkPlug.builder();

    SparkPlug.Builder getSparkPlug() {
        return sparkPlug;
    }

    @ConfigurationBuilder(prefixes = "with", configurationPrefix = "spark-plug") // (4)
    void setSparkPlug(SparkPlug.Builder sparkPlug) {
        this.sparkPlug = sparkPlug;
    }
}
@ConfigurationBuilder Example
import io.micronaut.context.annotation.ConfigurationBuilder
import io.micronaut.context.annotation.ConfigurationProperties

@ConfigurationProperties('my.engine') // (1)
class EngineConfig {

    @ConfigurationBuilder(prefixes = "with") // (2)
    EngineImpl.Builder builder = EngineImpl.builder()

    @ConfigurationBuilder(prefixes = "with", configurationPrefix = "crank-shaft") // (3)
    CrankShaft.Builder crankShaft = CrankShaft.builder()

    SparkPlug.Builder sparkPlug = SparkPlug.builder()

    @ConfigurationBuilder(prefixes = "with", configurationPrefix = "spark-plug") // (4)
    void setSparkPlug(SparkPlug.Builder sparkPlug) {
        this.sparkPlug = sparkPlug
    }
}
@ConfigurationBuilder Example
import io.micronaut.context.annotation.ConfigurationBuilder
import io.micronaut.context.annotation.ConfigurationProperties

@ConfigurationProperties("my.engine") // (1)
internal class EngineConfig {

    @ConfigurationBuilder(prefixes = ["with"])  // (2)
    val builder = EngineImpl.builder()

    @ConfigurationBuilder(prefixes = ["with"], configurationPrefix = "crank-shaft") // (3)
    val crankShaft = CrankShaft.builder()

    @set:ConfigurationBuilder(prefixes = ["with"], configurationPrefix = "spark-plug") // (4)
    var sparkPlug = SparkPlug.builder()
}
1 The @ConfigurationProperties annotation takes the configuration prefix
2 The first builder can be configured without the class configuration prefix; it inherits from the above.
3 The second builder can be configured with the class configuration prefix + the configurationPrefix value.
4 The third builder demonstrates that the annotation can be applied to a method as well as a property.
By default, only single-argument builder methods are supported. For methods with no arguments, set the allowZeroArgs parameter of the annotation to true.

Like in the previous example, we can construct an EngineImpl. Since we are using a builder, we can use a factory class to build the engine from the builder.

Factory Bean
import io.micronaut.context.annotation.Factory;

import jakarta.inject.Singleton;

@Factory
class EngineFactory {

    @Singleton
    EngineImpl buildEngine(EngineConfig engineConfig) {
        return engineConfig.builder.build(engineConfig.crankShaft, engineConfig.getSparkPlug());
    }
}
Factory Bean
import io.micronaut.context.annotation.Factory

import jakarta.inject.Singleton

@Factory
class EngineFactory {

    @Singleton
    EngineImpl buildEngine(EngineConfig engineConfig) {
        engineConfig.builder.build(engineConfig.crankShaft, engineConfig.sparkPlug)
    }
}
Factory Bean
import io.micronaut.context.annotation.Factory
import jakarta.inject.Singleton

@Factory
internal class EngineFactory {

    @Singleton
    fun buildEngine(engineConfig: EngineConfig): EngineImpl {
        return engineConfig.builder.build(engineConfig.crankShaft, engineConfig.sparkPlug)
    }
}

The engine that was returned can then be injected anywhere an engine is required.

Configuration values can be supplied from one of the PropertySource instances. For example:

Supply Configuration
        Map<String, Object> properties = new HashMap<>();
        properties.put("my.engine.cylinders"             ,"4");
        properties.put("my.engine.manufacturer"          , "Subaru");
        properties.put("my.engine.crank-shaft.rod-length", 4);
        properties.put("my.engine.spark-plug.name"       , "6619 LFR6AIX");
        properties.put("my.engine.spark-plug.type"       , "Iridium");
        properties.put("my.engine.spark-plug.companyName", "NGK");
        ApplicationContext applicationContext = ApplicationContext.run(properties, "test");

        Vehicle vehicle = applicationContext.getBean(Vehicle.class);
        System.out.println(vehicle.start());
Supply Configuration
        ApplicationContext applicationContext = ApplicationContext.run(
                ['my.engine.cylinders'             : '4',
                 'my.engine.manufacturer'          : 'Subaru',
                 'my.engine.crank-shaft.rod-length': 4,
                 'my.engine.spark-plug.name'       : '6619 LFR6AIX',
                 'my.engine.spark-plug.type'       : 'Iridium',
                 'my.engine.spark-plug.companyName': 'NGK'
                ],
                "test"
        )

        Vehicle vehicle = applicationContext.getBean(Vehicle)
        println(vehicle.start())
Supply Configuration
        val applicationContext = ApplicationContext.run(
                mapOf(
                        "my.engine.cylinders" to "4",
                        "my.engine.manufacturer" to "Subaru",
                        "my.engine.crank-shaft.rod-length" to 4,
                        "my.engine.spark-plug.name" to "6619 LFR6AIX",
                        "my.engine.spark-plug.type" to "Iridium",
                        "my.engine.spark-plug.company" to "NGK"
                ),
                "test"
        )

        val vehicle = applicationContext.getBean(Vehicle::class.java)
        println(vehicle.start())

The above example prints: "Subaru Engine Starting V4 [rodLength=4.0, sparkPlug=Iridium(NGK 6619 LFR6AIX)]"

MapFormat

For some use cases it may be desirable to accept a map of arbitrary configuration properties that can be supplied to a bean, especially if the bean represents a third-party API where not all the possible configuration properties are known. For example, a datasource may accept a map of configuration properties specific to a particular database driver, allowing the user to specify any desired options in the map without coding each property explicitly.

For this purpose, the MapFormat annotation lets you bind a map to a single configuration property, and specify whether to accept a flat map of keys to values, or a nested map (where the values may be additional maps).

@MapFormat Example
import io.micronaut.context.annotation.ConfigurationProperties;
import io.micronaut.core.convert.format.MapFormat;

import jakarta.validation.constraints.Min;
import java.util.Map;

@ConfigurationProperties("my.engine")
public class EngineConfig {

    @Min(1L)
    private int cylinders;

    @MapFormat(transformation = MapFormat.MapTransformation.FLAT) //(1)
    private Map<Integer, String> sensors;

    public int getCylinders() {
        return cylinders;
    }

    public void setCylinders(int cylinders) {
        this.cylinders = cylinders;
    }

    public Map<Integer, String> getSensors() {
        return sensors;
    }

    public void setSensors(Map<Integer, String> sensors) {
        this.sensors = sensors;
    }
}
@MapFormat Example
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.core.convert.format.MapFormat

import jakarta.validation.constraints.Min

@ConfigurationProperties('my.engine')
class EngineConfig {

    @Min(1L)
    int cylinders

    @MapFormat(transformation = MapFormat.MapTransformation.FLAT) //(1)
    Map<Integer, String> sensors
}
@MapFormat Example
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.core.convert.format.MapFormat
import jakarta.validation.constraints.Min

@ConfigurationProperties("my.engine")
class EngineConfig {

    @Min(1L)
    var cylinders: Int = 0

    @MapFormat(transformation = MapFormat.MapTransformation.FLAT) //(1)
    var sensors: Map<Int, String>? = null
}
1 Note the transformation argument to the annotation; possible values are MapTransformation.FLAT (for flat maps) and MapTransformation.NESTED (for nested maps)
EngineImpl
@Singleton
public class EngineImpl implements Engine {

    @Inject
    EngineConfig config;

    @Override
    public Map getSensors() {
        return config.getSensors();
    }

    @Override
    public String start() {
        return "Engine Starting V" + getConfig().getCylinders() +
               " [sensors=" + getSensors().size() + "]";
    }

    public EngineConfig getConfig() {
        return config;
    }

    public void setConfig(EngineConfig config) {
        this.config = config;
    }
}
EngineImpl
@Singleton
class EngineImpl implements Engine {

    @Inject EngineConfig config

    @Override
    Map getSensors() {
        config.sensors
    }

    @Override
    String start() {
        "Engine Starting V$config.cylinders [sensors=${sensors.size()}]"
    }
}
EngineImpl
@Singleton
class EngineImpl : Engine {

    override val sensors: Map<*, *>?
        get() = config!!.sensors

    @Inject
    var config: EngineConfig? = null

    override fun start(): String {
        return "Engine Starting V${config!!.cylinders} [sensors=${sensors!!.size}]"
    }
}

Now a map of properties can be supplied to the my.engine.sensors configuration property.

Use Map Configuration
Map<String, Object> map = new LinkedHashMap<>(2);
map.put("my.engine.cylinders", "8");

Map<Integer, String> map1 = new LinkedHashMap<>(2);
map1.put(0, "thermostat");
map1.put(1, "fuel pressure");

map.put("my.engine.sensors", map1);

ApplicationContext applicationContext = ApplicationContext.run(map, "test");

Vehicle vehicle = applicationContext.getBean(Vehicle.class);
System.out.println(vehicle.start());
Use Map Configuration
ApplicationContext applicationContext = ApplicationContext.run(
        ['my.engine.cylinders': '8',
         'my.engine.sensors'  : [0: 'thermostat',
                                 1: 'fuel pressure']],
        "test"
)

def vehicle = applicationContext.getBean(Vehicle)
println(vehicle.start())
Use Map Configuration
val subMap = mapOf(
    0 to "thermostat",
    1 to "fuel pressure"
)
val map = mapOf(
    "my.engine.cylinders" to "8",
    "my.engine.sensors" to subMap
)

val applicationContext = ApplicationContext.run(map, "test")

val vehicle = applicationContext.getBean(Vehicle::class.java)
println(vehicle.start())

The above example prints: "Engine Starting V8 [sensors=2]"

See the guide for @Configuration and @ConfigurationBuilder to learn more.

4.6 Custom Type Converters

The Micronaut framework includes an extensible type conversion mechanism. To add additional type converters you register beans of type TypeConverter.

The following example shows how to use one of the built-in converters (Map to an Object) or create your own.

Consider the following ConfigurationProperties:

@ConfigurationProperties(MyConfigurationProperties.PREFIX)
public class MyConfigurationProperties {

    public static final String PREFIX = "myapp";

    protected LocalDate updatedAt;

    public LocalDate getUpdatedAt() {
        return updatedAt;
    }
}
@ConfigurationProperties(MyConfigurationProperties.PREFIX)
class MyConfigurationProperties {

    public static final String PREFIX = "myapp"

    protected LocalDate updatedAt

    LocalDate getUpdatedAt() {
        updatedAt
    }
}
@ConfigurationProperties(MyConfigurationProperties.PREFIX)
class MyConfigurationProperties {

    var updatedAt: LocalDate? = null
        protected set

    companion object {
        const val PREFIX = "myapp"
    }
}

The type MyConfigurationProperties has a property named updatedAt of type LocalDate.

To bind this property from a map via configuration:

private static ApplicationContext ctx;

@BeforeAll
static void setupCtx() {
    ctx = ApplicationContext.run(
            new LinkedHashMap<>() {{
                put("myapp.updatedAt", // (1)
                        new LinkedHashMap<String, Integer>() {{
                            put("day", 28);
                            put("month", 10);
                            put("year", 1982);
                        }}
                );
            }}
    );
}

@AfterAll
static void teardownCtx() {
    if(ctx != null) {
        ctx.stop();
    }
}
@AutoCleanup
@Shared
ApplicationContext ctx = ApplicationContext.run(
        "myapp.updatedAt": [day: 28, month: 10, year: 1982]  // (1)
)
lateinit var ctx: ApplicationContext

@BeforeEach
fun setup() {
    ctx = ApplicationContext.run(
        mapOf(
            "myapp.updatedAt" to mapOf( // (1)
                "day" to 28,
                "month" to 10,
                "year" to 1982
            )
        )
    )
}

@AfterEach
fun teardown() {
    ctx.close()
}
1 Note how we match the myapp prefix and updatedAt property name in our MyConfigurationProperties class above

This won’t work by default, since there is no built-in conversion from Map to LocalDate. To resolve this, define a custom TypeConverter:

import io.micronaut.context.annotation.Prototype;
import io.micronaut.core.convert.ConversionContext;
import io.micronaut.core.convert.ConversionService;
import io.micronaut.core.convert.TypeConverter;

import java.time.DateTimeException;
import java.time.LocalDate;
import java.util.Map;
import java.util.Optional;

@Prototype
public class MapToLocalDateConverter implements TypeConverter<Map, LocalDate> { // (1)

    private final ConversionService  conversionService;

    public MapToLocalDateConverter(ConversionService conversionService) { // (2)
        this.conversionService = conversionService;
    }

    @Override
    public Optional<LocalDate> convert(Map propertyMap, Class<LocalDate> targetType, ConversionContext context) {
        Optional<Integer> day = conversionService.convert(propertyMap.get("day"), Integer.class);
        Optional<Integer> month = conversionService.convert(propertyMap.get("month"), Integer.class);
        Optional<Integer> year = conversionService.convert(propertyMap.get("year"), Integer.class);
        if (day.isPresent() && month.isPresent() && year.isPresent()) {
            try {
                return Optional.of(LocalDate.of(year.get(), month.get(), day.get())); // (3)
            } catch (DateTimeException e) {
                context.reject(propertyMap, e); // (4)
                return Optional.empty();
            }
        }

        return Optional.empty();
    }
}
import io.micronaut.core.convert.ConversionService
import io.micronaut.core.convert.TypeConverter

import java.time.DateTimeException
import java.time.LocalDate

@Prototype
class MapToLocalDateConverter implements TypeConverter<Map, LocalDate> { // (1)

    final ConversionService  conversionService

    MapToLocalDateConverter(ConversionService conversionService) { // (2)
        this.conversionService = conversionService;
    }

    @Override
    Optional<LocalDate> convert(Map propertyMap, Class<LocalDate> targetType, ConversionContext context) {
        Optional<Integer> day = conversionService.convert(propertyMap.day, Integer)
        Optional<Integer> month = conversionService.convert(propertyMap.month, Integer)
        Optional<Integer> year = conversionService.convert(propertyMap.year, Integer)
        if (day.present && month.present && year.present) {
            try {
                return Optional.of(LocalDate.of(year.get(), month.get(), day.get())) // (3)
            } catch (DateTimeException e) {
                context.reject(propertyMap, e) // (4)
                return Optional.empty()
            }
        }
        return Optional.empty()
    }
}
import io.micronaut.context.annotation.Prototype
import io.micronaut.core.convert.ConversionContext
import io.micronaut.core.convert.ConversionService
import io.micronaut.core.convert.TypeConverter
import java.time.DateTimeException
import java.time.LocalDate
import java.util.Optional

@Prototype
class MapToLocalDateConverter(
    private val conversionService: ConversionService // (2)
)
    : TypeConverter<Map<*, *>, LocalDate> { // (1)

    override fun convert(propertyMap: Map<*, *>, targetType: Class<LocalDate>, context: ConversionContext): Optional<LocalDate> {
        val day = conversionService.convert(propertyMap["day"], Int::class.java)
        val month = conversionService.convert(propertyMap["month"], Int::class.java)
        val year = conversionService.convert(propertyMap["year"], Int::class.java)
        if (day.isPresent && month.isPresent && year.isPresent) {
            try {
                return Optional.of(LocalDate.of(year.get(), month.get(), day.get())) // (3)
            } catch (e: DateTimeException) {
                context.reject(propertyMap, e) // (4)
                return Optional.empty()
            }
        }

        return Optional.empty()
    }
}
1 The class implements TypeConverter which has two generic arguments, the type you are converting from, and the type you are converting to
2 The constructor injects a bean of type ConversionService, introduced in Micronaut 4, instead of making static calls to ConversionService.SHARED used in previous versions
3 The implementation delegates to the injected conversion service to convert the values from the Map used to create a LocalDate
4 If an exception occurs during binding, call reject(..) which propagates additional information to the container
It’s possible to add a custom type converter into ConversionService.SHARED by registering it in a TypeConverterRegistrar via the service loader.

4.7 Using @EachProperty to Drive Configuration

The @ConfigurationProperties annotation is great for a single configuration class, but sometimes you want multiple instances, each with its own distinct configuration. That is where EachProperty comes in.

The @EachProperty annotation creates a ConfigurationProperties bean for each sub-property within the given name. As an example consider the following class:

Using @EachProperty
import java.net.URI;
import java.net.URISyntaxException;

import io.micronaut.context.annotation.Parameter;
import io.micronaut.context.annotation.EachProperty;

@EachProperty("test.datasource")  // (1)
public class DataSourceConfiguration {

    private final String name;
    private URI url = new URI("localhost");

    public DataSourceConfiguration(@Parameter String name) // (2)
            throws URISyntaxException {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public URI getUrl() { // (3)
        return url;
    }

    public void setUrl(URI url) {
        this.url = url;
    }
}
Using @EachProperty
import io.micronaut.context.annotation.EachProperty
import io.micronaut.context.annotation.Parameter

@EachProperty("test.datasource") // (1)
class DataSourceConfiguration {

    final String name
    URI url = new URI("localhost") // (3)

    DataSourceConfiguration(@Parameter String name) // (2)
            throws URISyntaxException {
        this.name = name
    }
}
Using @EachProperty
import io.micronaut.context.annotation.EachProperty
import io.micronaut.context.annotation.Parameter
import java.net.URI
import java.net.URISyntaxException

@EachProperty("test.datasource")  // (1)
class DataSourceConfiguration
@Throws(URISyntaxException::class)
constructor(@param:Parameter val name: String) { // (2)
    var url = URI("localhost") // (3)
}
1 The @EachProperty annotation defines the property name to be handled.
2 The @Parameter annotation can be used to inject the name of the sub-property that defines the name of the bean (which is also the bean qualifier)
3 Each property of the bean is bound to configuration.
Micronaut configuration uses kebap case, not lower camel case. For example, using @EachProperty("my-bean") works, but @EachProperty("myBean") fails.

The above DataSourceConfiguration defines a url property to configure one or more data sources. The URLs themselves can be configured using any of the PropertySource instances evaluated to Micronaut:

Providing Configuration to @EachProperty
ApplicationContext applicationContext = ApplicationContext.run(PropertySource.of(
        "test",
        CollectionUtils.mapOf(
                "test.datasource.one.url", "jdbc:mysql://localhost/one",
                "test.datasource.two.url", "jdbc:mysql://localhost/two")
));
Providing Configuration to @EachProperty
ApplicationContext applicationContext = ApplicationContext.run(PropertySource.of(
        "test",
        [
                "test.datasource.one.url": "jdbc:mysql://localhost/one",
                "test.datasource.two.url": "jdbc:mysql://localhost/two"
        ]
))
Providing Configuration to @EachProperty
val applicationContext = ApplicationContext.run(
    PropertySource.of(
        "test",
        mapOf(
            "test.datasource.one.url" to "jdbc:mysql://localhost/one",
            "test.datasource.two.url" to "jdbc:mysql://localhost/two"
        )
    )
)

In the above example two data sources (called one and two) are defined under the test.datasource prefix defined earlier in the @EachProperty annotation. Each of these configuration entries triggers the creation of a new DataSourceConfiguration bean such that the following test succeeds:

Evaluating Beans Built by @EachProperty
Collection<DataSourceConfiguration> beansOfType = applicationContext.getBeansOfType(DataSourceConfiguration.class);
assertEquals(2, beansOfType.size()); // (1)

DataSourceConfiguration firstConfig = applicationContext.getBean(
        DataSourceConfiguration.class,
        Qualifiers.byName("one") // (2)
);

assertEquals(new URI("jdbc:mysql://localhost/one"), firstConfig.getUrl());
Evaluating Beans Built by @EachProperty
when:
Collection<DataSourceConfiguration> beansOfType = applicationContext.getBeansOfType(DataSourceConfiguration.class)

then:
beansOfType.size() == 2 // (1)

when:
DataSourceConfiguration firstConfig = applicationContext.getBean(
        DataSourceConfiguration.class,
        Qualifiers.byName("one") // (2)
)

then:
new URI("jdbc:mysql://localhost/one") == firstConfig.getUrl()
Evaluating Beans Built by @EachProperty
val beansOfType = applicationContext.getBeansOfType(DataSourceConfiguration::class.java)
beansOfType.size shouldBe 2 // (1)

val firstConfig = applicationContext.getBean(
    DataSourceConfiguration::class.java,
    Qualifiers.byName("one") // (2)
)

firstConfig.url shouldBe URI("jdbc:mysql://localhost/one")
1 All beans of type DataSourceConfiguration can be retrieved using getBeansOfType
2 Individual beans can be retrieved by using the byName qualifier.

List-Based Binding

The default behavior of @EachProperty is to bind from a map style of configuration, where the key is the named qualifier of the bean and the value is the data to bind from. For cases where map style configuration doesn’t make sense, it is possible to inform the Micronaut framework that the class is bound from a list. Simply set the list member on the annotation to true.

@EachProperty List Example
import io.micronaut.context.annotation.EachProperty;
import io.micronaut.context.annotation.Parameter;
import io.micronaut.core.order.Ordered;

import java.time.Duration;

@EachProperty(value = "ratelimits", list = true) // (1)
public class RateLimitsConfiguration implements Ordered { // (2)

    private final Integer index;
    private Duration period;
    private Integer limit;

    RateLimitsConfiguration(@Parameter Integer index) { // (3)
        this.index = index;
    }

    @Override
    public int getOrder() {
        return index;
    }

    public Duration getPeriod() {
        return period;
    }

    public void setPeriod(Duration period) {
        this.period = period;
    }

    public Integer getLimit() {
        return limit;
    }

    public void setLimit(Integer limit) {
        this.limit = limit;
    }
}
@EachProperty List Example
import io.micronaut.context.annotation.EachProperty
import io.micronaut.context.annotation.Parameter
import io.micronaut.core.order.Ordered

import java.time.Duration

@EachProperty(value = "ratelimits", list = true) // (1)
class RateLimitsConfiguration implements Ordered { // (2)

    private final Integer index
    Duration period
    Integer limit

    RateLimitsConfiguration(@Parameter Integer index) { // (3)
        this.index = index
    }

    @Override
    int getOrder() {
        index
    }
}
@EachProperty List Example
import io.micronaut.context.annotation.EachProperty
import io.micronaut.context.annotation.Parameter
import io.micronaut.core.order.Ordered
import java.time.Duration

@EachProperty(value = "ratelimits", list = true) // (1)
class RateLimitsConfiguration
    constructor(@param:Parameter private val index: Int) // (3)
    : Ordered { // (2)

    var period: Duration? = null
    var limit: Int? = null

    override fun getOrder(): Int {
        return index
    }
}
1 The list member of the annotation is set to true
2 Implement Ordered if order matters when retrieving the beans
3 The index is injected into the constructor

4.8 Using @EachBean to Drive Configuration

The @EachProperty annotation is a great way to drive dynamic configuration, but typically you want to inject that configuration into another bean that depends on it. Injecting a single instance with a hard-coded qualifier is not a great solution, hence @EachProperty is typically used in combination with @EachBean:

Using @EachBean
@Factory // (1)
public class DataSourceFactory {

    @EachBean(DataSourceConfiguration.class) // (2)
    DataSource dataSource(DataSourceConfiguration configuration) { // (3)
        URI url = configuration.getUrl();
        return new DataSource(url);
    }
Using @EachBean
@Factory // (1)
class DataSourceFactory {

    @EachBean(DataSourceConfiguration) // (2)
    DataSource dataSource(DataSourceConfiguration configuration) { // (3)
        URI url = configuration.url
        return new DataSource(url)
    }
Using @EachBean
@Factory // (1)
class DataSourceFactory {

    @EachBean(DataSourceConfiguration::class) // (2)
    internal fun dataSource(configuration: DataSourceConfiguration): DataSource { // (3)
        val url = configuration.url
        return DataSource(url)
    }
1 The above example defines a bean Factory that creates instances of javax.sql.DataSource.
2 The @EachBean annotation indicates that a new DataSource bean will be created for each DataSourceConfiguration defined in the previous section.
3 The DataSourceConfiguration instance is injected as a method argument and used to drive the configuration of each javax.sql.DataSource
@EachBean requires that the parent bean has a @Named qualifier, since the qualifier is inherited by each bean created by @EachBean.

In other words, to retrieve the DataSource created by test.datasource.one you can do:

Using a Qualifier
Collection<DataSource> beansOfType = applicationContext.getBeansOfType(DataSource.class);
assertEquals(2, beansOfType.size()); // (1)

DataSource firstConfig = applicationContext.getBean(
        DataSource.class,
        Qualifiers.byName("one") // (2)
);
Using a Qualifier
when:
Collection<DataSource> beansOfType = applicationContext.getBeansOfType(DataSource)

then:
beansOfType.size() == 2 // (1)

when:
DataSource firstConfig = applicationContext.getBean(
        DataSource,
        Qualifiers.byName("one") // (2)
)
Using a Qualifier
val beansOfType = applicationContext.getBeansOfType(DataSource::class.java)
beansOfType.size shouldBe 2 // (1)

val firstConfig = applicationContext.getBean(
        DataSource::class.java,
        Qualifiers.byName("one") // (2)
)
1 We demonstrate here that there are indeed two data sources. How can we get one in particular?
2 By using Qualifiers.byName("one"), we can select which of the two beans we’d like to reference.

4.9 Immutable Configuration

Since 1.3, Micronaut framework supports the definition of immutable configuration. Immutable configuration with an interface requires the Micronaut Context dependency.

implementation("io.micronaut:micronaut-context")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-context</artifactId>
</dependency>

micronaut-context is a transitive dependency of micronaut-http. If you use a Micronaut HTTP runtime, your project already includes the Micronaut-context dependency.

There are two ways to define immutable configuration. The preferred way is to define an interface annotated with @ConfigurationProperties. For example:

@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationProperties;
import io.micronaut.core.bind.annotation.Bindable;

import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.NotNull;
import java.util.Optional;

@ConfigurationProperties("my.engine") // (1)
public interface EngineConfig {

    @Bindable(defaultValue = "Ford") // (2)
    @NotBlank // (3)
    String getManufacturer();

    @Min(1L)
    int getCylinders();

    @NotNull
    CrankShaft getCrankShaft(); // (4)

    @ConfigurationProperties("crank-shaft")
    interface CrankShaft { // (5)
        Optional<Double> getRodLength(); // (6)
    }
}
@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.core.bind.annotation.Bindable

import jakarta.validation.constraints.Min
import jakarta.validation.constraints.NotBlank
import jakarta.validation.constraints.NotNull

@ConfigurationProperties("my.engine") // (1)
interface EngineConfig {

    @Bindable(defaultValue = "Ford") // (2)
    @NotBlank // (3)
    String getManufacturer()

    @Min(1L)
    int getCylinders()

    @NotNull
    CrankShaft getCrankShaft() // (4)

    @ConfigurationProperties("crank-shaft")
    static interface CrankShaft { // (5)
        Optional<Double> getRodLength() // (6)
    }
}
@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.core.bind.annotation.Bindable
import jakarta.validation.constraints.Min
import jakarta.validation.constraints.NotBlank
import jakarta.validation.constraints.NotNull

@ConfigurationProperties("my.engine") // (1)
interface EngineConfig {

    @get:Bindable(defaultValue = "Ford") // (2)
    @get:NotBlank // (3)
    val manufacturer: String

    @get:Min(1L)
    val cylinders: Int

    @get:NotNull
    val crankShaft: CrankShaft // (4)

    @ConfigurationProperties("crank-shaft")
    interface CrankShaft { // (5)
        val rodLength: Double? // (6)
    }
}
1 The @ConfigurationProperties annotation takes the configuration prefix and is declared on an interface
2 You can use @Bindable to set a default value
3 Validation annotations can also be used
4 You can also specify references to other @ConfigurationProperties beans.
5 You can nest immutable configuration
6 Optional configuration can be indicated by returning an Optional or specifying @Nullable

In this case the Micronaut framework provides a compile-time implementation that delegates all getters to call the getProperty(..) method of the Environment interface.

This has the advantage that if the application configuration is refreshed (for example by invoking the /refresh endpoint), the injected interface automatically sees the new values.

If you try to specify any other abstract method other than a getter, a compilation error occurs (default methods are supported).

Another way to implement immutable configuration is to define a class and use the @ConfigurationInject annotation on a constructor of a @ConfigurationProperties or @EachProperty bean.

For example:

@ConfigurationProperties Example
import io.micronaut.core.bind.annotation.Bindable;
import io.micronaut.core.annotation.Nullable;
import io.micronaut.context.annotation.ConfigurationInject;
import io.micronaut.context.annotation.ConfigurationProperties;

import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.NotNull;
import java.util.Optional;

@ConfigurationProperties("my.engine") // (1)
public class EngineConfig {

    private final String manufacturer;
    private final int cylinders;
    private final CrankShaft crankShaft;

    @ConfigurationInject // (2)
    public EngineConfig(
            @Bindable(defaultValue = "Ford") @NotBlank String manufacturer, // (3)
            @Min(1L) int cylinders, // (4)
            @NotNull CrankShaft crankShaft) {
        this.manufacturer = manufacturer;
        this.cylinders = cylinders;
        this.crankShaft = crankShaft;
    }

    public String getManufacturer() {
        return manufacturer;
    }

    public int getCylinders() {
        return cylinders;
    }

    public CrankShaft getCrankShaft() {
        return crankShaft;
    }

    @ConfigurationProperties("crank-shaft")
    public static class CrankShaft { // (5)
        private final Double rodLength; // (6)

        @ConfigurationInject
        public CrankShaft(@Nullable Double rodLength) {
            this.rodLength = rodLength;
        }

        public Optional<Double> getRodLength() {
            return Optional.ofNullable(rodLength);
        }
    }
}
@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationInject
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.core.bind.annotation.Bindable

import jakarta.annotation.Nullable
import jakarta.validation.constraints.Min
import jakarta.validation.constraints.NotBlank
import jakarta.validation.constraints.NotNull

@ConfigurationProperties("my.engine") // (1)
class EngineConfig {

    final String manufacturer
    final int cylinders
    final CrankShaft crankShaft

    @ConfigurationInject // (2)
    EngineConfig(
            @Bindable(defaultValue = "Ford") @NotBlank String manufacturer, // (3)
            @Min(1L) int cylinders, // (4)
            @NotNull CrankShaft crankShaft) {
        this.manufacturer = manufacturer
        this.cylinders = cylinders
        this.crankShaft = crankShaft
    }

    @ConfigurationProperties("crank-shaft")
    static class CrankShaft { // (5)
        private final Double rodLength // (6)

        @ConfigurationInject
        CrankShaft(@Nullable Double rodLength) {
            this.rodLength = rodLength
        }

        Optional<Double> getRodLength() {
            Optional.ofNullable(rodLength)
        }
    }
}
@ConfigurationProperties Example
import io.micronaut.context.annotation.ConfigurationInject
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.core.bind.annotation.Bindable
import java.util.Optional
import jakarta.validation.constraints.Min
import jakarta.validation.constraints.NotBlank
import jakarta.validation.constraints.NotNull

@ConfigurationProperties("my.engine") // (1)
data class EngineConfig @ConfigurationInject // (2)
    constructor(
        @Bindable(defaultValue = "Ford") @NotBlank val manufacturer: String, // (3)
        @Min(1) val cylinders: Int, // (4)
        @NotNull val crankShaft: CrankShaft) {

    @ConfigurationProperties("crank-shaft")
    data class CrankShaft @ConfigurationInject
    constructor(// (5)
            private val rodLength: Double? // (6)
    ) {

        fun getRodLength(): Optional<Double> {
            return Optional.ofNullable(rodLength)
        }
    }
}
1 The @ConfigurationProperties annotation takes the configuration prefix
2 The @ConfigurationInject annotation is defined on the constructor
3 You can use @Bindable to set a default value
4 Validation annotations can be used too
5 You can nest immutable configuration
6 Optional configuration can be indicated with @Nullable

The @ConfigurationInject annotation provides a hint to the Micronaut framework to prioritize binding values from configuration instead of injecting beans.

Using this approach, to make the configuration refreshable, add the @Refreshable annotation to the class as well. This allows the bean to be re-created in the case of a runtime configuration refresh event.

There are a few exceptions to this rule. Micronaut framework will not perform configuration binding for a parameter if any of these conditions is met:

  • The parameter is annotated with @Value (explicit binding)

  • The parameter is annotated with @Property (explicit binding)

  • The parameter is annotated with @Parameter (parameterized bean handling)

  • The parameter is annotated with @Inject (generic bean injection)

  • The type of the parameter is annotated with a bean scope (such as @Singleton)

Once you have prepared a type-safe configuration it can be injected into your beans like any other bean:

@ConfigurationProperties Dependency Injection
@Singleton
public class Engine {
    private final EngineConfig config;

    public Engine(EngineConfig config) {// (1)
        this.config = config;
    }

    public int getCylinders() {
        return config.getCylinders();
    }

    public String start() {// (2)
        return getConfig().getManufacturer() + " Engine Starting V" + getConfig().getCylinders() +
                " [rodLength=" + getConfig().getCrankShaft().getRodLength().orElse(6.0d) + "]";
    }

    public final EngineConfig getConfig() {
        return config;
    }
}
@ConfigurationProperties Dependency Injection
@Singleton
class Engine {
    private final EngineConfig config

    Engine(EngineConfig config) {// (1)
        this.config = config
    }

    int getCylinders() {
        return config.cylinders
    }

    String start() {// (2)
        return "$config.manufacturer Engine Starting V$config.cylinders [rodLength=${config.crankShaft.rodLength.orElse(6.0d)}]"
    }

    final EngineConfig getConfig() {
        return config
    }
}
@ConfigurationProperties Dependency Injection
@Singleton
class Engine(val config: EngineConfig)// (1)
{
    val cylinders: Int
        get() = config.cylinders

    fun start(): String {// (2)
        return  "${config.manufacturer} Engine Starting V${config.cylinders} [rodLength=${config.crankShaft.getRodLength().orElse(6.0)}]"
    }
}
1 Inject the EngineConfig bean
2 Use the configuration properties

Configuration values can then be supplied when running the application. For example:

Supply Configuration
ApplicationContext applicationContext = ApplicationContext.run(CollectionUtils.mapOf(
        "my.engine.cylinders", "8",
        "my.engine.crank-shaft.rod-length", "7.0"
));

Vehicle vehicle = applicationContext.getBean(Vehicle.class);
System.out.println(vehicle.start());
Supply Configuration
ApplicationContext applicationContext = ApplicationContext.run(
        "my.engine.cylinders": "8",
        "my.engine.crank-shaft.rod-length": "7.0"
)

Vehicle vehicle = applicationContext.getBean(Vehicle)
System.out.println(vehicle.start())
Supply Configuration
val map = mapOf(
        "my.engine.cylinders" to "8",
        "my.engine.crank-shaft.rod-length" to "7.0"
)
val applicationContext = ApplicationContext.run(map)

val vehicle = applicationContext.getBean(Vehicle::class.java)
println(vehicle.start())

The above example prints: "Ford Engine Starting V8 [rodLength=7B.0]"

Using Java Record Classes to define immutable configuration

For Java language applications, it’s also possible to use Java Record Classes for immutable configuration with @ConfigurationProperties. For example:

Java Record Example
import io.micronaut.context.annotation.ConfigurationProperties;
import io.micronaut.context.annotation.Requires;
import io.micronaut.core.annotation.NonNull;
import jakarta.validation.constraints.NotNull;
import java.math.BigDecimal;

@ConfigurationProperties("vat")
public record ValueAddedTaxConfiguration(
    @NonNull @NotNull BigDecimal percentage) { // (1)
}
1 The percentage field defines a configuration property for "vat"
From a performance perspective Java records are better than interfaces.

Customizing accessors

As already explained in Change accessors style, it is also possible to customize the accessors when creating immutable configuration properties:

import io.micronaut.context.annotation.ConfigurationProperties;
import io.micronaut.core.annotation.AccessorsStyle;
import io.micronaut.core.bind.annotation.Bindable;

import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.NotNull;
import java.util.Optional;

@ConfigurationProperties("my.engine") (1)
@AccessorsStyle(readPrefixes = "read") (2)
public interface EngineConfigAccessors {

    @Bindable(defaultValue = "Ford")
    @NotBlank
    String readManufacturer(); (3)

    @Min(1L)
    int readCylinders(); (3)

    @NotNull
    CrankShaft readCrankShaft(); (3)

    @ConfigurationProperties("crank-shaft")
    @AccessorsStyle(readPrefixes = "read") (4)
    interface CrankShaft {
        Optional<Double> readRodLength(); (5)
    }
}
1 The @ConfigurationProperties annotation takes the configuration prefix and is declared on an interface
2 The @AccessorsStyle annotation defines the readPrefixes as read.
3 The getters are all prefixed with read.
4 Nested immutable configuration can also be annotated with @ConfigurationProperties.
5 The getter is prefixed with read.

4.10 Bootstrap Configuration

Most application configuration is stored in your configuration file (e.g application.yml), environment-specific files like application-{environment}.{extension}, environment and system properties, etc. These configure the application context. But during application startup, before the application context is created, a "bootstrap" context can be created to store configuration necessary to retrieve additional configuration for the main context. Typically, that additional configuration is in some remote source.

The bootstrap context is enabled depending on the following conditions. The conditions are checked in the following order:

  • If The BOOTSTRAP_CONTEXT_PROPERTY system property is set, that value determines if the bootstrap context is enabled.

  • If The application context builder option bootstrapEnvironment is set, that value determines if the bootstrap context is enabled.

  • If a BootstrapPropertySourceLocator bean is present the bootstrap context is enabled. Normally this comes from the micronaut-discovery-client dependency.

Configuration properties that must be present before application context configuration properties are resolved, for example when using distributed configuration, are stored in a bootstrap configuration file. Once it is determined the bootstrap context is enabled (as described above), the bootstrap configuration files are read using the same rules as regular application configuration. See the property source documentation for the details. The only difference is the prefix (bootstrap instead of application).

The file name prefix bootstrap is configurable with a system property micronaut.bootstrap.name.

The bootstrap context configuration is carried over to the main context automatically, so it is not necessary for configuration properties to be duplicated in the main context. In addition, the bootstrap context configuration has a higher precedence than the main context, meaning if a configuration property appears in both contexts, then the value will be taken from the bootstrap context first.

That means if a configuration property is needed in both places, it should go into the bootstrap context configuration.

See the distributed configuration section of the documentation for the list of integrations with common distributed configuration solutions.

Bootstrap Context Beans

In order for a bean to be resolvable in the bootstrap context it must be annotated with @BootstrapContextCompatible. If any given bean is not annotated then it will not be able to be resolved in the bootstrap context. Typically, any bean that is participating in the process of retrieving distributed configuration needs to be annotated.

4.11 JMX Support

Micronaut framework provides basic support for JMX.

For more information, see the documentation for the micronaut-jmx project.

5 Aspect Oriented Programming

Aspect-Oriented Programming (AOP) has historically had many incarnations and some very complicated implementations. Generally AOP can be thought of as a way to define cross-cutting concerns (logging, transactions, tracing, etc.) separate from application code in the form of aspects that define advice.

There are typically two forms of advice:

  • Around Advice - decorates a method or class

  • Introduction Advice - introduces new behaviour to a class.

In modern Java applications, declaring advice typically takes the form of an annotation. The most well-known annotation advice in the Java world is probably @Transactional, which demarcates transaction boundaries in Spring and Grails applications.

The disadvantage of traditional approaches to AOP is the heavy reliance on runtime proxy creation and reflection, which slows application performance, makes debugging harder and increases memory consumption.

Micronaut framework tries to address these concerns by providing a simple compile-time AOP API that does not use reflection.

5.1 Around Advice

The most common type of advice you may want to apply is "Around" advice, which lets you decorate a method’s behaviour.

Writing Around Advice

The first step is to define an annotation that will trigger a MethodInterceptor:

Around Advice Annotation Example
import io.micronaut.aop.Around;
import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Documented
@Retention(RUNTIME) // (1)
@Target({TYPE, METHOD}) // (2)
@Around // (3)
public @interface NotNull {
}
Around Advice Annotation Example
import io.micronaut.aop.Around
import java.lang.annotation.*
import static java.lang.annotation.ElementType.*
import static java.lang.annotation.RetentionPolicy.RUNTIME

@Documented
@Retention(RUNTIME) // (1)
@Target([TYPE, METHOD]) // (2)
@Around // (3)
@interface NotNull {
}
Around Advice Annotation Example
import io.micronaut.aop.Around
import kotlin.annotation.AnnotationRetention.RUNTIME
import kotlin.annotation.AnnotationTarget.CLASS
import kotlin.annotation.AnnotationTarget.FILE
import kotlin.annotation.AnnotationTarget.FUNCTION
import kotlin.annotation.AnnotationTarget.PROPERTY_GETTER
import kotlin.annotation.AnnotationTarget.PROPERTY_SETTER

@MustBeDocumented
@Retention(RUNTIME) // (1)
@Target(CLASS, FILE, FUNCTION, PROPERTY_GETTER, PROPERTY_SETTER) // (2)
@Around // (3)
annotation class NotNull
1 The retention policy of the annotation should be RUNTIME
2 Generally you want to be able to apply advice at the class or method level so the target types are TYPE and METHOD
3 The @Around annotation is added to tell the Micronaut framework that the annotation is Around advice

The next step to defining Around advice is to implement a MethodInterceptor. For example the following interceptor disallows parameters with null values:

MethodInterceptor Example
import io.micronaut.aop.InterceptorBean;
import io.micronaut.aop.MethodInterceptor;
import io.micronaut.aop.MethodInvocationContext;
import io.micronaut.core.annotation.Nullable;
import io.micronaut.core.type.MutableArgumentValue;

import jakarta.inject.Singleton;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;

@Singleton
@InterceptorBean(NotNull.class) // (1)
public class NotNullInterceptor implements MethodInterceptor<Object, Object> { // (2)
    @Nullable
    @Override
    public Object intercept(MethodInvocationContext<Object, Object> context) {
        Optional<Map.Entry<String, MutableArgumentValue<?>>> nullParam = context.getParameters()
            .entrySet()
            .stream()
            .filter(entry -> {
                MutableArgumentValue<?> argumentValue = entry.getValue();
                return Objects.isNull(argumentValue.getValue());
            })
            .findFirst(); // (3)
        if (nullParam.isPresent()) {
            throw new IllegalArgumentException("Null parameter [" + nullParam.get().getKey() + "] not allowed"); // (4)
        }
        return context.proceed(); // (5)
    }
}
MethodInterceptor Example
import io.micronaut.aop.InterceptorBean
import io.micronaut.aop.MethodInterceptor
import io.micronaut.aop.MethodInvocationContext
import io.micronaut.core.annotation.Nullable
import io.micronaut.core.type.MutableArgumentValue

import jakarta.inject.Singleton

@Singleton
@InterceptorBean(NotNull) // (1)
class NotNullInterceptor implements MethodInterceptor<Object, Object> { // (2)
    @Nullable
    @Override
    Object intercept(MethodInvocationContext<Object, Object> context) {
        Optional<Map.Entry<String, MutableArgumentValue<?>>> nullParam = context.parameters
            .entrySet()
            .stream()
            .filter({entry ->
                MutableArgumentValue<?> argumentValue = entry.value
                return Objects.isNull(argumentValue.value)
            })
            .findFirst() // (3)
        if (nullParam.present) {
            throw new IllegalArgumentException("Null parameter [${nullParam.get().key}] not allowed") // (4)
        }
        return context.proceed() // (5)
    }
}
MethodInterceptor Example
import io.micronaut.aop.InterceptorBean
import io.micronaut.aop.MethodInterceptor
import io.micronaut.aop.MethodInvocationContext
import java.util.Objects
import jakarta.inject.Singleton

@Singleton
@InterceptorBean(NotNull::class) // (1)
class NotNullInterceptor : MethodInterceptor<Any, Any> { // (2)
    override fun intercept(context: MethodInvocationContext<Any, Any>): Any? {
        val nullParam = context.parameters
                .entries
                .stream()
                .filter { entry ->
                    val argumentValue = entry.value
                    Objects.isNull(argumentValue.value)
                }
                .findFirst() // (3)
        return if (nullParam.isPresent) {
            throw IllegalArgumentException("Null parameter [${nullParam.get().key}] not allowed") // (4)
        } else {
            context.proceed() // (5)
        }
    }
}
1 The @InterceptorBean annotation is used to indicate what annotation the interceptor is associated with. Note that @InterceptorBean is meta-annotated with a default scope of @Singleton therefore if you want a new interceptor created and associated with each intercepted bean you should annotate the interceptor with @Prototype.
2 An interceptor implements the MethodInterceptor interface.
3 The passed MethodInvocationContext is used to find the first parameter that is null
4 If a null parameter is found an exception is thrown
5 Otherwise proceed() is called to proceed with the method invocation.
Micronaut AOP interceptors use no reflection which improves performance and reducing stack trace sizes, thus improving debugging.

Apply the annotation to target classes to put the new MethodInterceptor to work:

Around Advice Usage Example
import jakarta.inject.Singleton;

@Singleton
public class NotNullExample {

    @NotNull
    void doWork(String taskName) {
        System.out.println("Doing job: " + taskName);
    }
}
Around Advice Usage Example
import jakarta.inject.Singleton

@Singleton
class NotNullExample {

    @NotNull
    void doWork(String taskName) {
        println "Doing job: $taskName"
    }
}
Around Advice Usage Example
import jakarta.inject.Singleton

@Singleton
open class NotNullExample {

    @NotNull
    open fun doWork(taskName: String?) {
        println("Doing job: $taskName")
    }
}

Whenever the type NotNullExample is injected into a class, a compile-time-generated proxy is injected that decorates method calls with the @NotNull advice defined earlier. You can verify that the advice works by writing a test. The following test verifies that the expected exception is thrown when the argument is null:

Around Advice Test
Around Advice Test
void "test not null"() {
    when:
    def applicationContext = ApplicationContext.run()
    def exampleBean = applicationContext.getBean(NotNullExample)

    exampleBean.doWork(null)

    then:
    IllegalArgumentException e = thrown()
    e.message == 'Null parameter [taskName] not allowed'

    cleanup:
    applicationContext.close()
}
Around Advice Test
@Test
fun testNotNull() {
    val applicationContext = ApplicationContext.run()
    val exampleBean = applicationContext.getBean(NotNullExample::class.java)

    val exception = shouldThrow<IllegalArgumentException> {
        exampleBean.doWork(null)
    }
    exception.message shouldBe "Null parameter [taskName] not allowed"
    applicationContext.close()
}
Since Micronaut injection happens at compile time, generally the advice should be packaged in a dependent JAR file that is on the classpath when the above test is compiled. It should not be in the same codebase since you don’t want the test to be compiled before the advice itself is compiled.

Customizing Proxy Generation

The default behaviour of the Around annotation is to generate a proxy at compile time that is a subclass of the proxied class. In other words, in the previous example a compile-time subclass of the NotNullExample class will be produced where proxied methods are decorated with interceptor handling, and the original behaviour is invoked via a call to super.

This behaviour is more efficient as only one instance of the bean is required, however depending on the use case you may wish to alter this behaviour. The @Around annotation supports various attributes that allow you to alter this behaviour, including:

  • proxyTarget (defaults to false) - If set to true, instead of a subclass that calls super, the proxy delegates to the original bean instance

  • hotswap (defaults to false) - Same as proxyTarget=true, but in addition the proxy implements HotSwappableInterceptedProxy which wraps each method call in a ReentrantReadWriteLock and allows swapping the target instance at runtime.

  • lazy (defaults to false) - By default the Micronaut framework eagerly initializes the proxy target when the proxy is created. If set to true the proxy target is instead resolved lazily for each method call.

AOP Advice on @Factory Beans

The semantics of AOP advice when applied to Bean Factories differs from regular beans, with the following rules applying:

  1. AOP advice applied at the class level of a @Factory bean applies the advice to the factory itself and not to any beans defined with the @Bean annotation.

  2. AOP advice applied on a method annotated with a bean scope applies the AOP advice to the bean that the factory produces.

Consider the following two examples:

AOP Advice at the type level of a @Factory
@Timed
@Factory
public class MyFactory {

    @Prototype
    public MyBean myBean() {
        return new MyBean();
    }
}
AOP Advice at the type level of a @Factory
@Timed
@Factory
class MyFactory {

    @Prototype
    MyBean myBean() {
        new MyBean()
    }
}
AOP Advice at the type level of a @Factory
@Timed
@Factory
open class MyFactory {

    @Prototype
    open fun myBean(): MyBean {
        return MyBean()
    }
}

The above example logs the time it takes to create the MyBean bean.

Now consider this example:

AOP Advice at the method level of a @Factory
@Factory
public class MyFactory {

    @Prototype
    @Timed
    public MyBean myBean() {
        return new MyBean();
    }
}
AOP Advice at the method level of a @Factory
@Factory
class MyFactory {

    @Prototype
    @Timed
    MyBean myBean() {
        new MyBean()
    }
}
AOP Advice at the method level of a @Factory
@Factory
open class MyFactory {

    @Prototype
    @Timed
    open fun myBean(): MyBean {
        return MyBean()
    }
}

The above example logs the time it takes to execute the public methods of the MyBean bean, but not the bean creation.

The rationale for this behaviour is that you may at times wish to apply advice to a factory and at other times apply advice to the bean produced by the factory.

Note that there is currently no way to apply advice at the method level to a @Factory bean, and all advice for factories must be applied at the type level. You can control which methods have advice applied by defining methods as non-public which do not have advice applied.

5.2 Introduction Advice

Introduction advice is distinct from Around advice in that it involves providing an implementation instead of decorating.

Examples of introduction advice includes Spring Data which implements persistence logic for you.

Micronaut Client annotation is another example of introduction advice where the Micronaut framework implements HTTP client interfaces for you at compile time.

The way you implement Introduction advice is very similar to how you implement Around advice.

You start by defining an annotation that powers the introduction advice. As an example, say you want to implement advice to return a stubbed value for every method in an interface (a common requirement in testing frameworks). Consider the following @Stub annotation:

Introduction Advice Annotation Example
import io.micronaut.aop.Introduction;
import io.micronaut.context.annotation.Bean;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Introduction // (1)
@Bean // (2)
@Documented
@Retention(RUNTIME)
@Target({TYPE, ANNOTATION_TYPE, METHOD})
public @interface Stub {
    String value() default "";
}
Introduction Advice Annotation Example
import io.micronaut.aop.Introduction
import io.micronaut.context.annotation.Bean

import java.lang.annotation.Documented
import java.lang.annotation.Retention
import java.lang.annotation.Target

import static java.lang.annotation.ElementType.ANNOTATION_TYPE
import static java.lang.annotation.ElementType.METHOD
import static java.lang.annotation.ElementType.TYPE
import static java.lang.annotation.RetentionPolicy.RUNTIME

@Introduction // (1)
@Bean // (2)
@Documented
@Retention(RUNTIME)
@Target([TYPE, ANNOTATION_TYPE, METHOD])
@interface Stub {
    String value() default ""
}
Introduction Advice Annotation Example
import io.micronaut.aop.Introduction
import io.micronaut.context.annotation.Bean
import kotlin.annotation.AnnotationRetention.RUNTIME
import kotlin.annotation.AnnotationTarget.ANNOTATION_CLASS
import kotlin.annotation.AnnotationTarget.CLASS
import kotlin.annotation.AnnotationTarget.FILE
import kotlin.annotation.AnnotationTarget.FUNCTION
import kotlin.annotation.AnnotationTarget.PROPERTY_GETTER
import kotlin.annotation.AnnotationTarget.PROPERTY_SETTER

@Introduction // (1)
@Bean // (2)
@MustBeDocumented
@Retention(RUNTIME)
@Target(CLASS, FILE, ANNOTATION_CLASS, FUNCTION, PROPERTY_GETTER, PROPERTY_SETTER)
annotation class Stub(val value: String = "")
1 The introduction advice is annotated with Introduction
2 The Bean annotation is added so that all types annotated with @Stub become beans

The StubIntroduction class referred to in the previous example must then implement the MethodInterceptor interface, just like around advice.

The following is an example implementation:

StubIntroduction
import io.micronaut.aop.*;
import io.micronaut.core.annotation.Nullable;
import jakarta.inject.Singleton;

@Singleton
@InterceptorBean(Stub.class) // (1)
public class StubIntroduction implements MethodInterceptor<Object, Object> { // (2)

    @Nullable
    @Override
    public Object intercept(MethodInvocationContext<Object, Object> context) {
        return context.getValue( // (3)
                Stub.class,
                context.getReturnType().getType()
        ).orElse(null); // (4)
    }
}
StubIntroduction
import io.micronaut.aop.MethodInterceptor
import io.micronaut.aop.MethodInvocationContext
import io.micronaut.aop.InterceptorBean
import io.micronaut.core.annotation.Nullable
import jakarta.inject.Singleton

@Singleton
@InterceptorBean(Stub) // (1)
class StubIntroduction implements MethodInterceptor<Object,Object> { // (2)

    @Nullable
    @Override
    Object intercept(MethodInvocationContext<Object, Object> context) {
        context.getValue( // (3)
                Stub,
                context.returnType.type
        ).orElse(null) // (4)
    }
}
StubIntroduction
import io.micronaut.aop.*
import jakarta.inject.Singleton

@Singleton
@InterceptorBean(Stub::class) // (1)
class StubIntroduction : MethodInterceptor<Any, Any> { // (2)

    override fun intercept(context: MethodInvocationContext<Any, Any>): Any? {
        return context.getValue<Any>( // (3)
                Stub::class.java,
                context.returnType.type
        ).orElse(null) // (4)
    }
}
1 The InterceptorBean annotation is used to associate the interceptor with the @Stub annotation
2 The class is annotated with @Singleton and implements the MethodInterceptor interface
3 The value of the @Stub annotation is read from the context and an attempt made to convert the value to the return type
4 Otherwise null is returned

To now use this introduction advice in an application, annotate your abstract classes or interfaces with @Stub:

StubExample
@Stub
public interface StubExample {

    @Stub("10")
    int getNumber();

    LocalDateTime getDate();
}
StubExample
@Stub
interface StubExample {

    @Stub("10")
    int getNumber()

    LocalDateTime getDate()
}
StubExample
@Stub
interface StubExample {

    @get:Stub("10")
    val number: Int

    val date: LocalDateTime?
}

All abstract methods delegate to the StubIntroduction class to be implemented.

The following test demonstrates the behaviour or StubIntroduction:

Testing Introduction Advice
StubExample stubExample = applicationContext.getBean(StubExample.class);

assertEquals(10, stubExample.getNumber());
assertNull(stubExample.getDate());
Testing Introduction Advice
when:
def stubExample = applicationContext.getBean(StubExample)

then:
stubExample.number == 10
stubExample.date == null
Testing Introduction Advice
val stubExample = applicationContext.getBean(StubExample::class.java)

stubExample.number.shouldBe(10)
stubExample.date.shouldBe(null)

Note that if the introduction advice cannot implement the method, call the proceed method of the MethodInvocationContext. This lets other introduction advice interceptors implement the method, and an UnsupportedOperationException will be thrown if no advice can implement the method.

In addition, if multiple introduction advice are present you may wish to override the getOrder() method of MethodInterceptor to control the priority of advice.

The following sections cover core advice types provided by Micronaut.

5.3 Method Adapter Advice

There are cases where you want to introduce a new bean based on the presence of an annotation on a method. An example of this is the @EventListener annotation which produces an implementation of ApplicationEventListener for each annotated method that invokes the annotated method.

For example the following snippet runs the logic contained within the method when the ApplicationContext starts up:

import io.micronaut.context.event.StartupEvent;
import io.micronaut.runtime.event.annotation.EventListener;
...

@EventListener
void onStartup(StartupEvent event) {
    // startup logic here
}

The presence of the @EventListener annotation causes the Micronaut framework to create a new class that implements ApplicationEventListener and invokes the onStartup method defined in the bean above.

The actual implementation of the @EventListener is trivial; it simply uses the @Adapter annotation to specify which SAM (single abstract method) type it adapts:

import io.micronaut.aop.Adapter;
import io.micronaut.context.event.ApplicationEventListener;
import io.micronaut.core.annotation.Indexed;

import java.lang.annotation.*;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Documented
@Retention(RUNTIME)
@Target({ElementType.ANNOTATION_TYPE, ElementType.METHOD})
@Adapter(ApplicationEventListener.class) (1)
@Indexed(ApplicationEventListener.class)
@Inherited
public @interface EventListener {
}
1 The @Adapter annotation indicates which SAM type to adapt, in this case ApplicationEventListener.
The Micronaut framework also automatically aligns the generic types for the SAM interface if they are specified.

Using this mechanism you can define custom annotations that use the @Adapter annotation and a SAM interface to automatically implement beans for you at compile time.

5.4 Bean Life Cycle Advice

Sometimes you may need to apply advice to a bean’s lifecycle. There are 3 types of advice that are applicable in this case:

  • Interception of the construction of the bean

  • Interception of the bean’s @PostConstruct invocation

  • Interception of a bean’s @PreDestroy invocation

The Micronaut framework supports these 3 use cases by allowing the definition of additional @InterceptorBinding meta-annotations.

Consider the following annotation definition:

AroundConstruct example
import io.micronaut.aop.*;
import io.micronaut.context.annotation.Prototype;
import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@AroundConstruct // (1)
@InterceptorBinding(kind = InterceptorKind.POST_CONSTRUCT) // (2)
@InterceptorBinding(kind = InterceptorKind.PRE_DESTROY) // (3)
@Prototype // (4)
public @interface ProductBean {
}
AroundConstruct example
import io.micronaut.aop.*
import io.micronaut.context.annotation.Prototype
import java.lang.annotation.*

@Retention(RetentionPolicy.RUNTIME)
@AroundConstruct // (1)
@InterceptorBinding(kind = InterceptorKind.POST_CONSTRUCT) // (2)
@InterceptorBinding(kind = InterceptorKind.PRE_DESTROY) // (3)
@Prototype // (4)
@interface ProductBean {
}
AroundConstruct example
import io.micronaut.aop.AroundConstruct
import io.micronaut.aop.InterceptorBinding
import io.micronaut.aop.InterceptorBindingDefinitions
import io.micronaut.aop.InterceptorKind
import io.micronaut.context.annotation.Prototype

@Retention(AnnotationRetention.RUNTIME)
@AroundConstruct // (1)
@InterceptorBindingDefinitions(
    InterceptorBinding(kind = InterceptorKind.POST_CONSTRUCT), // (2)
    InterceptorBinding(kind = InterceptorKind.PRE_DESTROY) // (3)
)
@Prototype // (4)
annotation class ProductBean
1 The @AroundConstruct annotation is added to indicate that interception of the constructor should occur
2 An @InterceptorBinding definition is used to indicate that @PostConstruct interception should occur
3 An @InterceptorBinding definition is used to indicate that @PreDestroy interception should occur
4 The bean is defined as @Prototype so a new instance is required for each injection point

Note that if you do not need @PostConstruct and @PreDestroy interception you can simply remove those bindings.

The @ProductBean annotation can then be used on the target class:

Using an AroundConstruct meta-annotation
import io.micronaut.context.annotation.Parameter;

import jakarta.annotation.PreDestroy;

@ProductBean // (1)
public class Product {
    private final String productName;
    private boolean active = false;

    public Product(@Parameter String productName) { // (2)
        this.productName = productName;
    }

    public String getProductName() {
        return productName;
    }

    public boolean isActive() {
        return active;
    }

    public void setActive(boolean active) {
        this.active = active;
    }

    @PreDestroy // (3)
    void disable() {
        active = false;
    }
}
Using an AroundConstruct meta-annotation
import io.micronaut.context.annotation.Parameter
import jakarta.annotation.PreDestroy

@ProductBean // (1)
class Product {
    final String productName
    boolean active = false

    Product(@Parameter String productName) { // (2)
        this.productName = productName
    }

    @PreDestroy // (3)
    void disable() {
        active = false
    }
}
Using an AroundConstruct meta-annotation
import io.micronaut.context.annotation.Parameter
import jakarta.annotation.PreDestroy

@ProductBean // (1)
class Product(@param:Parameter val productName: String ) { // (2)

    var active: Boolean = false
    @PreDestroy
    fun disable() { // (3)
        active = false
    }
}
1 The @ProductBean annotation is defined on a class of type Product
2 The @Parameter annotation indicates that this bean requires an argument to complete constructions
3 Any @PreDestroy or @PostConstruct methods are executed last in the interceptor chain

Now you can define ConstructorInterceptor beans for constructor interception and MethodInterceptor beans for @PostConstruct or @PreDestroy interception.

The following factory defines a ConstructorInterceptor that intercepts construction of Product instances and registers them with a hypothetical ProductService validating the product name first:

Defining a constructor interceptor
import io.micronaut.aop.*;
import io.micronaut.context.annotation.Factory;

@Factory
public class ProductInterceptors {
    private final ProductService productService;

    public ProductInterceptors(ProductService productService) {
        this.productService = productService;
    }
}

@InterceptorBean(ProductBean.class)
ConstructorInterceptor<Product> aroundConstruct() { // (1)
    return context -> {
        final Object[] parameterValues = context.getParameterValues(); // (2)
        final Object parameterValue = parameterValues[0];
        if (parameterValue == null || parameterValues[0].toString().isEmpty()) {
            throw new IllegalArgumentException("Invalid product name");
        }
        String productName = parameterValues[0].toString().toUpperCase();
        parameterValues[0] = productName;
        final Product product = context.proceed(); // (3)
        productService.addProduct(product);
        return product;
    };
}
Defining a constructor interceptor
import io.micronaut.aop.*
import io.micronaut.context.annotation.Factory


@Factory
class ProductInterceptors {
    private final ProductService productService

    ProductInterceptors(ProductService productService) {
        this.productService = productService
    }
}

@InterceptorBean(ProductBean.class)
ConstructorInterceptor<Product> aroundConstruct() { // (1)
    return  { context ->
        final Object[] parameterValues = context.parameterValues // (2)
        final Object parameterValue = parameterValues[0]
        if (parameterValue == null || parameterValues[0].toString().isEmpty()) {
            throw new IllegalArgumentException("Invalid product name")
        }
        String productName = parameterValues[0].toString().toUpperCase()
        parameterValues[0] = productName
        final Product product = context.proceed() // (3)
        productService.addProduct(product)
        return product
    }
}
Defining a constructor interceptor
import io.micronaut.aop.*
import io.micronaut.context.annotation.Factory

@Factory
class ProductInterceptors(private val productService: ProductService) {
}

@InterceptorBean(ProductBean::class)
fun aroundConstruct(): ConstructorInterceptor<Product> { // (1)
    return ConstructorInterceptor { context: ConstructorInvocationContext<Product> ->
        val parameterValues = context.parameterValues // (2)
        val parameterValue = parameterValues[0]
        require(!(parameterValue == null || parameterValues[0].toString().isEmpty())) { "Invalid product name" }
        val productName = parameterValues[0].toString().uppercase()
        parameterValues[0] = productName
        val product = context.proceed() // (3)
        productService.addProduct(product)
        product
    }
}
1 A new @InterceptorBean is defined that is a ConstructorInterceptor
2 The constructor parameter values can be retrieved and modified as needed
3 The constructor can be invoked with the proceed() method

Defining MethodInterceptor instances that interceptor the @PostConstruct and @PreDestroy methods is no different from defining interceptors for regular methods. Note however that you can use the passed MethodInvocationContext to identify what kind of interception is occurring and adapt the code accordingly like in the following example:

Defining a constructor interceptor
@InterceptorBean(ProductBean.class) // (1)
MethodInterceptor<Product, Object> aroundInvoke() {
    return context -> {
        final Product product = context.getTarget();
        switch (context.getKind()) {
            case POST_CONSTRUCT: // (2)
                product.setActive(true);
                return context.proceed();
            case PRE_DESTROY: // (3)
                productService.removeProduct(product);
                return context.proceed();
            default:
                return context.proceed();
        }
    };
}
Defining a constructor interceptor
@InterceptorBean(ProductBean.class) // (1)
MethodInterceptor<Product, Object> aroundInvoke() {
    return { context ->
        final Product product = context.getTarget()
        switch (context.kind) {
            case InterceptorKind.POST_CONSTRUCT: // (2)
                product.setActive(true)
                return context.proceed()
            case InterceptorKind.PRE_DESTROY: // (3)
                productService.removeProduct(product)
                return context.proceed()
            default:
                return context.proceed()
        }
    }
}
Defining a constructor interceptor
@InterceptorBean(ProductBean::class)
fun  aroundInvoke(): MethodInterceptor<Product, Any> { // (1)
    return MethodInterceptor { context: MethodInvocationContext<Product, Any> ->
        val product = context.target
        return@MethodInterceptor when (context.kind) {
            InterceptorKind.POST_CONSTRUCT -> { // (2)
                product.active = true
                context.proceed()
            }
            InterceptorKind.PRE_DESTROY -> { // (3)
                productService.removeProduct(product)
                context.proceed()
            }
            else -> context.proceed()
        }
    }
}
1 A new @InterceptorBean is defined that is a MethodInterceptor
2 @PostConstruct interception is handled
3 @PreDestroy interception is handled

5.5 Validation Advice

Validation advice is one of the most common advice types you are likely to want to use in your application.

Validation advice is built on Bean Validation JSR 380, a specification of the Java API for bean validation which ensures that the properties of a bean meet specific criteria, using jakarta.validation annotations such as @NotNull, @Min, and @Max.

The Micronaut framework provides native support for the jakarta.validation annotations with the micronaut-validation dependency:

annotationProcessor("io.micronaut.validation:micronaut-validation-processor")
<annotationProcessorPaths>
    <path>
        <groupId>io.micronaut.validation</groupId>
        <artifactId>micronaut-validation-processor</artifactId>
    </path>
</annotationProcessorPaths>

implementation("io.micronaut.validation:micronaut-validation")
<dependency>
    <groupId>io.micronaut.validation</groupId>
    <artifactId>micronaut-validation</artifactId>
</dependency>

Or full JSR 380 compliance with the micronaut-hibernate-validator dependency:

implementation("io.micronaut:micronaut-hibernate-validator")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-hibernate-validator</artifactId>
</dependency>

See the section on Bean Validation for more information on how to apply validation rules to your bean classes.

5.6 Cache Advice

Like Spring and Grails, the Micronaut framework provides caching annotations in the io.micronaut.cache package.

The CacheManager interface allows different cache implementations to be plugged in as necessary.

The SyncCache interface provides a synchronous API for caching, whilst the AsyncCache API allows non-blocking operation.

Cache Annotations

The following cache annotations are supported:

  • @Cacheable - Indicates a method is cacheable in the specified cache

  • @CachePut - Indicates that the return value of a method invocation should be cached. Unlike @Cacheable the original operation is never skipped.

  • @CacheInvalidate - Indicates the invocation of a method should cause the invalidation of one or more caches.

Using one of these annotations activates the CacheInterceptor, which in the case of @Cacheable caches the return value of the method.

The emitted result is cached if the method return type is a non-blocking type (either CompletableFuture or an instance of Publisher) .

In addition, if the underlying Cache implementation supports non-blocking cache operations, cache values are read without blocking, resulting in non-blocking cache operations.

Configuring Caches

By default, Caffeine is used to create caches from application configuration. For example:

Cache Configuration Example
micronaut.caches.my-cache.maximum-size=20
micronaut:
  caches:
    my-cache:
      maximum-size: 20
[micronaut]
  [micronaut.caches]
    [micronaut.caches.my-cache]
      maximum-size=20
micronaut {
  caches {
    myCache {
      maximumSize = 20
    }
  }
}
{
  micronaut {
    caches {
      my-cache {
        maximum-size = 20
      }
    }
  }
}
{
  "micronaut": {
    "caches": {
      "my-cache": {
        "maximum-size": 20
      }
    }
  }
}

The above example configures a cache called "my-cache" with a maximum size of 20.

Naming Caches

Define names of caches under micronaut.caches in kebab case (lowercase and hyphen separated); if you use camel case, the names are normalized to kebab case. For example myCache becomes my-cache. The kebab-case form must be used when referencing caches in the @Cacheable annotation.

To configure a weigher to be used with the maximumWeight configuration, create a bean that implements io.micronaut.caffeine.cache.Weigher. To associate a given weigher with only a specific cache, annotate the bean with @Named(<cache name>). Weighers without a named qualifier apply to all caches that don’t have a named weigher. If no beans are found, a default implementation is used.

See the configuration reference for all available configuration options.

Dynamic Cache Creation

A DynamicCacheManager bean can be registered for use cases where caches cannot be configured ahead of time. When a cache is attempted to be retrieved that was not predefined, the dynamic cache manager is invoked to create and return a cache.

By default, if there is no other dynamic cache manager defined in the application, the Micronaut framework registers an instance of DefaultDynamicCacheManager that creates Caffeine caches with default values.

Other Cache Implementations

Check the Micronaut Cache project for more information.

5.7 Retry Advice

In distributed systems and microservice environments, failure is something you have to plan for, and it is common to want to attempt to retry an operation if it fails. If first you don’t succeed try again!

With this in mind, the Micronaut framework includes a Retryable annotation.

Retry Dependency

Since Micronaut Framework 4.0 to use the Retry functionality you need to add the following dependency:

implementation("io.micronaut:micronaut-retry")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-retry</artifactId>
</dependency>

Simple Retry

The simplest form of retry is just to add the @Retryable annotation to a type or method. The default behaviour of @Retryable is to retry three times with a linear delay of one second between each retry. (first attempt with 1s delay, second attempt with 2s delay, third attempt with 3s delay).

For example:

Simple Retry Example
@Retryable
public List<Book> listBooks() {
    // ...
Simple Retry Example
@Retryable
List<Book> listBooks() {
    // ...
Simple Retry Example
@Retryable
open fun listBooks(): List<Book> {
    // ...

With the above example if the listBooks() method throws a RuntimeException, it is retried until the maximum number of attempts is reached.

The multiplier value of the @Retryable annotation can be used to configure a multiplier used to calculate the delay between retries, allowing exponential retry support.

To customize retry behaviour, set the attempts and delay members, For example to configure five attempts with a two seconds delay:

Setting Retry Attempts
@Retryable(attempts = "5",
           delay = "2s")
public Book findBook(String title) {
    // ...
Setting Retry Attempts
@Retryable(attempts = "5",
           delay = "2s")
Book findBook(String title) {
    // ...
Setting Retry Attempts
@Retryable(attempts = "5",
           delay = "2s")
open fun findBook(title: String): Book {
    // ...

Notice how both attempts and delay are defined as strings. This is to support configurability through annotation metadata. For example, you can allow the retry policy to be configured using property placeholder resolution:

Setting Retry via Configuration
@Retryable(attempts = "${book.retry.attempts:3}",
           delay = "${book.retry.delay:1s}")
public Book getBook(String title) {
    // ...
Setting Retry via Configuration
@Retryable(attempts = '${book.retry.attempts:3}',
           delay = '${book.retry.delay:1s}')
Book getBook(String title) {
    // ...
Setting Retry via Configuration
@Retryable(attempts = "\${book.retry.attempts:3}",
           delay = "\${book.retry.delay:1s}")
open fun getBook(title: String): Book {
    // ...

With the above in place, if book.retry.attempts is specified in configuration it is bound to the value of the attempts member of the @Retryable annotation via annotation metadata.

Reactive Retry

@Retryable advice can also be applied to methods that return reactive types, such as Publisher (Project Reactor's Flux or RxJava's Flowable). For example:

Applying Retry Policy to Reactive Types
@Retryable
public Publisher<Book> streamBooks() {
    // ...
Applying Retry Policy to Reactive Types
@Retryable
Flux<Book> streamBooks() {
    // ...
Applying Retry Policy to Reactive Types
@Retryable
open fun streamBooks(): Flux<Book> {
    // ...

In this case @Retryable advice applies the retry policy to the reactive type.

Circuit Breaker

Retry is useful in a microservice environment, but in some cases excessive retries can overwhelm the system as clients repeatedly re-attempt failing operations.

The Circuit Breaker pattern is designed to resolve this issue by allowing a certain number of failing requests and then opening a circuit that remains open for a period before allowing additional retry attempts.

The CircuitBreaker annotation is a variation of the @Retryable annotation that supports a reset member which indicates how long the circuit should remain open before it is reset (the default is 20 seconds).

Applying CircuitBreaker Advice
@CircuitBreaker(reset = "30s")
public List<Book> findBooks() {
    // ...
Applying CircuitBreaker Advice
@CircuitBreaker(reset = "30s")
List<Book> findBooks() {
    // ...
Applying CircuitBreaker Advice
@CircuitBreaker(reset = "30s")
open fun findBooks(): List<Book> {
    // ...

The above example retries the findBooks method three times and then opens the circuit for 30 seconds, rethrowing the original exception and preventing potential downstream traffic such as HTTP requests and I/O operations flooding the system.

Factory Bean Retry

When @Retryable is applied to bean factory methods, it behaves as if the annotation was placed on the type being returned. The retry behavior applies when the methods on the returned object are invoked. Note that the bean factory method itself is not retried. If you want the functionality of creating the bean to be retried, it should be delegated to another singleton that has the @Retryable annotation applied.

For example:

@Factory (1)
public class Neo4jDriverFactory {
    ...
    @Retryable(ServiceUnavailableException.class) (2)
    @Bean(preDestroy = "close")
    public Driver buildDriver() {
        ...
    }
}
1 A factory bean is created that defines methods that create beans
2 The @Retryable annotation is used to catch exceptions thrown from methods executed on the Driver.

Retry Events

You can register RetryEventListener instances as beans to listen for RetryEvent events that are published every time an operation is retried.

In addition, you can register event listeners for CircuitOpenEvent to be notified when a circuit breaker circuit is opened, or CircuitClosedEvent for when a circuit is closed.

5.8 Scheduled Tasks

Like Spring and Grails, the Micronaut framework features a Scheduled annotation for scheduling background tasks.

Using the @Scheduled Annotation

The Scheduled annotation can be added to any method of a bean, and you should set one of the fixedRate, fixedDelay, or cron members. Scheduling requires the Micronaut Context dependency:

implementation("io.micronaut:micronaut-context")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-context</artifactId>
</dependency>

micronaut-context is a transitive dependency of micronaut-http. If you use a Micronaut HTTP runtime, your project already includes the Micronaut-context dependency.

Remember that the scope of a bean impacts behaviour. A @Singleton bean shares state (the fields of the instance) each time the scheduled method is executed, while for a @Prototype bean a new instance is created for each execution.

Scheduling at a Fixed Rate

To schedule a task at a fixed rate, use the fixedRate member. For example:

Fixed Rate Example
@Scheduled(fixedRate = "5m")
void everyFiveMinutes() {
    System.out.println("Executing everyFiveMinutes()");
}
Fixed Rate Example
@Scheduled(fixedRate = "5m")
void everyFiveMinutes() {
    println "Executing everyFiveMinutes()"
}
Fixed Rate Example
@Scheduled(fixedRate = "5m")
internal fun everyFiveMinutes() {
    println("Executing everyFiveMinutes()")
}

The task above executes every five minutes.

Scheduling with a Fixed Delay

To schedule a task, so it runs five minutes after the termination of the previous task use the fixedDelay member. For example:

Fixed Delay Example
@Scheduled(fixedDelay = "5m")
void fiveMinutesAfterLastExecution() {
    System.out.println("Executing fiveMinutesAfterLastExecution()");
}
Fixed Delay Example
@Scheduled(fixedDelay = "5m")
void fiveMinutesAfterLastExecution() {
    println "Executing fiveMinutesAfterLastExecution()"
}
Fixed Delay Example
@Scheduled(fixedDelay = "5m")
internal fun fiveMinutesAfterLastExecution() {
    println("Executing fiveMinutesAfterLastExecution()")
}

Scheduling a Cron Task

To schedule a Cron task use the cron member:

Cron Example
@Scheduled(cron = "0 15 10 ? * MON")
void everyMondayAtTenFifteenAm() {
    System.out.println("Executing everyMondayAtTenFifteenAm()");
}
Cron Example
@Scheduled(cron = "0 15 10 ? * MON")
void everyMondayAtTenFifteenAm() {
    println "Executing everyMondayAtTenFifteenAm()"
}
Cron Example
@Scheduled(cron = "0 15 10 ? * MON")
internal fun everyMondayAtTenFifteenAm() {
    println("Executing everyMondayAtTenFifteenAm()")
}

The above example runs the task every Monday morning at 10:15AM in the time zone of the server.

Scheduling with only an Initial Delay

To schedule a task, so it runs once after the server starts, use the initialDelay member:

Initial Delay Example
@Scheduled(initialDelay = "1m")
void onceOneMinuteAfterStartup() {
    System.out.println("Executing onceOneMinuteAfterStartup()");
}
Initial Delay Example
@Scheduled(initialDelay = "1m")
void onceOneMinuteAfterStartup() {
    println "Executing onceOneMinuteAfterStartup()"
}
Initial Delay Example
@Scheduled(initialDelay = "1m")
internal fun onceOneMinuteAfterStartup() {
    println("Executing onceOneMinuteAfterStartup()")
}

The above example only runs once, one minute after the server starts.

Programmatically Scheduling Tasks

To programmatically schedule tasks, use the TaskScheduler bean which can be injected as follows:

@Inject
@Named(TaskExecutors.SCHEDULED)
TaskScheduler taskScheduler;
@Inject
@Named(TaskExecutors.SCHEDULED)
TaskScheduler taskScheduler
@Inject
@Named(TaskExecutors.SCHEDULED)
lateinit var taskScheduler: TaskScheduler

Configuring Scheduled Tasks with Annotation Metadata

To make your application’s tasks configurable, you can use annotation metadata and property placeholder configuration. For example:

Allow tasks to be configured
@Scheduled(fixedRate = "${my.task.rate:5m}",
        initialDelay = "${my.task.delay:1m}")
void configuredTask() {
    System.out.println("Executing configuredTask()");
}
Allow tasks to be configured
@Scheduled(fixedRate = '${my.task.rate:5m}',
        initialDelay = '${my.task.delay:1m}')
void configuredTask() {
    println "Executing configuredTask()"
}
Allow tasks to be configured
@Scheduled(fixedRate = "\${my.task.rate:5m}",
        initialDelay = "\${my.task.delay:1m}")
internal fun configuredTask() {
    println("Executing configuredTask()")
}

The above example allows the task execution frequency to be configured with the property my.task.rate, and the initial delay to be configured with the property my.task.delay.

Configuring the Scheduled Task Thread Pool

Tasks executed by @Scheduled are run by default on a ScheduledExecutorService configured to have twice the number of threads as available processors.

You can configure this thread pool in your configuration file (e.g application.yml):

Configuring Scheduled Task Thread Pool
micronaut.executors.scheduled.type=scheduled
micronaut.executors.scheduled.core-pool-size=30
micronaut:
  executors:
    scheduled:
      type: scheduled
      core-pool-size: 30
[micronaut]
  [micronaut.executors]
    [micronaut.executors.scheduled]
      type="scheduled"
      core-pool-size=30
micronaut {
  executors {
    scheduled {
      type = "scheduled"
      corePoolSize = 30
    }
  }
}
{
  micronaut {
    executors {
      scheduled {
        type = "scheduled"
        core-pool-size = 30
      }
    }
  }
}
{
  "micronaut": {
    "executors": {
      "scheduled": {
        "type": "scheduled",
        "core-pool-size": 30
      }
    }
  }
}
🔗
Table 1. Configuration Properties for UserExecutorConfiguration
Property Type Description

micronaut.executors.*.n-threads

java.lang.Integer

number of threads

micronaut.executors.*.type

ExecutorType

the type

micronaut.executors.*.parallelism

java.lang.Integer

the parallelism

micronaut.executors.*.core-pool-size

java.lang.Integer

the core pool size

micronaut.executors.*.virtual

java.lang.Boolean

whether to use virtual threads

micronaut.executors.*.thread-factory-class

java.lang.Class

the thread factory class

micronaut.executors.*.name

java.lang.String

Sets the executor name.

micronaut.executors.*.number-of-threads

java.lang.Integer

Sets the number of threads for FIXED. Default value (2 * Number of processors available to the Java virtual machine).

Handling Exceptions

By default, the Micronaut framework includes a DefaultTaskExceptionHandler bean that implements the TaskExceptionHandler interface and simply logs the exception if an error occurs invoking a scheduled task.

If you have custom requirements you can replace this bean with your own implementation (for example to send an email or shutdown the context to fail fast). To do so, write your own TaskExceptionHandler and annotate it with @Replaces(DefaultTaskExceptionHandler.class).

5.9 Bridging Spring AOP

Although the Micronaut framework’s design is based on a compile-time approach and does not rely on Spring dependency injection, there is still a lot of value in the Spring ecosystem that does not depend directly on the Spring container.

You may wish to use existing Spring projects within the Micronaut framework and configure beans to be used within the Micronaut framework.

You may also wish to leverage existing AOP advice from Spring. One example of this is Spring’s support for declarative transactions with @Transactional.

The Micronaut framework provides support for Spring-based transaction management without requiring Spring itself. Simply add the spring module to your application dependencies:

implementation("io.micronaut.spring:micronaut-spring")
<dependency>
    <groupId>io.micronaut.spring</groupId>
    <artifactId>micronaut-spring</artifactId>
</dependency>

This also requires adding the spring-annotation module dependency as an annotation processor:

annotationProcessor("io.micronaut.spring:micronaut-spring-annotation")
<annotationProcessorPaths>
    <path>
        <groupId>io.micronaut.spring</groupId>
        <artifactId>micronaut-spring-annotation</artifactId>
    </path>
</annotationProcessorPaths>

If you use Micronaut’s Hibernate support you already get this dependency and a HibernateTransactionManager is configured for you.

This is done by intercepting method calls annotated with Spring’s @Transactional with TransactionInterceptor.

The benefit here is you can use Micronaut’s compile-time, reflection-free AOP to declare programmatic Spring transactions. For example:

Using @Transactional
import org.springframework.transaction.annotation.Transactional;
...

@Transactional
public Book saveBook(String title) {
    ...
}

6 The HTTP Server

Using the CLI

If you create your project using the Micronaut CLI create-app command, the http-server dependency is included by default.

Micronaut framework includes both non-blocking HTTP server and client APIs based on Netty.

The goal of the HTTP server is to make it as easy as possible to expose services to be consumed by HTTP clients and web pages, regardless of the language they are written in. There is support for rendering HTML from a variety of templating frameworks in the Micronaut Views module. To use the HTTP server you need the http-server-netty dependency in your build:

implementation("io.micronaut:micronaut-http-server-netty")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-http-server-netty</artifactId>
</dependency>

A "Hello World" server application can be seen below:

import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;

@Controller("/hello") // (1)
public class HelloController {

    @Get(produces = MediaType.TEXT_PLAIN) // (2)
    public String index() {
        return "Hello World"; // (3)
    }
}
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller('/hello') // (1)
class HelloController {

    @Get(produces = MediaType.TEXT_PLAIN) // (2)
    String index() {
        'Hello World' // (3)
    }
}
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/hello") // (1)
class HelloController {

    @Get(produces = [MediaType.TEXT_PLAIN]) // (2)
    fun index(): String {
        return "Hello World" // (3)
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /hello
2 The method responds to a GET requests to /hello and returns a response with a text/plain content type
3 By defining a method named index, by convention the method is exposed via the /hello URI

6.1 Running the Embedded Server

To run the server, create an Application class with a static void main method, for example:

Micronaut Application Class
import io.micronaut.runtime.Micronaut;

public class Application {

    public static void main(String[] args) {
        Micronaut.run(Application.class);
    }
}
Micronaut Application Class
import io.micronaut.runtime.Micronaut

class Application {

    static void main(String... args) {
        Micronaut.run Application
    }
}
Micronaut Application Class
import io.micronaut.runtime.Micronaut

object Application {

    @JvmStatic
    fun main(args: Array<String>) {
        Micronaut.run(Application.javaClass)
    }
}

To run the application from a unit test, use the EmbeddedServer interface:

Micronaut Test Case
import io.micronaut.http.HttpRequest;
import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.runtime.server.EmbeddedServer;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;

import jakarta.inject.Inject;

import static org.junit.jupiter.api.Assertions.assertEquals;

@MicronautTest
public class HelloControllerSpec {

    @Inject
    EmbeddedServer server; // (1)

    @Inject
    @Client("/")
    HttpClient client; // (2)

    @Test
    void testHelloWorldResponse() {
        String response = client.toBlocking() // (3)
                .retrieve(HttpRequest.GET("/hello"));
        assertEquals("Hello World", response); // (4)
    }
}
Micronaut Test Case
import io.micronaut.http.HttpRequest
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import spock.lang.Specification

import jakarta.inject.Inject

@MicronautTest
class HelloControllerSpec extends Specification {

    @Inject
    EmbeddedServer embeddedServer // (1)

    @Inject
    @Client("/")
    HttpClient client // (2)

    void "test hello world response"() {
        expect:
            client.toBlocking() // (3)
                    .retrieve(HttpRequest.GET('/hello')) == "Hello World" // (4)
    }
}
Micronaut Test Case
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import jakarta.inject.Inject

@MicronautTest
class HelloControllerSpec {

    @Inject
    lateinit var server: EmbeddedServer // (1)

    @Inject
    @field:Client("/")
    lateinit var client: HttpClient // (2)

    @Test
    fun testHelloWorldResponse() {
        val rsp: String = client.toBlocking() // (3)
                .retrieve("/hello")
        assertEquals("Hello World", rsp) // (4)
    }
}
1 The EmbeddedServer is run and the Spock @AutoCleanup annotation ensures the server is stopped after the specification completes.
2 The EmbeddedServer interface provides the URL of the server under test which runs on a random port.
3 The test uses the Micronaut HTTP client to make the call
4 The retrieve method returns the response of the controller as a String
Without explicit port configuration, the port will be 8080, unless the application is run under the test environment where the port is random. When the application context starts from the context of a test class, the test environment is added automatically.

6.2 Running Server on a Specific Port

By default, the server runs on port 8080. However, you can set the server to run on a specific port:

micronaut.server.port=8086
micronaut:
  server:
    port: 8086
[micronaut]
  [micronaut.server]
    port=8086
micronaut {
  server {
    port = 8086
  }
}
{
  micronaut {
    server {
      port = 8086
    }
  }
}
{
  "micronaut": {
    "server": {
      "port": 8086
    }
  }
}
This is also configurable from an environment variable, e.g. MICRONAUT_SERVER_PORT=8086

To run on a random port:

micronaut.server.port=-1
micronaut:
  server:
    port: -1
[micronaut]
  [micronaut.server]
    port=-1
micronaut {
  server {
    port = -1
  }
}
{
  micronaut {
    server {
      port = -1
    }
  }
}
{
  "micronaut": {
    "server": {
      "port": -1
    }
  }
}
Setting an explicit port may cause tests to fail if multiple servers start simultaneously on the same port. To prevent that, specify a random port in the test environment configuration.

6.3 HTTP Routing

The @Controller annotation used in the previous section is one of several annotations that allow you to control the construction of HTTP routes.

URI Paths

The value of the @Controller annotation is an RFC-6570 URI template, so you can embed URI variables within the path using the syntax defined by the URI template specification.

Many other frameworks, including Spring, implement the URI template specification

The actual implementation is handled by the UriMatchTemplate class, which extends UriTemplate.

You can use this class in your applications to build URIs, for example:

Using a UriTemplate
UriMatchTemplate template = UriMatchTemplate.of("/hello/{name}");

assertTrue(template.match("/hello/John").isPresent()); // (1)
assertEquals("/hello/John", template.expand( // (2)
        Collections.singletonMap("name", "John")
));
Using a UriTemplate
given:
UriMatchTemplate template = UriMatchTemplate.of("/hello/{name}")

expect:
template.match("/hello/John").isPresent() // (1)
template.expand(["name": "John"]) == "/hello/John" // (2)
Using a UriTemplate
val template = UriMatchTemplate.of("/hello/{name}")

template.match("/hello/John").isPresent.shouldBeTrue() // (1)
template.expand(mapOf("name" to "John")) shouldBe "/hello/John"  // (2)
1 Use the match method to match a path
2 Use the expand method to expand a template into a URI

You can use UriTemplate to build paths to include in your responses.

URI Path Variables

URI variables can be referenced via method arguments. When the path variable matches the method argument name, they are bound together automatically. If you want to use different names or specify a default value for a missing URI Variable, the PathVariable annotation can be used. The following example illustrates these options:

URI Variables Example
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.annotation.PathVariable;

@Controller("/issues") // (1)
public class IssuesController {

    @Get("/{number}") // (2)
    public String issue(Integer number) { // (3)
        return "Issue # " + number + "!"; // (4)
    }

    @Get("/issue/{number}")
    public String issueFromId(@PathVariable("number") Integer id) { // (5)
        return "Issue # " + id + "!";
    }

}
URI Variables Example
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.PathVariable

@Controller("/issues") // (1)
class IssuesController {

    @Get("/{number}") // (2)
    String issue(Integer number) { // (3)
        "Issue # " + number + "!" // (4)
    }

    @Get("/issue/{number}")
    String issueFromId(@PathVariable("number") Integer id) { // (5)
        "Issue # " + id + "!"
    }

}
URI Variables Example
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.PathVariable

@Controller("/issues") // (1)
class IssuesController {

    @Get("/{number}") // (2)
    fun issue(number: Int): String { // (3)
        return "Issue # $number!" // (4)
    }

    @Get("/issue/{number}")
    fun issueById(@PathVariable("number") id: Int): String {
        return "Issue # $id!" // (5)
    }

}
1 The @Controller annotation is specified with a base URI of /issues
2 The Get annotation maps the method to an HTTP GET with a URI variable embedded in the URI named number
3 The method argument number is bound automatically to the path variable {number} because the names match
4 The value of the URI variable is referenced in the implementation
5 The method argument requires the PathVariable annotation when method argument and path variable names don’t match

The Micronaut framework maps the URI /issues/{number} for the above controller. We can assert this is the case by writing unit tests:

Testing URI Variables
import io.micronaut.context.ApplicationContext;
import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.exceptions.HttpClientResponseException;
import io.micronaut.runtime.server.EmbeddedServer;
import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertNotNull;
import static org.junit.jupiter.api.Assertions.assertThrows;

class IssuesControllerTest {

    private static EmbeddedServer server;
    private static HttpClient client;

    @BeforeAll // (1)
    static void setupServer() {
        server = ApplicationContext.run(EmbeddedServer.class);
        client = server
                    .getApplicationContext()
                    .createBean(HttpClient.class, server.getURL());
    }

    @AfterAll // (2)
    static void stopServer() {
        if (server != null) {
            server.stop();
        }
        if (client != null) {
            client.stop();
        }
    }

    @Test
    void testIssue() {
        String body = client.toBlocking().retrieve("/issues/12"); // (3)

        assertNotNull(body);
        assertEquals("Issue # 12!", body); // (4)
    }

    @Test
    void testIssueFromId() {
        String body = client.toBlocking().retrieve("/issues/issue/13");

        assertNotNull(body);
        assertEquals("Issue # 13!", body); // (5)
    }

    @Test
    void testShowWithInvalidInteger() {
        HttpClientResponseException e = assertThrows(HttpClientResponseException.class, () ->
                client.toBlocking().exchange("/issues/hello"));

        assertEquals(400, e.getStatus().getCode()); // (6)
    }

    @Test
    void testIssueWithoutNumber() {
        HttpClientResponseException e = assertThrows(HttpClientResponseException.class, () ->
                client.toBlocking().exchange("/issues/"));

        assertEquals(404, e.getStatus().getCode()); // (7)
    }

}
Testing URI Variables
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.exceptions.HttpClientResponseException
import io.micronaut.runtime.server.EmbeddedServer
import spock.lang.AutoCleanup
import spock.lang.Shared
import spock.lang.Specification


class IssuesControllerTest extends Specification {

    @Shared
    @AutoCleanup // (2)
    EmbeddedServer embeddedServer = ApplicationContext.run(EmbeddedServer) // (1)

    @Shared
    @AutoCleanup // (2)
    HttpClient client = HttpClient.create(embeddedServer.URL) // (1)

    void "test issue"() {
        when:
        String body = client.toBlocking().retrieve("/issues/12") // (3)

        then:
        body != null
        body == "Issue # 12!" // (4)
    }

    void "test issue from id"() {
        when:
        String body = client.toBlocking().retrieve("/issues/issue/13")

        then:
        body != null
        body == "Issue # 13!" // (5)
    }

    void "/issues/{number} with an invalid Integer number responds 400"() {
        when:
        client.toBlocking().exchange("/issues/hello")

        then:
        def e = thrown(HttpClientResponseException)
        e.status.code == 400 // (6)
    }

    void "/issues/{number} without number responds 404"() {
        when:
        client.toBlocking().exchange("/issues/")

        then:
        def e = thrown(HttpClientResponseException)
        e.status.code == 404 // (7)
    }

}
Testing URI Variables
import io.kotest.matchers.shouldBe
import io.kotest.matchers.shouldNotBe
import io.kotest.assertions.throwables.shouldThrow
import io.kotest.core.spec.style.StringSpec
import io.micronaut.context.ApplicationContext
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.exceptions.HttpClientResponseException
import io.micronaut.runtime.server.EmbeddedServer

class IssuesControllerTest: StringSpec() {

    val embeddedServer = autoClose( // (2)
        ApplicationContext.run(EmbeddedServer::class.java) // (1)
    )

    val client = autoClose( // (2)
        embeddedServer.applicationContext.createBean(
            HttpClient::class.java,
            embeddedServer.url) // (1)
    )

    init {
        "test issue" {
            val body = client.toBlocking().retrieve("/issues/12") // (3)

            body shouldNotBe null
            body shouldBe "Issue # 12!" // (4)
        }

        "test issue from id" {
            val body = client.toBlocking().retrieve("/issues/issue/13")

            body shouldNotBe null
            body shouldBe "Issue # 13!" // (5)
        }

        "test issue with invalid integer" {
            val e = shouldThrow<HttpClientResponseException> {
                client.toBlocking().exchange<Any>("/issues/hello")
            }

            e.status.code shouldBe 400 // (6)
        }

        "test issue without number" {
            val e = shouldThrow<HttpClientResponseException> {
                client.toBlocking().exchange<Any>("/issues/")
            }

            e.status.code shouldBe 404 // (7)
        }

    }
}
1 The embedded server and HTTP client are started
2 The server and client are cleaned up after the tests finish
3 The tests send a request to the URI /issues/12
4 And then asserts the response is "Issue # 12"
5 Another test using the end point defined with @PathVariable asserts the response is "Issue # 13"
6 Another test asserts a 400 response is returned when an invalid number is sent in the URL
7 Another test asserts a 404 response is returned when no number is provided in the URL. The variable being present is required for the route to be executed.

Note that the URI template in the previous example requires that the number variable is specified. You can specify optional URI templates with the syntax: /issues{/number} and by annotating the number parameter with @Nullable. Alternatively, you can use the defaultValue element of the PathVariable annotation to specify a default value when the URI variable is missing. For example:

@Get("/default{/number}") // (1)
public String issueFromIdOrDefault(@PathVariable(defaultValue = "0") Integer number) { // (2)
    return "Issue # " + number + "!";
}
@Get("/default{/number}") // (1)
String issueFromIdOrDefault(@PathVariable(defaultValue = "0") Integer number) { // (2)
    "Issue # " + number + "!"
}
@Get("/default{/number}") // (1)
fun issueFromIdOrDefault(@PathVariable(defaultValue = "0") number: Int): String { // (2)
    return "Issue # $number!"
}
1 The forward slash inside the braces designates number as an optional URI variable
2 The defaultValue attribute specifies the default value for number when the URI variable is missing
@Test
void testDefaultIssue() {
    String body = client.toBlocking().retrieve("/issues/default");

    assertNotNull(body);
    assertEquals("Issue # 0!", body); // (1)
}

@Test
void testNotDefaultIssue() {
    String body = client.toBlocking().retrieve("/issues/default/1");

    assertNotNull(body);
    assertEquals("Issue # 1!", body); // (2)
}
void "test default issue"() {
    when:
    String body = client.toBlocking().retrieve("/issues/default")

    then:
    body != null
    body == "Issue # 0!" // (1)
}

void "test not default issue"() {
    when:
    String body = client.toBlocking().retrieve("/issues/default/1")

    then:
    body != null
    body == "Issue # 1!" // (2)
}
"test issue from id" {
    val body = client.toBlocking().retrieve("/issues/default")

    body shouldNotBe null
    body shouldBe "Issue # 0!" // (1)
}

"test issue from id" {
    val body = client.toBlocking().retrieve("/issues/default/1")

    body shouldNotBe null
    body shouldBe "Issue # 1!" // (2)
}
1 This test illustrates the substitution of a default `PathVariable' value when the URI variable is missing
2 And another test to illustrate when the optional URI variable is provided

The following table provides examples of URI templates and what they match:

Table 1. URI Template Matching
Template Description Matching URI

/books/{id}

Simple match

/books/1

/books/{id:2}

A variable of two characters max

/books/10

/books{/id}

An optional URI variable

/books/10 or /books

/book{/id:[a-zA-Z]+}

An optional URI variable with regex

/books/foo

/books{?max,offset}

Optional query parameters

/books?max=10&offset=10

/books{/path:.*}{.ext}

Regex path match with extension

/books/foo/bar.xml

URI Reserved Character Matching

By default, URI variables as defined by the RFC-6570 URI template spec cannot include reserved characters such as /, ? etc.

This can be problematic if you wish to match or expand entire paths. As per section 3.2.3 of the specification, you can use reserved expansion or matching using the + operator.

For example the URI /books/{+path} matches both /books/foo and /books/foo/bar since the + indicates that the variable path should include reserved characters (in this case /).

Routing Annotations

The previous example uses the @Get annotation to add a method that accepts HTTP GET requests. The following table summarizes the available annotations and how they map to HTTP methods:

Table 2. HTTP Routing Annotations
Annotation HTTP Method

@Delete

DELETE

@Get

GET

@Head

HEAD

@Options

OPTIONS

@Patch

PATCH

@Put

PUT

@Post

POST

@Trace

TRACE

All the method annotations default to /.

@Options

CORS support handles OPTIONS preflight requests. However, if you want to dispatch OPTIONS requests without an Origin HTTP Header, you can enable it via:

micronaut.server.dispatch-options-requests=true
micronaut:
  server:
    dispatch-options-requests: true
[micronaut]
  [micronaut.server]
    dispatch-options-requests=true
micronaut {
  server {
    dispatchOptionsRequests = true
  }
}
{
  micronaut {
    server {
      dispatch-options-requests = true
    }
  }
}
{
  "micronaut": {
    "server": {
      "dispatch-options-requests": true
    }
  }
}

Multiple URIs

Each of the routing annotations supports multiple URI templates. For each template, a route is created. This feature is useful for example to change the path of the API and leave the existing path as is for backwards compatibility. For example:

Multiple URIs
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;

@Controller("/hello")
public class BackwardCompatibleController {

    @Get(uris = {"/{name}", "/person/{name}"}) // (1)
    public String hello(String name) { // (2)
        return "Hello, " + name;
    }
}
Multiple URIs
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/hello")
class BackwardCompatibleController {

    @Get(uris = ["/{name}", "/person/{name}"]) // (1)
    String hello(String name) { // (2)
        "Hello, $name"
    }
}
Multiple URIs
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/hello")
class BackwardCompatibleController {

    @Get(uris = ["/{name}", "/person/{name}"]) // (1)
    fun hello(name: String): String { // (2)
        return "Hello, $name"
    }
}
1 Specify multiple templates
2 Bind to the template arguments as normal
Route validation is more complicated with multiple templates. If a variable that would normally be required does not exist in all templates, that variable is considered optional since it may not exist for every execution of the method.

Building Routes Programmatically

If you prefer to not use annotations and instead declare all routes in code then never fear, the Micronaut framework has a flexible RouteBuilder API that makes it a breeze to define routes programmatically.

To start, subclass DefaultRouteBuilder and inject the controller to route to into the method, and define your routes:

URI Variables Example
import io.micronaut.context.ExecutionHandleLocator;
import io.micronaut.web.router.DefaultRouteBuilder;

import jakarta.inject.Inject;
import jakarta.inject.Singleton;

@Singleton
public class MyRoutes extends DefaultRouteBuilder { // (1)

    public MyRoutes(ExecutionHandleLocator executionHandleLocator,
                    UriNamingStrategy uriNamingStrategy) {
        super(executionHandleLocator, uriNamingStrategy);
    }

    @Inject
    void issuesRoutes(IssuesController issuesController) { // (2)
        GET("/issues/show/{number}", issuesController, "issue", Integer.class); // (3)
    }
}
URI Variables Example
import io.micronaut.context.ExecutionHandleLocator
import io.micronaut.core.convert.ConversionService
import io.micronaut.web.router.GroovyRouteBuilder

import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class MyRoutes extends GroovyRouteBuilder { // (1)

    MyRoutes(ExecutionHandleLocator executionHandleLocator,
             UriNamingStrategy uriNamingStrategy,
             ConversionService conversionService) {
        super(executionHandleLocator, uriNamingStrategy, conversionService)
    }

    @Inject
    void issuesRoutes(IssuesController issuesController) { // (2)
        GET("/issues/show/{number}", issuesController.&issue) // (3)
    }
}
URI Variables Example
import io.micronaut.context.ExecutionHandleLocator
import io.micronaut.web.router.DefaultRouteBuilder
import io.micronaut.web.router.RouteBuilder
import jakarta.inject.Inject
import jakarta.inject.Singleton

@Singleton
class MyRoutes(executionHandleLocator: ExecutionHandleLocator,
               uriNamingStrategy: RouteBuilder.UriNamingStrategy) :
        DefaultRouteBuilder(executionHandleLocator, uriNamingStrategy) { // (1)

    @Inject
    fun issuesRoutes(issuesController: IssuesController) { // (2)
        GET("/issues/show/{number}", issuesController, "issue", Int::class.java) // (3)
    }
}
1 Route definitions should subclass DefaultRouteBuilder
2 Use @Inject to inject a method with the controller to route to
3 Use methods such as RouteBuilder::GET(String,Class,String,Class…​) to route to controller methods. Note that even though the issues controller is used, the route has no knowledge of its @Controller annotation and thus the full path must be specified.
Unfortunately due to type erasure, a Java method lambda reference cannot be used with the API. For Groovy there is a GroovyRouteBuilder class which can be subclassed that allows passing Groovy method references.

Route Compile-Time Validation

The Micronaut framework supports validating route arguments at compile time with the validation library. To get started, add the micronaut-http-validation dependency to your build:

annotationProcessor("io.micronaut:micronaut-http-validation")
<annotationProcessorPaths>
    <path>
        <groupId>io.micronaut</groupId>
        <artifactId>micronaut-http-validation</artifactId>
    </path>
</annotationProcessorPaths>

With the correct dependency on your classpath, route arguments will automatically be checked at compile time. Compilation will fail if any of the following conditions are met:

  • The URI template contains a variable that is optional, but the method parameter is not annotated with @Nullable or is an java.util.Optional.

An optional variable is one that allows the route to match a URI even if the value is not present. For example /foo{/bar} matches requests to /foo and /foo/abc. The non-optional variant would be /foo/{bar}. See the URI Path Variables section for more information.

  • The URI template contains a variable that is missing from the method arguments.

To disable route compile-time validation, set the system property -Dmicronaut.route.validation=false. For Java and Kotlin users using Gradle, the same effect can be achieved by removing the micronaut-http-validation dependency from the annotationProcessor/kapt scope.

Routing non-standard HTTP methods

The @CustomHttpMethod annotation supports non-standard HTTP methods for a client or server. Specifications like RFC-4918 Webdav require additional methods like REPORT or LOCK for example.

RoutingExample
@CustomHttpMethod(method = "LOCK", value = "/{name}")
String lock(String name)

The annotation can be used anywhere the standard method annotations can be used, including controllers and declarative HTTP clients.

RouteMatch

The RouteMatch API provides information about an executable Route.

Given a request you can retrieve a RouteMatch with:

String index(HttpRequest<?> request) {
    RouteMatch<?> routeMatch = request.getAttribute(HttpAttributes.ROUTE_MATCH, RouteMatch.class)
            .orElse(null);
String index(HttpRequest<?> request) {
    RouteMatch<?> routeMatch = request.getAttribute(HttpAttributes.ROUTE_MATCH, RouteMatch.class)
            .orElse(null)
fun index(request: HttpRequest<*>): String? {
    val routeMatch = request.getAttribute(HttpAttributes.ROUTE_MATCH, RouteMatch::class.java)
        .orElse(null)

6.4 Simple Request Binding

The examples in the previous section demonstrate how the Micronaut framework lets you bind method parameters from URI path variables. This section shows how to bind arguments from other parts of the request.

Binding Annotations

All binding annotations support customization of the name of the variable being bound from with their name member.

The following table summarizes the annotations and their purpose, and provides examples:

Table 1. Parameter Binding Annotations
Annotation Description Example

@Body

Binds from the body of the request

@Body String body

@CookieValue

Binds a parameter from a cookie

@CookieValue String myCookie

@Header

Binds a parameter from an HTTP header

@Header String requestId

@QueryValue

Binds from a request query parameter

@QueryValue String myParam

@Part

Binds from a part of a multipart request

@Part CompletedFileUpload file

@RequestAttribute

Binds from an attribute of the request. Attributes are typically created in filters

@RequestAttribute String myAttribute

@PathVariable

Binds from the path of the request

@PathVariable String id

@RequestBean

Binds any Bindable value to single Bean object

@RequestBean MyBean bean

The method parameter name is used when a value is not specified in a binding annotation. In other words the following two methods are equivalent and both bind from a cookie named myCookie:

@Get("/cookieName")
public String cookieName(@CookieValue("myCookie") String myCookie) {
    // ...
}

@Get("/cookieInferred")
public String cookieInferred(@CookieValue String myCookie) {
    // ...
}
@Get("/cookieName")
String cookieName(@CookieValue("myCookie") String myCookie) {
    // ...
}

@Get("/cookieInferred")
String cookieInferred(@CookieValue String myCookie) {
    // ...
}
@Get("/cookieName")
fun cookieName(@CookieValue("myCookie") myCookie: String): String {
    // ...
}

@Get("/cookieInferred")
fun cookieInferred(@CookieValue myCookie: String): String {
    // ...
}

Because hyphens are not allowed in variable names, it may be necessary to set the name in the annotation. The following definitions are equivalent:

@Get("/headerName")
public String headerName(@Header("Content-Type") String contentType) {
    // ...
}

@Get("/headerInferred")
public String headerInferred(@Header String contentType) {
    // ...
}
@Get("/headerName")
String headerName(@Header("Content-Type") String contentType) {
    // ...
}

@Get("/headerInferred")
String headerInferred(@Header String contentType) {
    // ...
}
@Get("/headerName")
fun headerName(@Header("Content-Type") contentType: String): String {
    // ...
}

@Get("/headerInferred")
fun headerInferred(@Header contentType: String): String {
    // ...
}

Stream Support

The Micronaut framework also supports binding the body to an InputStream. If the method is reading the stream, the method execution must be offloaded to another thread pool to avoid blocking the event loop.

Performing Blocking I/O With InputStream
@Post(value = "/read", processes = MediaType.TEXT_PLAIN)
@ExecuteOn(TaskExecutors.IO) // (1)
String read(@Body InputStream inputStream) throws IOException { // (2)
    return IOUtils.readText(new BufferedReader(new InputStreamReader(inputStream))); // (3)
}
Performing Blocking I/O With InputStream
@Post(value = "/read", processes = MediaType.TEXT_PLAIN)
@ExecuteOn(TaskExecutors.IO) // (1)
String read(@Body InputStream inputStream) throws IOException { // (2)
    IOUtils.readText(new BufferedReader(new InputStreamReader(inputStream))) // (3)
}
Performing Blocking I/O With InputStream
@Post(value = "/read", processes = [MediaType.TEXT_PLAIN])
@ExecuteOn(TaskExecutors.IO) // (1)
fun read(@Body inputStream: InputStream): String { // (2)
    return IOUtils.readText(BufferedReader(InputStreamReader(inputStream))) // (3)
}
1 The controller method is executed on the IO thread pool
2 The body is passed to the method as an input stream
3 The stream is read

Binding from Multiple Query values

Instead of binding from a single section of the request, it may be desirable to bind all query values for example to a POJO. This can be achieved by using the exploded operator (?pojo*) in the URI template. For example:

Binding Request parameters to POJO
import io.micronaut.http.HttpStatus;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.core.annotation.Nullable;

import jakarta.validation.Valid;

@Controller("/api")
public class BookmarkController {

    @Get("/bookmarks/list{?paginationCommand*}")
    public HttpStatus list(@Valid @Nullable PaginationCommand paginationCommand) {
        return HttpStatus.OK;
    }
}
Binding Request parameters to POJO
import io.micronaut.http.HttpStatus
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

import jakarta.annotation.Nullable
import jakarta.validation.Valid

@Controller("/api")
class BookmarkController {

    @Get("/bookmarks/list{?paginationCommand*}")
    HttpStatus list(@Valid @Nullable PaginationCommand paginationCommand) {
        HttpStatus.OK
    }
}
Binding Request parameters to POJO
import io.micronaut.http.HttpStatus
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import jakarta.validation.Valid

@Controller("/api")
open class BookmarkController {

    @Get("/bookmarks/list{?paginationCommand*}")
    open fun list(@Valid paginationCommand: PaginationCommand): HttpStatus {
        return HttpStatus.OK
    }
}

Binding from Multiple Bindable values

Instead of binding just query values, it is also possible to bind any Bindable value to a POJO (e.g. to bind HttpRequest, @PathVariable, @QueryValue and @Header to a single POJO). This can be achieved with the @RequestBean annotation and a custom Bean class with fields with Bindable annotations, or fields that can be bound by type (e.g. HttpRequest, BasicAuth, Authentication, etc).

For example:

Binding Bindable values to POJO
@Controller("/api")
public class MovieTicketController {

    // You can also omit query parameters like:
    // @Get("/movie/ticket/{movieId}
    @Get("/movie/ticket/{movieId}{?minPrice,maxPrice}")
    public HttpStatus list(@Valid @RequestBean MovieTicketBean bean) {
        return HttpStatus.OK;
    }
}
Binding Bindable values to POJO
@Controller("/api")
class MovieTicketController {

    // You can also omit query parameters like:
    // @Get("/movie/ticket/{movieId}
    @Get("/movie/ticket/{movieId}{?minPrice,maxPrice}")
    HttpStatus list(@Valid @RequestBean MovieTicketBean bean) {
        HttpStatus.OK
    }
}
Binding Bindable values to POJO
@Controller("/api")
open class MovieTicketController {

    // You can also omit query parameters like:
    // @Get("/movie/ticket/{movieId}
    @Get("/movie/ticket/{movieId}{?minPrice,maxPrice}")
    open fun list(@Valid @RequestBean bean: MovieTicketBean): HttpStatus {
        return HttpStatus.OK
    }

}

which uses this bean class:

Bean definition
@Introspected
public class MovieTicketBean {

    private HttpRequest<?> httpRequest;

    @PathVariable
    private String movieId;

    @Nullable
    @QueryValue
    @PositiveOrZero
    private Double minPrice;

    @Nullable
    @QueryValue
    @PositiveOrZero
    private Double maxPrice;

    public MovieTicketBean(HttpRequest<?> httpRequest,
                           String movieId,
                           Double minPrice,
                           Double maxPrice) {
        this.httpRequest = httpRequest;
        this.movieId = movieId;
        this.minPrice = minPrice;
        this.maxPrice = maxPrice;
    }

    public HttpRequest<?> getHttpRequest() {
        return httpRequest;
    }

    public String getMovieId() {
        return movieId;
    }

    @Nullable
    public Double getMaxPrice() {
        return maxPrice;
    }

    @Nullable
    public Double getMinPrice() {
        return minPrice;
    }
}
Bean definition
@Introspected
class MovieTicketBean {

    private HttpRequest<?> httpRequest

    @PathVariable
    String movieId

    @Nullable
    @QueryValue
    @PositiveOrZero
    Double minPrice

    @Nullable
    @QueryValue
    @PositiveOrZero
    Double maxPrice
}
Bean definition
@Introspected
data class MovieTicketBean(
    val httpRequest: HttpRequest<Any>,
    @field:PathVariable val movieId: String,
    @field:QueryValue @field:PositiveOrZero @field:Nullable val minPrice: Double,
    @field:QueryValue @field:PositiveOrZero @field:Nullable val maxPrice: Double
)

The bean class has to be introspected with @Introspected. It can be one of:

  1. Mutable Bean class with setters and getters

  2. Immutable Bean class with getters and an all-argument constructor (or @Creator annotation on a constructor or static method). Arguments of the constructor must match field names so the object can be instantiated without reflection.

Since Java does not retain argument names in bytecode, you must compile code with -parameters to use an immutable bean class from another jar. Another option is to extend Bean class in your source.

Bindable Types

Generally any type that can be converted from a String representation to a Java type via the ConversionService API can be bound to.

This includes most common Java types. However, you can simply add additional TypeConverter either by defining it as a bean or by registering it in a TypeConverterRegistrar via the service loader.

The handling of nullability deserves special mention. Consider for example the following example:

@Get("/headerInferred")
public String headerInferred(@Header String contentType) {
    // ...
}
@Get("/headerInferred")
String headerInferred(@Header String contentType) {
    // ...
}
@Get("/headerInferred")
fun headerInferred(@Header contentType: String): String {
    // ...
}

In this case, if the HTTP header Content-Type is not present in the request, the route is considered invalid, since it cannot be satisfied, and an HTTP 400 BAD REQUEST is returned.

To make the Content-Type header optional, you can instead write:

@Get("/headerNullable")
public String headerNullable(@Nullable @Header String contentType) {
    // ...
}
@Get("/headerNullable")
String headerNullable(@Nullable @Header String contentType) {
    // ...
}
@Get("/headerNullable")
fun headerNullable(@Header contentType: String?): String? {
    // ...
}

A null string is passed if the header is absent from the request.

java.util.Optional can also be used, but that is discouraged for method parameters.

Additionally, any DateTime that conforms to RFC-1123 can be bound to a parameter. Alternatively the format can be customized with the Format annotation:

@Get("/date")
public String date(@Header ZonedDateTime date) {
    // ...
}

@Get("/dateFormat")
public String dateFormat(@Format("dd/MM/yyyy hh:mm:ss a z") @Header ZonedDateTime date) {
    // ...
}
@Get("/date")
String date(@Header ZonedDateTime date) {
    // ...
}

@Get("/dateFormat")
String dateFormat(@Format("dd/MM/yyyy hh:mm:ss a z") @Header ZonedDateTime date) {
    // ...
}
@Get("/date")
fun date(@Header date: ZonedDateTime): String {
    // ...
}

@Get("/dateFormat")
fun dateFormat(@Format("dd/MM/yyyy hh:mm:ss a z") @Header date: ZonedDateTime): String {
    // ...
}

Type-Based Binding Parameters

Some parameters are recognized by their type instead of their annotation. The following table summarizes the parameter types, their purpose, and provides an example:

Type Description Example

BasicAuth

Allows binding of basic authorization credentials

BasicAuth basicAuth

Variable resolution

The Micronaut framework tries to populate method arguments in the following order:

  1. URI variables like /{id}.

  2. From query parameters if the request is a GET request (e.g. ?foo=bar).

  3. If there is a @Body and request allows the body, bind the body to it.

  4. If the request can have a body and no @Body is defined then try to parse the body (either JSON or form data) and bind the method arguments from the body (see the example).

  5. Finally, if the method arguments cannot be populated return 400 BAD REQUEST.

Binding Method Arguments From Body with no @Body
@Controller("/point")
public class PointController {

    @Post(uri = "/no-body-json")
    @Status(HttpStatus.CREATED)
    Point noBodyJson(Integer x, Integer y) { // (1)
        return new Point(x,y);
    }

    @Consumes(MediaType.APPLICATION_FORM_URLENCODED)
    @Post("/no-body-form")
    @Status(HttpStatus.CREATED)
    Point noBodyForm(Integer x, Integer y) {  // (2)
        return new Point(x,y);
    }
}
Binding Method Arguments From Body with no @Body
@Controller("/point")
class PointController {

    @Post(uri = "/no-body-json")
    @Status(HttpStatus.CREATED)
    Point noBodyJson(Integer x, Integer y) { // (1)
        new Point(x,y)
    }

    @Consumes(MediaType.APPLICATION_FORM_URLENCODED)
    @Post("/no-body-form")
    @Status(HttpStatus.CREATED)
    Point noBodyForm(Integer x, Integer y) {  // (2)
        new Point(x,y)
    }
}
Binding Method Arguments From Body with no @Body
@Controller("/point")
class PointController {

    @Post(uri = "/no-body-json")
    @Status(HttpStatus.CREATED)
    fun noBodyJson(x: Int, y: Int) = Point(x,y) // (1)

    @Consumes(MediaType.APPLICATION_FORM_URLENCODED)
    @Post("/no-body-form")
    @Status(HttpStatus.CREATED)
    fun noBodyForm(x: Int, y: Int) = Point(x,y)  // (2)
}
1 JSON request body binds to method controller arguments, e.g. '{"x":10,"y":20}' (with "application/json")
2 Form data also works, e.g. 'x=10&y=20' (with "application/x-www-form-urlencoded")

6.5 Custom Argument Binding

The Micronaut framework uses an ArgumentBinderRegistry to look up ArgumentBinder beans capable of binding to the arguments in controller methods. The default implementation looks for an annotation on the argument that is meta-annotated with @Bindable. If one exists the argument binder registry searches for an argument binder that supports that annotation.

If no fitting annotation is found, the Micronaut framework tries to find an argument binder that supports the argument type.

An argument binder returns a ArgumentBinder.BindingResult. The binding result gives the Micronaut framework more information than just the value. Binding results are either satisfied or unsatisfied, and either empty or not empty. If an argument binder returns an unsatisfied result, the binder may be called again at different times in request processing. Argument binders are initially called before the body is read and before any filters are executed. If a binder relies on any of that data, and it is not present, return a ArgumentBinder.BindingResult#UNSATISFIED result. Returning an ArgumentBinder.BindingResult#EMPTY or satisfied result will be the final result and the binder will not be called again for that request.

At the end of processing if the result is still ArgumentBinder.BindingResult#UNSATISFIED, it is considered ArgumentBinder.BindingResult#EMPTY.

Key interfaces are:

AnnotatedRequestArgumentBinder

Argument binders that bind based on the presence of an annotation must implement AnnotatedRequestArgumentBinder, and can be used by creating an annotation that is annotated with Bindable. For example:

An example of a binding annotation
import io.micronaut.context.annotation.AliasFor;
import io.micronaut.core.bind.annotation.Bindable;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Target({FIELD, PARAMETER, ANNOTATION_TYPE})
@Retention(RUNTIME)
@Bindable //(1)
public @interface ShoppingCart {

    @AliasFor(annotation = Bindable.class, member = "value")
    String value() default "";
}
An example of a binding annotation
import groovy.transform.CompileStatic
import io.micronaut.context.annotation.AliasFor
import io.micronaut.core.bind.annotation.Bindable

import java.lang.annotation.Retention
import java.lang.annotation.Target

import static java.lang.annotation.ElementType.ANNOTATION_TYPE
import static java.lang.annotation.ElementType.FIELD
import static java.lang.annotation.ElementType.PARAMETER
import static java.lang.annotation.RetentionPolicy.RUNTIME

@CompileStatic
@Target([FIELD, PARAMETER, ANNOTATION_TYPE])
@Retention(RUNTIME)
@Bindable //(1)
@interface ShoppingCart {
    @AliasFor(annotation = Bindable, member = "value")
    String value() default ""
}
An example of a binding annotation
import io.micronaut.core.bind.annotation.Bindable
import kotlin.annotation.AnnotationRetention.RUNTIME
import kotlin.annotation.AnnotationTarget.ANNOTATION_CLASS
import kotlin.annotation.AnnotationTarget.FIELD
import kotlin.annotation.AnnotationTarget.VALUE_PARAMETER

@Target(FIELD, VALUE_PARAMETER, ANNOTATION_CLASS)
@Retention(RUNTIME)
@Bindable //(1)
annotation class ShoppingCart(val value: String = "")
1 The binding annotation must itself be annotated as Bindable
Example of annotated data binding
import io.micronaut.core.convert.ArgumentConversionContext;
import io.micronaut.core.convert.ConversionService;
import io.micronaut.core.type.Argument;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.bind.binders.AnnotatedRequestArgumentBinder;
import io.micronaut.http.cookie.Cookie;
import io.micronaut.jackson.serialize.JacksonObjectSerializer;

import jakarta.inject.Singleton;
import java.util.Map;
import java.util.Optional;

@Singleton
public class ShoppingCartRequestArgumentBinder
        implements AnnotatedRequestArgumentBinder<ShoppingCart, Object> { //(1)

    private final ConversionService conversionService;
    private final JacksonObjectSerializer objectSerializer;

    public ShoppingCartRequestArgumentBinder(ConversionService conversionService,
                                             JacksonObjectSerializer objectSerializer) {
        this.conversionService = conversionService;
        this.objectSerializer = objectSerializer;
    }

    @Override
    public Class<ShoppingCart> getAnnotationType() {
        return ShoppingCart.class;
    }

    @Override
    public BindingResult<Object> bind(
            ArgumentConversionContext<Object> context,
            HttpRequest<?> source) { //(2)

        String parameterName = context.getAnnotationMetadata()
                .stringValue(ShoppingCart.class)
                .orElse(context.getArgument().getName());

        Cookie cookie = source.getCookies().get("shoppingCart");
        if (cookie == null) {
            return BindingResult.EMPTY;
        }

        Optional<Map<String, Object>> cookieValue = objectSerializer.deserialize(
                cookie.getValue().getBytes(),
                Argument.mapOf(String.class, Object.class));

        return () -> cookieValue.flatMap(map -> {
            Object obj = map.get(parameterName);
            return conversionService.convert(obj, context);
        });
    }
}
Example of annotated data binding
import groovy.transform.CompileStatic
import io.micronaut.core.convert.ArgumentConversionContext
import io.micronaut.core.convert.ConversionService
import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.bind.binders.AnnotatedRequestArgumentBinder
import io.micronaut.http.cookie.Cookie
import io.micronaut.jackson.serialize.JacksonObjectSerializer

import jakarta.inject.Singleton

@CompileStatic
@Singleton
class ShoppingCartRequestArgumentBinder
        implements AnnotatedRequestArgumentBinder<ShoppingCart, Object> { //(1)

    private final ConversionService conversionService
    private final JacksonObjectSerializer objectSerializer

    ShoppingCartRequestArgumentBinder(
            ConversionService conversionService,
            JacksonObjectSerializer objectSerializer) {
        this.conversionService = conversionService
        this.objectSerializer = objectSerializer
    }

    @Override
    Class<ShoppingCart> getAnnotationType() {
        ShoppingCart
    }

    @Override
    BindingResult<Object> bind(
            ArgumentConversionContext<Object> context,
            HttpRequest<?> source) { //(2)

        String parameterName = context.annotationMetadata
                .stringValue(ShoppingCart)
                .orElse(context.argument.name)

        Cookie cookie = source.cookies.get("shoppingCart")
        if (!cookie) {
            return BindingResult.EMPTY
        }

        Optional<Map<String, Object>> cookieValue = objectSerializer.deserialize(
                cookie.value.bytes,
                Argument.mapOf(String, Object))

        return (BindingResult) { ->
            cookieValue.flatMap({value ->
                conversionService.convert(value.get(parameterName), context)
            })
        }
    }
}
Example of annotated data binding
import io.micronaut.core.bind.ArgumentBinder.BindingResult
import io.micronaut.core.convert.ArgumentConversionContext
import io.micronaut.core.convert.ConversionService
import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.bind.binders.AnnotatedRequestArgumentBinder
import io.micronaut.jackson.serialize.JacksonObjectSerializer
import java.util.Optional
import jakarta.inject.Singleton

@Singleton
class ShoppingCartRequestArgumentBinder(
        private val conversionService: ConversionService,
        private val objectSerializer: JacksonObjectSerializer
) : AnnotatedRequestArgumentBinder<ShoppingCart, Any> { //(1)

    override fun getAnnotationType(): Class<ShoppingCart> {
        return ShoppingCart::class.java
    }

    override fun bind(context: ArgumentConversionContext<Any>,
                      source: HttpRequest<*>): BindingResult<Any> { //(2)

        val parameterName = context.annotationMetadata
            .stringValue(ShoppingCart::class.java)
            .orElse(context.argument.name)

        val cookie = source.cookies.get("shoppingCart") ?: return BindingResult.EMPTY

        val cookieValue: Optional<Map<String, Any>> = objectSerializer.deserialize(
                cookie.value.toByteArray(),
                Argument.mapOf(String::class.java, Any::class.java))

        return BindingResult {
            cookieValue.flatMap { map: Map<String, Any> ->
                conversionService.convert(map[parameterName], context)
            }
        }
    }
}
1 The custom argument binder must implement AnnotatedRequestArgumentBinder, including both the annotation type to trigger the binder (in this case, MyBindingAnnotation) and the type of the argument expected (in this case, Object)
2 Override the bind method with the custom argument binding logic - in this case, we resolve the name of the annotated argument, extract a value from a cookie with that same name, and convert that value to the argument type
It is common to use ConversionService to convert the data to the type of the argument.

Once the binder is created, we can annotate an argument in our controller method which will be bound using the custom logic we’ve specified.

A controller operation with this annotated binding
    @Get("/annotated")
    HttpResponse<String> checkSession(@ShoppingCart Long sessionId) { //(1)
        return HttpResponse.ok("Session:" + sessionId);
    }
    // end::method
}
A controller operation with this annotated binding
    @Get("/annotated")
    HttpResponse<String> checkSession(@ShoppingCart Long sessionId) { //(1)
        HttpResponse.ok("Session:" + sessionId)
    }
    // end::method
}
A controller operation with this annotated binding
@Get("/annotated")
fun checkSession(@ShoppingCart sessionId: Long): HttpResponse<String> { //(1)
    return HttpResponse.ok("Session:$sessionId")
}
1 The parameter is bound with the binder associated with MyBindingAnnotation. This takes precedence over a type-based binder, if applicable.

TypedRequestArgumentBinder

Argument binders that bind based on the type of the argument must implement TypedRequestArgumentBinder. For example, given this class:

Example of POJO
import io.micronaut.core.annotation.Introspected;

@Introspected
public class ShoppingCart {

    private String sessionId;
    private Integer total;

    public String getSessionId() {
        return sessionId;
    }

    public void setSessionId(String sessionId) {
        this.sessionId = sessionId;
    }

    public Integer getTotal() {
        return total;
    }

    public void setTotal(Integer total) {
        this.total = total;
    }
}
Example of POJO
import io.micronaut.core.annotation.Introspected

@Introspected
class ShoppingCart {
    String sessionId
    Integer total
}
Example of POJO
import io.micronaut.core.annotation.Introspected

@Introspected
class ShoppingCart {
    var sessionId: String? = null
    var total: Int? = null
}

We can define a TypedRequestArgumentBinder for this class, as seen below:

Example of typed data binding
import io.micronaut.core.convert.ArgumentConversionContext;
import io.micronaut.core.type.Argument;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.bind.binders.TypedRequestArgumentBinder;
import io.micronaut.http.cookie.Cookie;
import io.micronaut.jackson.serialize.JacksonObjectSerializer;

import jakarta.inject.Singleton;
import java.util.Optional;

@Singleton
public class ShoppingCartRequestArgumentBinder
        implements TypedRequestArgumentBinder<ShoppingCart> {

    private final JacksonObjectSerializer objectSerializer;

    public ShoppingCartRequestArgumentBinder(JacksonObjectSerializer objectSerializer) {
        this.objectSerializer = objectSerializer;
    }

    @Override
    public BindingResult<ShoppingCart> bind(ArgumentConversionContext<ShoppingCart> context,
                                            HttpRequest<?> source) { //(1)

        Cookie cookie = source.getCookies().get("shoppingCart");
        if (cookie == null) {
            return Optional::empty;
        }

        return () -> objectSerializer.deserialize( //(2)
                cookie.getValue().getBytes(),
                ShoppingCart.class);
    }

    @Override
    public Argument<ShoppingCart> argumentType() {
        return Argument.of(ShoppingCart.class); //(3)
    }
}
Example of typed data binding
import io.micronaut.core.convert.ArgumentConversionContext
import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.bind.binders.TypedRequestArgumentBinder
import io.micronaut.http.cookie.Cookie
import io.micronaut.jackson.serialize.JacksonObjectSerializer

import jakarta.inject.Singleton

@Singleton
class ShoppingCartRequestArgumentBinder
        implements TypedRequestArgumentBinder<ShoppingCart> {

    private final JacksonObjectSerializer objectSerializer

    ShoppingCartRequestArgumentBinder(JacksonObjectSerializer objectSerializer) {
        this.objectSerializer = objectSerializer
    }

    @Override
    BindingResult<ShoppingCart> bind(ArgumentConversionContext<ShoppingCart> context,
                                     HttpRequest<?> source) { //(1)

        Cookie cookie = source.cookies.get("shoppingCart")
        if (!cookie) {
            return BindingResult.EMPTY
        }

        return () -> objectSerializer.deserialize( //(2)
                cookie.value.bytes,
                ShoppingCart)
    }

    @Override
    Argument<ShoppingCart> argumentType() {
        Argument.of(ShoppingCart) //(3)
    }
}
Example of typed data binding
import io.micronaut.core.bind.ArgumentBinder
import io.micronaut.core.bind.ArgumentBinder.BindingResult
import io.micronaut.core.convert.ArgumentConversionContext
import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.bind.binders.TypedRequestArgumentBinder
import io.micronaut.jackson.serialize.JacksonObjectSerializer
import java.util.Optional
import jakarta.inject.Singleton

@Singleton
class ShoppingCartRequestArgumentBinder(private val objectSerializer: JacksonObjectSerializer) :
    TypedRequestArgumentBinder<ShoppingCart> {

    override fun bind(
        context: ArgumentConversionContext<ShoppingCart>,
        source: HttpRequest<*>
    ): BindingResult<ShoppingCart> { //(1)

        val cookie = source.cookies["shoppingCart"]

        return if (cookie == null)
            BindingResult {
                Optional.empty()
            }
        else {
            BindingResult {
                objectSerializer.deserialize( // (2)
                    cookie.value.toByteArray(),
                    ShoppingCart::class.java
                )
            }
        }
    }

    override fun argumentType(): Argument<ShoppingCart> {
        return Argument.of(ShoppingCart::class.java) //(3)
    }
}
1 Override the bind method with the data type to bind, in this example the ShoppingCart type
2 After retrieving the data (in this case, by deserializing JSON text from a cookie), return as a ArgumentBinder.BindingResult
3 Also override the argumentType method, which is used by the ArgumentBinderRegistry.

Once the binder is created, it is used for any controller argument of the associated type:

A controller operation with this typed binding
@Get("/typed")
public HttpResponse<?> loadCart(ShoppingCart shoppingCart) { //(1)
    Map<String, Object> responseMap = new HashMap<>();
    responseMap.put("sessionId", shoppingCart.getSessionId());
    responseMap.put("total", shoppingCart.getTotal());

    return HttpResponse.ok(responseMap);
}
A controller operation with this typed binding
@Get("/typed")
HttpResponse<Map<String, Object>> loadCart(ShoppingCart shoppingCart) { //(1)
    HttpResponse.ok(
            sessionId: shoppingCart.sessionId,
            total: shoppingCart.total)
}
A controller operation with this typed binding
@Get("/typed")
fun loadCart(shoppingCart: ShoppingCart): HttpResponse<*> { //(1)
    return HttpResponse.ok(mapOf(
        "sessionId" to shoppingCart.sessionId,
        "total" to shoppingCart.total))
}
1 The parameter is bound using the custom logic defined for this type in our TypedRequestArgumentBinder

6.6 Host Resolution

You may need to resolve the host name of the current server. The Micronaut framework includes an implementation of the HttpHostResolver interface.

The default implementation looks for host information in the following places in order:

  1. The supplied configuration

  2. The Forwarded header

  3. X-Forwarded- headers. If the X-Forwarded-Host header is not present, the other X-Forwarded headers are ignored.

  4. The Host header

  5. The properties on the request URI

  6. The properties on the embedded server URI

The behavior of which headers to pull the relevant data can be changed with the following configuration:

🔗
Table 1. Configuration Properties for HttpServerConfiguration$HostResolutionConfiguration
Property Type Description

micronaut.server.host-resolution.host-header

java.lang.String

micronaut.server.host-resolution.protocol-header

java.lang.String

micronaut.server.host-resolution.port-header

java.lang.String

micronaut.server.host-resolution.port-in-host

boolean

micronaut.server.host-resolution.allowed-hosts

java.util.List

The above configuration also supports an allowed host list. Configuring this list ensures any resolved host matches one of the supplied regular expression patterns. That is useful to prevent host cache poisoning attacks and is recommended to be configured.

6.7 Locale Resolution

The Micronaut framework supports several strategies for resolving locales for a given request. The getLocale-- method is available on the request, however it only supports parsing the Accept-Language header. For other use cases where the locale can be in a cookie, the user’s session, or should be set to a fixed value, HttpLocaleResolver can be used to determine the current locale.

The LocaleResolver API does not need to be used directly. Simply define a parameter to a controller method of type java.util.Locale and the locale will be resolved and injected automatically.

There are several configuration options to control how to resolve the locale:

🔗
Table 1. Configuration Properties for HttpServerConfiguration$HttpLocaleResolutionConfigurationProperties
Property Type Description

micronaut.server.locale-resolution.fixed

java.util.Locale

micronaut.server.locale-resolution.session-attribute

java.lang.String

micronaut.server.locale-resolution.default-locale

java.util.Locale

micronaut.server.locale-resolution.cookie-name

java.lang.String

micronaut.server.locale-resolution.header

boolean

Locales can be configured in the "en_GB" format, or in the BCP 47 (Language tag) format. If multiple methods are configured, the fixed locale takes precedence, followed by session/cookie, then header.

If any of the built-in methods do not meet your use case, create a bean of type HttpLocaleResolver and set its order (through the getOrder method) relative to the existing resolvers.

6.8 Client IP Address

You may need to resolve the originating IP address of an HTTP Request. The Micronaut framework includes an implementation of HttpClientAddressResolver.

The default implementation resolves the client address in the following places in order:

  1. The configured header

  2. The Forwarded header

  3. The X-Forwarded-For header

  4. The remote address on the request

The first priority header name can be configured with micronaut.server.client-address-header.

6.9 The HttpRequest and HttpResponse

If you need more control over request processing you can write a method that receives the complete HttpRequest.

In fact, there are several higher-level interfaces that can be bound to controller method parameters. These include:

Table 1. Bindable Micronaut Interfaces
Interface Description Example

HttpRequest

The full HttpRequest

String hello(HttpRequest request)

HttpHeaders

All HTTP headers present in the request

String hello(HttpHeaders headers)

HttpParameters

All HTTP parameters (either from URI variables or request parameters) present in the request

String hello(HttpParameters params)

Cookies

All Cookies present in the request

String hello(Cookies cookies)

The HttpRequest should be declared parametrized with a concrete generic type if the request body is needed, e.g. HttpRequest<MyClass> request. The body may not be available from the request otherwise.

In addition, for full control over the emitted HTTP response you can use the static factory methods of the HttpResponse class which return a MutableHttpResponse.

The following example implements the previous MessageController example using the HttpRequest and HttpResponse objects:

Request and Response Example
import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.context.ServerRequestContext;
import reactor.core.publisher.Mono;

@Controller("/request")
public class MessageController {

@Get("/hello") // (1)
public HttpResponse<String> hello(HttpRequest<?> request) {
    String name = request.getParameters()
                         .getFirst("name")
                         .orElse("Nobody"); // (2)

    return HttpResponse.ok("Hello " + name + "!!")
             .header("X-My-Header", "Foo"); // (3)
}

}
Request and Response Example
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.context.ServerRequestContext
import reactor.core.publisher.Mono


@Controller("/request")
class MessageController {

@Get("/hello") // (1)
HttpResponse<String> hello(HttpRequest<?> request) {
    String name = request.parameters
                         .getFirst("name")
                         .orElse("Nobody") // (2)

    HttpResponse.ok("Hello " + name + "!!")
             .header("X-My-Header", "Foo") // (3)
}

}
Request and Response Example
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.context.ServerRequestContext
import reactor.core.publisher.Mono
import reactor.util.context.ContextView


@Controller("/request")
class MessageController {

@Get("/hello") // (1)
fun hello(request: HttpRequest<*>): HttpResponse<String> {
    val name = request.parameters
                      .getFirst("name")
                      .orElse("Nobody") // (2)

    return HttpResponse.ok("Hello $name!!")
            .header("X-My-Header", "Foo") // (3)
}

}
1 The method is mapped to the URI /hello and accepts a HttpRequest
2 The HttpRequest is used to obtain the value of a query parameter named name.
3 The HttpResponse.ok(T) method returns a MutableHttpResponse with a text body. A header named X-My-Header is also added to the response.

The HttpRequest is also available from a static context via ServerRequestContext.

Using the ServerRequestContext
import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.context.ServerRequestContext;
import reactor.core.publisher.Mono;

@Controller("/request")
public class MessageController {

@Get("/hello-static") // (1)
public HttpResponse<String> helloStatic() {
    HttpRequest<?> request = ServerRequestContext.currentRequest() // (1)
            .orElseThrow(() -> new RuntimeException("No request present"));
    String name = request.getParameters()
            .getFirst("name")
            .orElse("Nobody");

    return HttpResponse.ok("Hello " + name + "!!")
            .header("X-My-Header", "Foo");
}

}
Using the ServerRequestContext
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.context.ServerRequestContext
import reactor.core.publisher.Mono


@Controller("/request")
class MessageController {

@Get("/hello-static") // (1)
HttpResponse<String> helloStatic() {
    HttpRequest<?> request = ServerRequestContext.currentRequest() // (1)
            .orElseThrow(() -> new RuntimeException("No request present"))
    String name = request.parameters
            .getFirst("name")
            .orElse("Nobody")

    HttpResponse.ok("Hello " + name + "!!")
            .header("X-My-Header", "Foo")
}

}
Using the ServerRequestContext
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.context.ServerRequestContext
import reactor.core.publisher.Mono
import reactor.util.context.ContextView


@Controller("/request")
class MessageController {

@Get("/hello-static") // (1)
fun helloStatic(): HttpResponse<String> {
    val request: HttpRequest<*> = ServerRequestContext.currentRequest<Any>() // (1)
        .orElseThrow { RuntimeException("No request present") }
    val name = request.parameters
        .getFirst("name")
        .orElse("Nobody")
    return HttpResponse.ok("Hello $name!!")
        .header("X-My-Header", "Foo")
}

}
1 The ServerRequestContext is used to retrieve the request.
Generally ServerRequestContext is available within reactive flow, but the recommended approach is consumed the request as an argument as shown in the previous example. If the request is needed in downstream methods it should be passed as an argument to those methods. There are cases where the context is not propagated because other threads are used to emit the data.

An alternative for users of Project Reactor to using the ServerRequestContext is to use the contextual features of Project Reactor to retrieve the request. Because the Micronaut Framework uses Project Reactor as it’s default reactive streams implementation, users of Project Reactor can benefit by being able to access the request in the context. For example:

Using the Project Reactor context
import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.context.ServerRequestContext;
import reactor.core.publisher.Mono;

@Controller("/request")
public class MessageController {

@Get("/hello-reactor")
public Mono<HttpResponse<String>> helloReactor() {
    return Mono.deferContextual(ctx -> { // (1)
        HttpRequest<?> request = ctx.get(ServerRequestContext.KEY); // (2)
        String name = request.getParameters()
                .getFirst("name")
                .orElse("Nobody");

        return Mono.just(HttpResponse.ok("Hello " + name + "!!")
                .header("X-My-Header", "Foo"));
    });
}

}
Using the Project Reactor context
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.context.ServerRequestContext
import reactor.core.publisher.Mono


@Controller("/request")
class MessageController {

@Get("/hello-reactor")
Mono<HttpResponse<String>> helloReactor() {
    Mono.deferContextual(ctx -> { // (1)
        HttpRequest<?> request = ctx.get(ServerRequestContext.KEY) // (2)
        String name = request.parameters
                .getFirst("name")
                .orElse("Nobody")

        Mono.just(HttpResponse.ok("Hello " + name + "!!")
                .header("X-My-Header", "Foo"))
    })
}

}
Using the Project Reactor context
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.context.ServerRequestContext
import reactor.core.publisher.Mono
import reactor.util.context.ContextView


@Controller("/request")
class MessageController {

@Get("/hello-reactor")
fun helloReactor(): Mono<HttpResponse<String>?>? {
    return Mono.deferContextual { ctx: ContextView ->  // (1)
        val request = ctx.get<HttpRequest<*>>(ServerRequestContext.KEY) // (2)
        val name = request.parameters
            .getFirst("name")
            .orElse("Nobody")
        Mono.just(HttpResponse.ok("Hello $name!!")
                .header("X-My-Header", "Foo"))
    }
}

}
1 The Mono is created with a reference to the context
2 The request is retrieved from the context

Using the context to retrieve the request is the best approach for reactive flows because Project Reactor propagates the context, and it does not rely on a thread local like ServerRequestContext.

6.10 Response Status

A Micronaut controller action responds with a 200 HTTP status code by default.

If the action returns an HttpResponse, configure the status code for the response with the status method.

@Get(value = "/http-response", produces = MediaType.TEXT_PLAIN)
public HttpResponse httpResponse() {
    return HttpResponse.status(HttpStatus.CREATED).body("success");
}
@Get(value = "/http-response", produces = MediaType.TEXT_PLAIN)
HttpResponse httpResponse() {
    HttpResponse.status(HttpStatus.CREATED).body("success")
}
@Get(value = "/http-response", produces = [MediaType.TEXT_PLAIN])
fun httpResponse(): HttpResponse<String> {
    return HttpResponse.status<String>(HttpStatus.CREATED).body("success")
}

You can also use the @Status annotation.

@Status(HttpStatus.CREATED)
@Get(produces = MediaType.TEXT_PLAIN)
public String index() {
    return "success";
}
@Status(HttpStatus.CREATED)
@Get(produces = MediaType.TEXT_PLAIN)
String index() {
    "success"
}
@Status(HttpStatus.CREATED)
@Get(produces = [MediaType.TEXT_PLAIN])
fun index(): String {
    return "success"
}

or even respond with an HttpStatus

@Get("/http-status")
public HttpStatus httpStatus() {
    return HttpStatus.CREATED;
}
@Get("/http-status")
HttpStatus httpStatus() {
    HttpStatus.CREATED
}
@Get("/http-status")
fun httpStatus(): HttpStatus {
    return HttpStatus.CREATED
}

6.11 Response Content-Type

A Micronaut controller action produces application/json by default. However, you can change the Content-Type of the response with the @Produces annotation or the produces member of the HTTP method annotations.

import io.micronaut.context.annotation.Requires;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.annotation.Produces;

@Controller("/produces")
public class ProducesController {

    @Get // (1)
    public HttpResponse index() {
        return HttpResponse.ok().body("{\"msg\":\"This is JSON\"}");
    }

    @Produces(MediaType.TEXT_HTML)
    @Get("/html") // (2)
    public String html() {
        return "<html><title><h1>HTML</h1></title><body></body></html>";
    }

    @Get(value = "/xml", produces = MediaType.TEXT_XML) // (3)
    public String xml() {
        return "<html><title><h1>XML</h1></title><body></body></html>";
    }
}
import io.micronaut.context.annotation.Requires
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Produces

@Controller("/produces")
class ProducesController {

    @Get // (1)
    HttpResponse index() {
        HttpResponse.ok().body('{"msg":"This is JSON"}')
    }

    @Produces(MediaType.TEXT_HTML) // (2)
    @Get("/html")
    String html() {
        "<html><title><h1>HTML</h1></title><body></body></html>"
    }

    @Get(value = "/xml", produces = MediaType.TEXT_XML) // (3)
    String xml() {
        "<html><title><h1>XML</h1></title><body></body></html>"
    }
}
import io.micronaut.context.annotation.Requires
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Produces

@Controller("/produces")
class ProducesController {

    @Get // (1)
    fun index(): HttpResponse<*> {
        return HttpResponse.ok<Any>().body("{\"msg\":\"This is JSON\"}")
    }

    @Produces(MediaType.TEXT_HTML)
    @Get("/html") // (2)
    fun html(): String {
        return "<html><title><h1>HTML</h1></title><body></body></html>"
    }

    @Get(value = "/xml", produces = [MediaType.TEXT_XML]) // (3)
    fun xml(): String {
        return "<html><title><h1>XML</h1></title><body></body></html>"
    }
}
1 The default content type is JSON
2 Annotate a controller action with @Produces to change the response content type.
3 Setting the produces member of the method annotation also changes the content type.

6.12 Accepted Request Content-Type

A Micronaut controller action consumes application/json by default. Consuming other content types is supported with the @Consumes annotation, or the consumes member of any HTTP method annotation.

import io.micronaut.context.annotation.Requires;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Consumes;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;

@Controller("/consumes")
public class ConsumesController {

    @Post // (1)
    public HttpResponse index() {
        return HttpResponse.ok();
    }

    @Consumes({MediaType.APPLICATION_FORM_URLENCODED, MediaType.APPLICATION_JSON}) // (2)
    @Post("/multiple")
    public HttpResponse multipleConsumes() {
        return HttpResponse.ok();
    }

    @Post(value = "/member", consumes = MediaType.TEXT_PLAIN) // (3)
    public HttpResponse consumesMember() {
        return HttpResponse.ok();
    }
}
import io.micronaut.context.annotation.Requires
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Consumes
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post

@Controller("/consumes")
class ConsumesController {

    @Post // (1)
    HttpResponse index() {
        HttpResponse.ok()
    }

    @Consumes([MediaType.APPLICATION_FORM_URLENCODED, MediaType.APPLICATION_JSON]) // (2)
    @Post("/multiple")
    HttpResponse multipleConsumes() {
        HttpResponse.ok()
    }

    @Post(value = "/member", consumes = MediaType.TEXT_PLAIN) // (3)
    HttpResponse consumesMember() {
        HttpResponse.ok()
    }
}
import io.micronaut.context.annotation.Requires
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Consumes
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post

@Controller("/consumes")
class ConsumesController {

    @Post // (1)
    fun index(): HttpResponse<*> {
        return HttpResponse.ok<Any>()
    }

    @Consumes(MediaType.APPLICATION_FORM_URLENCODED, MediaType.APPLICATION_JSON) // (2)
    @Post("/multiple")
    fun multipleConsumes(): HttpResponse<*> {
        return HttpResponse.ok<Any>()
    }

    @Post(value = "/member", consumes = [MediaType.TEXT_PLAIN]) // (3)
    fun consumesMember(): HttpResponse<*> {
        return HttpResponse.ok<Any>()
    }
}
1 By default, a controller action consumes request with Content-Type of type application/json.
2 The @Consumes annotation takes a String[] of supported media types for an incoming request.
3 Content types can also be specified with the consumes member of the method annotation.

Customizing Processed Content Types

Normally JSON parsing only happens if the content type is application/json. The other MediaTypeCodec classes behave similarly in that they have predefined content types they can process. To extend the list of media types that a given codec processes, provide configuration that will be stored in CodecConfiguration:

micronaut.codec.json.additional-types[0]=text/javascript
micronaut.codec.json.additional-types[1]=...
micronaut:
  codec:
    json:
      additional-types:
        - text/javascript
        - ...
[micronaut]
  [micronaut.codec]
    [micronaut.codec.json]
      additional-types=[
        "text/javascript",
        "..."
      ]
micronaut {
  codec {
    json {
      additionalTypes = ["text/javascript", "..."]
    }
  }
}
{
  micronaut {
    codec {
      json {
        additional-types = ["text/javascript", "..."]
      }
    }
  }
}
{
  "micronaut": {
    "codec": {
      "json": {
        "additional-types": ["text/javascript", "..."]
      }
    }
  }
}

The currently supported configuration prefixes are json, json-stream, text, and text-stream.

6.13 Reactive HTTP Request Processing

As mentioned previously, Micronaut framework is built on Netty which is designed around an Event loop model and non-blocking I/O. Micronaut executes code defined in @Controller beans in the same thread as the request thread (an Event Loop thread).

This makes it critical that if you do any blocking I/O operations (for example interactions with Hibernate/JPA or JDBC) that you offload those tasks to a separate thread pool that does not block the Event loop.

For example the following configuration configures the I/O thread pool as a fixed thread pool with 75 threads (similar to what a traditional blocking server such as Tomcat uses in the thread-per-request model):

Configuring the IO thread pool
micronaut.executors.io.type=fixed
micronaut.executors.io.nThreads=75
micronaut:
  executors:
    io:
      type: fixed
      nThreads: 75
[micronaut]
  [micronaut.executors]
    [micronaut.executors.io]
      type="fixed"
      nThreads=75
micronaut {
  executors {
    io {
      type = "fixed"
      nThreads = 75
    }
  }
}
{
  micronaut {
    executors {
      io {
        type = "fixed"
        nThreads = 75
      }
    }
  }
}
{
  "micronaut": {
    "executors": {
      "io": {
        "type": "fixed",
        "nThreads": 75
      }
    }
  }
}

To use this thread pool in a @Controller bean you have a number of options. The simplest is to use the @ExecuteOn annotation, which can be applied only to either a @Controller or @Filter at the type or method level. It indicates the configured thread pool to run method(s) of the controller or filter on:

Using @ExecuteOn
import io.micronaut.docs.http.server.reactive.PersonService;
import io.micronaut.docs.ioc.beans.Person;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.scheduling.TaskExecutors;
import io.micronaut.scheduling.annotation.ExecuteOn;

@Controller("/executeOn/people")
public class PersonController {

    private final PersonService personService;

    PersonController(PersonService personService) {
        this.personService = personService;
    }

    @Get("/{name}")
    @ExecuteOn(TaskExecutors.IO) // (1)
    Person byName(String name) {
        return personService.findByName(name);
    }
}
Using @ExecuteOn
import io.micronaut.docs.http.server.reactive.PersonService
import io.micronaut.docs.ioc.beans.Person
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.scheduling.TaskExecutors
import io.micronaut.scheduling.annotation.ExecuteOn

@Controller("/executeOn/people")
class PersonController {

    private final PersonService personService

    PersonController(PersonService personService) {
        this.personService = personService
    }

    @Get("/{name}")
    @ExecuteOn(TaskExecutors.IO) // (1)
    Person byName(String name) {
        personService.findByName(name)
    }
}
Using @ExecuteOn
import io.micronaut.docs.http.server.reactive.PersonService
import io.micronaut.docs.ioc.beans.Person
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.scheduling.TaskExecutors
import io.micronaut.scheduling.annotation.ExecuteOn

@Controller("/executeOn/people")
class PersonController (private val personService: PersonService) {

    @Get("/{name}")
    @ExecuteOn(TaskExecutors.IO) // (1)
    fun byName(name: String): Person {
        return personService.findByName(name)
    }
}
1 The @ExecuteOn annotation is used to execute the operation on the I/O thread pool

The value of the @ExecuteOn annotation can be any named executor defined under micronaut.executors.

Generally speaking for database operations you want a thread pool configured that matches the maximum number of connections specified in the database connection pool.

An alternative to the @ExecuteOn annotation is to use the facility provided by the reactive library you have chosen. Reactive implementations such as Project Reactor or RxJava feature a subscribeOn method which lets you alter which thread executes user code. For example:

Reactive subscribeOn Example
import io.micronaut.docs.ioc.beans.Person;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.scheduling.TaskExecutors;
import jakarta.inject.Named;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Scheduler;
import reactor.core.scheduler.Schedulers;
import io.micronaut.core.async.annotation.SingleResult;
import java.util.concurrent.ExecutorService;

@Controller("/subscribeOn/people")
public class PersonController {

    private final Scheduler scheduler;
    private final PersonService personService;

    PersonController(
            @Named(TaskExecutors.IO) ExecutorService executorService, // (1)
            PersonService personService) {
        this.scheduler = Schedulers.fromExecutorService(executorService);
        this.personService = personService;
    }

    @Get("/{name}")
    @SingleResult
    Publisher<Person> byName(String name) {
        return Mono
                .fromCallable(() -> personService.findByName(name)) // (2)
                .subscribeOn(scheduler); // (3)
    }
}
Reactive subscribeOn Example
import io.micronaut.docs.ioc.beans.Person
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.scheduling.TaskExecutors
import jakarta.inject.Named
import reactor.core.publisher.Mono
import reactor.core.scheduler.Scheduler
import reactor.core.scheduler.Schedulers
import java.util.concurrent.ExecutorService

@Controller("/subscribeOn/people")
class PersonController {

    private final Scheduler scheduler
    private final PersonService personService

    PersonController(
            @Named(TaskExecutors.IO) ExecutorService executorService, // (1)
            PersonService personService) {
        this.scheduler = Schedulers.fromExecutorService(executorService)
        this.personService = personService
    }

    @Get("/{name}")
    Mono<Person> byName(String name) {
        return Mono
                .fromCallable({ -> personService.findByName(name) }) // (2)
                .subscribeOn(scheduler) // (3)
    }
}
Reactive subscribeOn Example
import io.micronaut.docs.ioc.beans.Person
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.scheduling.TaskExecutors
import java.util.concurrent.ExecutorService
import jakarta.inject.Named
import reactor.core.publisher.Mono
import reactor.core.scheduler.Scheduler
import reactor.core.scheduler.Schedulers


@Controller("/subscribeOn/people")
class PersonController internal constructor(
    @Named(TaskExecutors.IO) executorService: ExecutorService, // (1)
    private val personService: PersonService) {

    private val scheduler: Scheduler = Schedulers.fromExecutorService(executorService)

    @Get("/{name}")
    fun byName(name: String): Mono<Person> {
        return Mono
            .fromCallable { personService.findByName(name) } // (2)
            .subscribeOn(scheduler) // (3)
    }
}
1 The configured I/O executor service is injected
2 The Mono::fromCallable method wraps the blocking operation
3 The Project Reactor subscribeOn method schedules the operation on the I/O thread pool

6.13.1 Using the @Body Annotation

To parse the request body, you first indicate to the Micronaut framework which parameter receives the data with the Body annotation.

The following example implements a simple echo server that echoes the body sent in the request:

Using the @Body annotation
import io.micronaut.http.HttpResponse;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Body;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import jakarta.validation.constraints.Size;

@Controller("/receive")
public class MessageController {

@Post(value = "/echo", consumes = MediaType.TEXT_PLAIN) // (1)
String echo(@Size(max = 1024) @Body String text) { // (2)
    return text; // (3)
}

}
Using the @Body annotation
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import jakarta.validation.constraints.Size

@Controller("/receive")
class MessageController {

@Post(value = "/echo", consumes = MediaType.TEXT_PLAIN) // (1)
String echo(@Size(max = 1024) @Body String text) { // (2)
    text // (3)
}

}
Using the @Body annotation
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import jakarta.validation.constraints.Size

@Controller("/receive")
open class MessageController {

@Post(value = "/echo", consumes = [MediaType.TEXT_PLAIN]) // (1)
open fun echo(@Size(max = 1024) @Body text: String): String { // (2)
    return text // (3)
}

}
1 The Post annotation is used with a MediaType of text/plain (the default is application/json).
2 The Body annotation is used with a jakarta.validation.constraints.Size that limits the size of the body to at most 1KB. This constraint does not limit the amount of data read/buffered by the server.
3 The body is returned as the result of the method

Note that reading the request body is done in a non-blocking manner in that the request contents are read as the data becomes available and accumulated into the String passed to the method.

The micronaut.server.maxRequestSize setting in your configuration file (e.g. application.yml) limits the size of the data (the default maximum request size is 10MB) read/buffered by the server. @Size is not a replacement for this setting.

Regardless of the limit, for a large amount of data accumulating the data into a String in-memory may lead to memory strain on the server. A better approach is to include a Reactive library in your project (such as Reactor, RxJava,or Akka) that supports the Reactive streams implementation and stream the data it becomes available:

Using Reactive Streams to Read the request body
import io.micronaut.http.HttpResponse;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Body;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import jakarta.validation.constraints.Size;

import org.reactivestreams.Publisher;
import reactor.core.publisher.Flux;
import io.micronaut.core.async.annotation.SingleResult;

@Controller("/receive")
public class MessageController {

@Post(value = "/echo-publisher", consumes = MediaType.TEXT_PLAIN) // (1)
@SingleResult
Publisher<HttpResponse<String>> echoFlow(@Body Publisher<String> text) { //(2)
    return Flux.from(text)
            .collect(StringBuffer::new, StringBuffer::append) // (3)
            .map(buffer -> HttpResponse.ok(buffer.toString()));
}

}
Using Reactive Streams to Read the request body
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import jakarta.validation.constraints.Size

import org.reactivestreams.Publisher
import io.micronaut.core.async.annotation.SingleResult
import reactor.core.publisher.Flux

@Controller("/receive")
class MessageController {

@Post(value = "/echo-publisher", consumes = MediaType.TEXT_PLAIN) // (1)
@SingleResult
Publisher<HttpResponse<String>> echoFlow(@Body Publisher<String> text) { // (2)
    return Flux.from(text)
            .collect({ x -> new StringBuffer() }, { StringBuffer sb, String s -> sb.append(s) }) // (3)
            .map({ buffer -> HttpResponse.ok(buffer.toString()) });
}

}
Using Reactive Streams to Read the request body
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import jakarta.validation.constraints.Size

import org.reactivestreams.Publisher
import io.micronaut.core.async.annotation.SingleResult
import reactor.core.publisher.Flux

@Controller("/receive")
open class MessageController {

@Post(value = "/echo-publisher", consumes = [MediaType.TEXT_PLAIN]) // (1)
@SingleResult
open fun echoFlow(@Body text: Publisher<String>): Publisher<HttpResponse<String>> { //(2)
    return Flux.from(text)
        .collect({ StringBuffer() }, { obj, str -> obj.append(str) }) // (3)
        .map { buffer -> HttpResponse.ok(buffer.toString()) }
}

}
1 In this case the method is altered to receive and return an Publisher type.
2 This example uses Project Reactor and returns a single item. Because of that the response type is annotated also with SingleResult. The Micronaut framework only emits the response once the operation completes without blocking.
3 The collect method is used to accumulate the data in this simulated example, but it could for example write the data to a logging service, database, etc. chunk by chunk
Body arguments of types that do not require conversion cause the Micronaut framework to skip decoding of the request!

6.13.2 Reactive Responses

The previous section introduced the notion of Reactive programming using Project Reactor and Micronaut.

The Micronaut framework supports returning common reactive types such as Mono (or Single Maybe Observable types from RxJava), an instance of Publisher or CompletableFuture from any controller method.

To use Project Reactor's Flux or Mono you need to add the Micronaut Reactor dependency to your project to include the necessary converters.
To use RxJava's Flowable, Single or Maybe you need to add the Micronaut RxJava dependency to your project to include the necessary converters.

The argument designated as the body of the request using the Body annotation can also be a reactive type or a CompletableFuture.

When returning a reactive type, The Micronaut framework subscribes to the returned reactive type on the same thread as the request (a Netty Event Loop thread). It is therefore important that if you perform any blocking operations, you offload those operations to an appropriately configured thread pool, for example using the Project Reactor or RxJava subscribeOn(..) facility or @ExecuteOn.

See the section on Configuring Thread Pools for information on the thread pools that the Micronaut framework sets up and how to configure them.

To summarize, the following table illustrates some common response types and their handling:

Table 1. Micronaut Response Types
Type Description Example Signature

Publisher

Any type that implements the Publisher interface

Publisher<String> hello()

CompletableFuture

A Java CompletableFuture instance

CompletableFuture<String> hello()

HttpResponse

An HttpResponse and optional response body

HttpResponse<Publisher<String>> hello()

CharSequence

Any implementation of CharSequence

String hello()

T

Any simple POJO type

Book show()

When returning a Reactive type, its type affects the returned response. For example, when returning a Flux, the Micronaut framework cannot know the size of the response, so Transfer-Encoding type of Chunked is used. Whilst for types that emit a single result such as Mono the Content-Length header is populated.

6.14 JSON Binding

The most common data interchange format nowadays is JSON.

By default, the Controller annotation specifies that the controllers in Micronaut framework consume and produce JSON by default.

Since Micronaut Framework 4.0, users must choose how they want to serialize (Jackson Databind or Micronaut Serialization). Both approaches allow the usage of Jackson Annotations.

With either approach, the Micronaut framework reads incoming JSON in a non-blocking manner.

Serialize using Micronaut Serialization

Micronaut Serialization offers reflection-free serialization using build-time Bean Introspections. It supports alternative formats such as JSON-P or JSON-B. You need to add the following dependencies:

annotationProcessor("io.micronaut.serde:micronaut-serde-processor")
<annotationProcessorPaths>
    <path>
        <groupId>io.micronaut.serde</groupId>
        <artifactId>micronaut-serde-processor</artifactId>
    </path>
</annotationProcessorPaths>
implementation("io.micronaut.serde:micronaut-serde-jackson")
<dependency>
    <groupId>io.micronaut.serde</groupId>
    <artifactId>micronaut-serde-jackson</artifactId>
</dependency>

Serialization using Jackson Databind

To serialize using Jackson Databind include the following dependency:

implementation("io.micronaut:micronaut-jackson-databind")
<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-jackson-databind</artifactId>
</dependency>

JsonMapper

You may be used to work with Jackson’s ObjectMapper. However, we don’t recommend using Jackson’s ObjectMapper directly; instead you should use JsonMapper, an API almost identical to Jackson’s ObjectMapper. Moreover, both Micronaut Serialization and Micronaut Jackson Databind implement JsonMapper.

You can inject a bean of type JsonMapper or manually instantiate one via JsonMapper.createDefault().

Binding using Reactive Frameworks

From a developer perspective however, you can generally just work with Plain Old Java Objects (POJOs) and can optionally use a Reactive framework such as RxJava or Project Reactor. The following is an example of a controller that reads and saves an incoming POJO in a non-blocking way from JSON:

Using Reactive Streams to Read the JSON
@Controller("/people")
public class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>();

@Post("/saveReactive")
@SingleResult
public Publisher<HttpResponse<Person>> save(@Body Publisher<Person> person) { // (1)
    return Mono.from(person).map(p -> {
                inMemoryDatastore.put(p.getFirstName(), p); // (2)
                return HttpResponse.created(p); // (3)
            }
    );
}

}
Using Reactive Streams to Read the JSON
@Controller("/people")
class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>()

@Post("/saveReactive")
@SingleResult
Publisher<HttpResponse<Person>> save(@Body Publisher<Person> person) { // (1)
    Mono.from(person).map({ p ->
        inMemoryDatastore.put(p.getFirstName(), p) // (2)
        HttpResponse.created(p) // (3)
    })
}

}
Using Reactive Streams to Read the JSON
@Controller("/people")
class PersonController {

    internal var inMemoryDatastore: MutableMap<String, Person> = ConcurrentHashMap()

@Post("/saveReactive")
@SingleResult
fun save(@Body person: Publisher<Person>): Publisher<HttpResponse<Person>> { // (1)
    return Mono.from(person).map { p ->
        inMemoryDatastore[p.firstName] = p // (2)
        HttpResponse.created(p) // (3)
    }
}

}
1 The method receives a Publisher which emits the POJO once the JSON has been read
2 The map method stores the instance in a Map
3 An HttpResponse is returned

Using cURL from the command line, you can POST JSON to the /people URI:

Using cURL to Post JSON
$ curl -X POST localhost:8080/people -d '{"firstName":"Fred","lastName":"Flintstone","age":45}'

Binding Using CompletableFuture

The same method as the previous example can also be written with the CompletableFuture API instead:

Using CompletableFuture to Read the JSON
@Controller("/people")
public class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>();

@Post("/saveFuture")
public CompletableFuture<HttpResponse<Person>> save(@Body CompletableFuture<Person> person) {
    return person.thenApply(p -> {
                inMemoryDatastore.put(p.getFirstName(), p);
                return HttpResponse.created(p);
            }
    );
}

}
Using CompletableFuture to Read the JSON
@Controller("/people")
class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>()

@Post("/saveFuture")
CompletableFuture<HttpResponse<Person>> save(@Body CompletableFuture<Person> person) {
    person.thenApply({ p ->
        inMemoryDatastore.put(p.getFirstName(), p)
        HttpResponse.created(p)
    })
}

}
Using CompletableFuture to Read the JSON
@Controller("/people")
class PersonController {

    internal var inMemoryDatastore: MutableMap<String, Person> = ConcurrentHashMap()

@Post("/saveFuture")
fun save(@Body person: CompletableFuture<Person>): CompletableFuture<HttpResponse<Person>> {
    return person.thenApply { p ->
        inMemoryDatastore[p.firstName] = p
        HttpResponse.created(p)
    }
}

}

The above example uses the thenApply method to achieve the same as the previous example.

Binding using POJOs

Note however you can just as easily write:

Binding JSON POJOs
@Controller("/people")
public class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>();

@Post
public HttpResponse<Person> save(@Body Person person) {
    inMemoryDatastore.put(person.getFirstName(), person);
    return HttpResponse.created(person);
}

}
Binding JSON POJOs
@Controller("/people")
class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>()

@Post
HttpResponse<Person> save(@Body Person person) {
    inMemoryDatastore.put(person.getFirstName(), person)
    HttpResponse.created(person)
}

}
Binding JSON POJOs
@Controller("/people")
class PersonController {

    internal var inMemoryDatastore: MutableMap<String, Person> = ConcurrentHashMap()

@Post
fun save(@Body person: Person): HttpResponse<Person> {
    inMemoryDatastore[person.firstName] = person
    return HttpResponse.created(person)
}

}

The Micronaut framework only executes your method once the data has been read in a non-blocking manner.

You can customize the output in various ways, such as using Jackson annotations.

Jackson Configuration

If you use micronaut-jackson-databind, the Jackson’s ObjectMapper can be configured through configuration with the JacksonConfiguration class.

All Jackson configuration keys start with jackson.

dateFormat

String

The date format

locale

String

Uses Locale.forLanguageTag. Example: en-US

timeZone

String

Uses TimeZone.getTimeZone. Example: PST

serializationInclusion

String

One of JsonInclude.Include. Example: ALWAYS

propertyNamingStrategy

String

Name of an instance of PropertyNamingStrategy. Example: SNAKE_CASE

defaultTyping

String

The global defaultTyping for polymorphic type handling from enum ObjectMapper.DefaultTyping. Example: NON_FINAL

Example:

jackson.serializationInclusion=ALWAYS
jackson:
  serializationInclusion: ALWAYS
[jackson]
  serializationInclusion="ALWAYS"
jackson {
  serializationInclusion = "ALWAYS"
}
{
  jackson {
    serializationInclusion = "ALWAYS"
  }
}
{
  "jackson": {
    "serializationInclusion": "ALWAYS"
  }
}

Features

If you use micronaut-jackson-databind, all Jackson’s features can be configured with their name as the key and a boolean to indicate enabled or disabled.

serialization

Map

SerializationFeature

deserialization

Map

DeserializationFeature

mapper

Map

MapperFeature

parser

Map

JsonParser.Feature

generator

Map

JsonGenerator.Feature

factory

Map

JsonFactory.Feature

Example:

jackson.serialization.indentOutput=true
jackson.serialization.writeDatesAsTimestamps=false
jackson.deserialization.useBigIntegerForInts=true
jackson.deserialization.failOnUnknownProperties=false
jackson:
  serialization:
    indentOutput: true
    writeDatesAsTimestamps: false
  deserialization:
    useBigIntegerForInts: true
    failOnUnknownProperties: false
[jackson]
  [jackson.serialization]
    indentOutput=true
    writeDatesAsTimestamps=false
  [jackson.deserialization]
    useBigIntegerForInts=true
    failOnUnknownProperties=false
jackson {
  serialization {
    indentOutput = true
    writeDatesAsTimestamps = false
  }
  deserialization {
    useBigIntegerForInts = true
    failOnUnknownProperties = false
  }
}
{
  jackson {
    serialization {
      indentOutput = true
      writeDatesAsTimestamps = false
    }
    deserialization {
      useBigIntegerForInts = true
      failOnUnknownProperties = false
    }
  }
}
{
  "jackson": {
    "serialization": {
      "indentOutput": true,
      "writeDatesAsTimestamps": false
    },
    "deserialization": {
      "useBigIntegerForInts": true,
      "failOnUnknownProperties": false
    }
  }
}

Further customising JsonFactory

If you use micronaut-jackson-databind, there may be situations where you wish to customise the JsonFactory used by the ObjectMapper beyond the configuration of features (for example to allow custom character escaping). This can be achieved by providing your own JsonFactory bean, or by providing a BeanCreatedEventListener<JsonFactory> which configures the default bean on startup.

Support for @JsonView

If you use micronaut-jackson-databind, you can use the @JsonView annotation on controller methods if you set jackson.json-view.enabled to true in your configuration file (e.g application.yml).

Jackson’s @JsonView annotation lets you control which properties are exposed on a per-response basis. See Jackson JSON Views for more information.

Beans

If you use micronaut-jackson-databind, in addition to configuration, beans can be registered to customize Jackson. All beans that extend any of the following classes are registered with the object mapper:

Service Loader

Any modules registered via the service loader are also added to the default object mapper.

Number Precision

During JSON parsing, the framework may convert any incoming data to an intermediate object model. By default, this model uses BigInteger, long and double for numeric values. This means some information that could be represented by BigDecimal may be lost. For example, numbers with many decimal places that cannot be represented by double may be truncated, even if the target type for deserialization uses BigDecimal. Metadata on the number of trailing zeroes (BigDecimal.precision()), e.g. the difference between 0.12 and 0.120, is also discarded.

If you need full accuracy for number types, use the following configuration:

jackson.deserialization.useBigIntegerForInts=true
jackson.deserialization.useBigDecimalForFloats=true
jackson:
  deserialization:
    useBigIntegerForInts: true
    useBigDecimalForFloats: true
[jackson]
  [jackson.deserialization]
    useBigIntegerForInts=true
    useBigDecimalForFloats=true
jackson {
  deserialization {
    useBigIntegerForInts = true
    useBigDecimalForFloats = true
  }
}
{
  jackson {
    deserialization {
      useBigIntegerForInts = true
      useBigDecimalForFloats = true
    }
  }
}
{
  "jackson": {
    "deserialization": {
      "useBigIntegerForInts": true,
      "useBigDecimalForFloats": true
    }
  }
}

6.15 Plain Text Responses

By default, a Micronaut Controller responds with content-type application/json. However, you can respond with content type text/plain by annotating the controller method with the @Produces annotation.

HTTP Response with text/plain Content-Type
@Controller("/txt")
public class TextPlainController {

    @Get("/date")
    @Produces(MediaType.TEXT_PLAIN) // (1)
    String date() {
        return new Calendar.Builder().setDate(2023,7,4).build().toString(); // (2)
    }

}
HTTP Response with text/plain Content-Type
@Controller('/txt')
class TextPlainController {

    @Get('/date')
    @Produces(MediaType.TEXT_PLAIN) // (1)
    String date() {
        new Calendar.Builder().setDate(2023,7,4).build().toString() // (2)
    }

}
HTTP Response with text/plain Content-Type
@Controller("/txt")
class TextPlainController {

    @Get("/date")
    @Produces(MediaType.TEXT_PLAIN) // (1)
    fun date(): String = Calendar.Builder().setDate(2023, 7, 4).build().toString() // (2)

}
1 The Controller endpoint specifies a response’s Content-Type of text/plain.
2 The endpoint returns type String, and the implementation explicitly converts the data to a string using the toString() method.
Micronaut Framework 4.x text/plain responses are more restrictive about allowed types than Micronaut Framework 3.x. To return plain text responses for answers other than java.lang.String, manually call the object toString() method. Alternatively, set the micronaut.http.legacy-text-conversion configuration option to true to restore the old – but not recommended – Micronaut Framework 3.x behavior.

6.16 Data Validation

It is easy to validate incoming data with Micronaut controllers using Validation Advice.

The Micronaut framework provides native support for the jakarta.validation annotations with the micronaut-validation dependency:

annotationProcessor("io.micronaut.validation:micronaut-validation-processor")
<annotationProcessorPaths>
    <path>
        <groupId>io.micronaut.validation</groupId>
        <artifactId>micronaut-validation-processor</artifactId>
    </path>
</annotationProcessorPaths>

implementation("io.micronaut.validation:micronaut-validation")
<dependency>
    <groupId>io.micronaut.validation</groupId>
    <artifactId>micronaut-validation</artifactId>
</dependency>

Or full JSR 380 compliance with the micronaut-hibernate-validator dependency:

implementation("io.micronaut.beanvalidation:micronaut-hibernate-validator")
<dependency>
    <groupId>io.micronaut.beanvalidation</groupId>
    <artifactId>micronaut-hibernate-validator</artifactId>
</dependency>

We can validate parameters using jakarta.validation annotations and the Validated annotation at the class level.

Example
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.validation.Validated;

import jakarta.validation.constraints.NotBlank;
import java.util.Collections;

@Validated // (1)
@Controller("/email")
public class EmailController {

    @Get("/send")
    public HttpResponse send(@NotBlank String recipient, // (2)
                             @NotBlank String subject) { // (2)
        return HttpResponse.ok(Collections.singletonMap("msg", "OK"));
    }
}
Example
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.validation.Validated

import jakarta.validation.constraints.NotBlank

@Validated // (1)
@Controller("/email")
class EmailController {

    @Get("/send")
    HttpResponse send(@NotBlank String recipient, // (2)
                      @NotBlank String subject) { // (2)
        HttpResponse.ok(msg: "OK")
    }
}
Example
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.validation.Validated
import jakarta.validation.constraints.NotBlank

@Validated // (1)
@Controller("/email")
open class EmailController {

    @Get("/send")
    open fun send(@NotBlank recipient: String, // (2)
                  @NotBlank subject: String): HttpResponse<*> { // (2)
        return HttpResponse.ok(mapOf("msg" to "OK"))
    }
}
1 Annotate controller with Validated
2 subject and recipient cannot be blank.

If a validation error occurs a jakarta.validation.ConstraintViolationException is thrown. By default, the integrated io.micronaut.validation.exception.ConstraintExceptionHandler handles the exception, leading to a behaviour as shown in the following test:

Example Test
@Test
void testParametersAreValidated() {
    HttpClientResponseException e = assertThrows(HttpClientResponseException.class, () ->
        client.toBlocking().exchange("/email/send?subject=Hi&recipient="));
    HttpResponse<?> response = e.getResponse();

    assertEquals(HttpStatus.BAD_REQUEST, response.getStatus());

    response = client.toBlocking().exchange("/email/send?subject=Hi&recipient=me@micronaut.example");

    assertEquals(HttpStatus.OK, response.getStatus());
}
Example Test
def "test parameter validation"() {
    when:
    client.toBlocking().exchange('/email/send?subject=Hi&recipient=')

    then:
    def e = thrown(HttpClientResponseException)
    def response = e.response
    response.status == HttpStatus.BAD_REQUEST

    when:
    response = client.toBlocking().exchange('/email/send?subject=Hi&recipient=me@micronaut.example')

    then:
    response.status == HttpStatus.OK
}
Example Test
"test params are validated"() {
    val e = shouldThrow<HttpClientResponseException> {
        client.toBlocking().exchange<Any>("/email/send?subject=Hi&recipient=")
    }
    var response = e.response

    response.status shouldBe HttpStatus.BAD_REQUEST

    response = client.toBlocking().exchange<Any>("/email/send?subject=Hi&recipient=me@micronaut.example")

    response.status shouldBe HttpStatus.OK
}

To use your own ExceptionHandler to handle the constraint exceptions, annotate it with @Replaces(ConstraintExceptionHandler.class)

Often you may want to use POJOs as controller method parameters.

import io.micronaut.core.annotation.Introspected;

import jakarta.validation.constraints.NotBlank;

@Introspected
public class Email {

    @NotBlank // (1)
    String subject;

    @NotBlank // (1)
    String recipient;

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public String getRecipient() {
        return recipient;
    }

    public void setRecipient(String recipient) {
        this.recipient = recipient;
    }
}
import io.micronaut.core.annotation.Introspected

import jakarta.validation.constraints.NotBlank

@Introspected
class Email {

    @NotBlank // (1)
    String subject

    @NotBlank // (1)
    String recipient
}
import io.micronaut.core.annotation.Introspected
import jakarta.validation.constraints.NotBlank

@Introspected
open class Email {

    @NotBlank // (1)
    var subject: String? = null

    @NotBlank // (1)
    var recipient: String? = null
}
1 You can use jakarta.validation annotations in your POJOs.

Annotate your controller with Validated, and annotate the binding POJO with @Valid.

Example
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Body;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import io.micronaut.validation.Validated;

import jakarta.validation.Valid;
import java.util.Collections;

@Validated // (1)
@Controller("/email")
public class EmailController {

    @Post("/send")
    public HttpResponse send(@Body @Valid Email email) { // (2)
        return HttpResponse.ok(Collections.singletonMap("msg", "OK"));
    }
}
Example
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.validation.Validated

import jakarta.validation.Valid

@Validated // (1)
@Controller("/email")
class EmailController {

    @Post("/send")
    HttpResponse send(@Body @Valid Email email) { // (2)
        HttpResponse.ok(msg: "OK")
    }
}
Example
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.validation.Validated
import jakarta.validation.Valid

@Validated // (1)
@Controller("/email")
open class EmailController {

    @Post("/send")
    open fun send(@Body @Valid email: Email): HttpResponse<*> { // (2)
        return HttpResponse.ok(mapOf("msg" to "OK"))
    }
}
1 Annotate the controller with Validated
2 Annotate the POJO to validate with @Valid

Validation of POJOs is shown in the following test:

@Test
void testPojoValidation() {
    HttpClientResponseException e = assertThrows(HttpClientResponseException.class, () -> {
        Email email = new Email();
        email.subject = "Hi";
        email.recipient = "";
        client.toBlocking().exchange(HttpRequest.POST("/email/send", email));
    });
    HttpResponse<?> response = e.getResponse();

    assertEquals(HttpStatus.BAD_REQUEST, response.getStatus());

    Email email = new Email();
    email.subject = "Hi";
    email.recipient = "me@micronaut.example";
    response = client.toBlocking().exchange(HttpRequest.POST("/email/send", email));

    assertEquals(HttpStatus.OK, response.getStatus());
}
def "invoking /email/send parse parameters in a POJO and validates"() {
    when:
    Email email = new Email(subject: 'Hi', recipient: '')
    client.toBlocking().exchange(HttpRequest.POST('/email/send', email))

    then:
    def e = thrown(HttpClientResponseException)
    def response = e.response
    response.status == HttpStatus.BAD_REQUEST

    when:
    email = new Email(subject: 'Hi', recipient: 'me@micronaut.example')
    response = client.toBlocking().exchange(HttpRequest.POST('/email/send', email))

    then:
    response.status == HttpStatus.OK
}
"test pojo validation" {
    val e = shouldThrow<HttpClientResponseException> {
        val email = Email()
        email.subject = "Hi"
        email.recipient = ""
        client.toBlocking().exchange<Email, Any>(HttpRequest.POST("/email/send", email))
    }
    var response = e.response

    response.status shouldBe HttpStatus.BAD_REQUEST

    val email = Email()
    email.subject = "Hi"
    email.recipient = "me@micronaut.example"
    response = client.toBlocking().exchange<Email, Any>(HttpRequest.POST("/email/send", email))

    response.status shouldBe HttpStatus.OK
}
Bean injection is supported in custom constraints with the Hibernate Validator configuration.

6.16.1 Validation Groups

You can enforce a subset of constraints using validation groups using groups on Validated. More information is available in the Bean Validation specification

import jakarta.validation.groups.Default;

public interface FinalValidation extends Default {} // (1)
import jakarta.validation.groups.Default

interface FinalValidation extends Default {} // (1)
import jakarta.validation.groups.Default

interface FinalValidation : Default {} // (1)
1 Define a custom validation group. This one extends Default so any validations done with this group will include constraints in the Default group.
import io.micronaut.core.annotation.Introspected;

import jakarta.validation.constraints.NotBlank;

@Introspected
public class Email {

    @NotBlank // (1)
    String subject;

    @NotBlank(groups = FinalValidation.class) // (2)
    String recipient;

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public String getRecipient() {
        return recipient;
    }

    public void setRecipient(String recipient) {
        this.recipient = recipient;
    }
}
import io.micronaut.core.annotation.Introspected

import jakarta.validation.constraints.NotBlank

@Introspected
class Email {

    @NotBlank // (1)
    String subject

    @NotBlank(groups = FinalValidation) // (2)
    String recipient
}
import io.micronaut.core.annotation.Introspected
import jakarta.validation.constraints.NotBlank

@Introspected
open class Email {

    @NotBlank // (1)
    var subject: String? = null

    @NotBlank(groups = [FinalValidation::class]) // (2)
    var recipient: String? = null
}
1 Specify a constraint using the Default validation group. This constraint will only be enforced when Default is active.
2 Specify a constraint using the custom FinalValidation validation group. This constraint will only be enforced when FinalValidation is active.

Annotate your controller with Validated, specifying the validation groups that will be active or letting it default to Default. Also annotate the binding POJO with @Valid.

Example
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Body;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import io.micronaut.validation.Validated;

import jakarta.validation.Valid;
import java.util.Collections;

@Validated // (1)
@Controller("/email")
public class EmailController {

    @Post("/createDraft")
    public HttpResponse createDraft(@Body @Valid Email email) { // (2)
        return HttpResponse.ok(Collections.singletonMap("msg", "OK"));
    }

    @Post("/send")
    @Validated(groups = FinalValidation.class) // (3)
    public HttpResponse send(@Body @Valid Email email) { // (4)
        return HttpResponse.ok(Collections.singletonMap("msg", "OK"));
    }
}
Example
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.validation.Validated

import jakarta.validation.Valid

@Validated // (1)
@Controller("/email")
class EmailController {

    @Post("/createDraft")
    HttpResponse createDraft(@Body @Valid Email email) { // (2)
        HttpResponse.ok(msg: "OK")
    }

    @Post("/send")
    @Validated(groups = [FinalValidation]) // (3)
    HttpResponse send(@Body @Valid Email email) { // (4)
        HttpResponse.ok(msg: "OK")
    }
}
Example
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.validation.Validated
import jakarta.validation.Valid

@Validated // (1)
@Controller("/email")
open class EmailController {

    @Post("/createDraft")
    open fun createDraft(@Body @Valid email: Email): HttpResponse<*> { // (2)
        return HttpResponse.ok(mapOf("msg" to "OK"))
    }

    @Post("/send")
    @Validated(groups = [FinalValidation::class]) // (3)
    open fun send(@Body @Valid email: Email): HttpResponse<*> { // (4)
        return HttpResponse.ok(mapOf("msg" to "OK"))
    }
}
1 Annotating with Validated without specifying groups means that the Default group will be active. Since this is defined on the class, it will apply to all methods.
2 Constraints in the Default validation group will be enforced, inheriting from the class. The effect is that @NotBlank on email.recipient will not be enforced when this method is called.
3 Specifying groups means that these validation groups will be enforced when this method is called. Note that FinalValidation extends Default so constraints from both groups will be enforced.
4 Constraints in the Default and FinalValidation validation groups will be enforced, since FinalValidation extends Default. The effect is that both @NotBlank constraints in email will be enforced when this method is called.

Validation of POJOs using the default validation group is shown in the following test:

@Test
void testPojoValidation_defaultGroup() {
    HttpClientResponseException e = assertThrows(HttpClientResponseException.class, () -> {
        Email email = new Email();
        email.subject = "";
        email.recipient = "";
        client.toBlocking().exchange(HttpRequest.POST("/email/createDraft", email));
    });
    HttpResponse<?> response = e.getResponse();

    assertEquals(HttpStatus.BAD_REQUEST, response.getStatus());

    Email email = new Email();
    email.subject = "Hi";
    email.recipient = "";
    response = client.toBlocking().exchange(HttpRequest.POST("/email/createDraft", email));

    assertEquals(HttpStatus.OK, response.getStatus());
}
def "invoking /email/createDraft parse parameters in a POJO and validates using default validation groups"() {
    when:
    Email email = new Email(subject: '', recipient: '')
    client.toBlocking().exchange(HttpRequest.POST('/email/createDraft', email))

    then:
    def e = thrown(HttpClientResponseException)
    def response = e.response
    response.status == HttpStatus.BAD_REQUEST

    when:
    email = new Email(subject: 'Hi', recipient: '')
    response = client.toBlocking().exchange(HttpRequest.POST('/email/createDraft', email))

    then:
    response.status == HttpStatus.OK
}
"test pojo validation using default validation groups" {
    val e = shouldThrow<HttpClientResponseException> {
        val email = Email()
        email.subject = ""
        email.recipient = ""
        client.toBlocking().exchange<Email, Any>(HttpRequest.POST("/email/createDraft", email))
    }
    var response = e.response

    response.status shouldBe HttpStatus.BAD_REQUEST

    val email = Email()
    email.subject = "Hi"
    email.recipient = ""
    response = client.toBlocking().exchange<Email, Any>(HttpRequest.POST("/email/createDraft", email))

    response.status shouldBe HttpStatus.OK
}

Validation of POJOs using the custom FinalValidation validation group is shown in the following test:

@Test
void testPojoValidation_finalValidationGroup() {
    HttpClientResponseException e = assertThrows(HttpClientResponseException.class, () -> {
        Email email = new Email();
        email.subject = "Hi";
        email.recipient = "";
        client.toBlocking().exchange(HttpRequest.POST("/email/send", email));
    });
    HttpResponse<?> response = e.getResponse();

    assertEquals(HttpStatus.BAD_REQUEST, response.getStatus());

    Email email = new Email();
    email.subject = "Hi";
    email.recipient = "me@micronaut.example";
    response = client.toBlocking().exchange(HttpRequest.POST("/email/send", email));

    assertEquals(HttpStatus.OK, response.getStatus());
}
def "invoking /email/send parse parameters in a POJO and validates using FinalValidation validation group"() {
    when:
    Email email = new Email(subject: 'Hi', recipient: '')
    client.toBlocking().exchange(HttpRequest.POST('/email/send', email))

    then:
    def e = thrown(HttpClientResponseException)
    def response = e.response
    response.status == HttpStatus.BAD_REQUEST

    when:
    email = new Email(subject: 'Hi', recipient: 'me@micronaut.example')
    response = client.toBlocking().exchange(HttpRequest.POST('/email/send', email))

    then:
    response.status == HttpStatus.OK
}
"test pojo validation using FinalValidation validation group" {
    val e = shouldThrow<HttpClientResponseException> {
        val email = Email()
        email.subject = "Hi"
        email.recipient = ""
        client.toBlocking().exchange<Email, Any>(HttpRequest.POST("/email/send", email))
    }
    var response = e.response

    response.status shouldBe HttpStatus.BAD_REQUEST

    val email = Email()
    email.subject = "Hi"
    email.recipient = "me@micronaut.example"
    response = client.toBlocking().exchange<Email, Any>(HttpRequest.POST("/email/send", email))

    response.status shouldBe HttpStatus.OK
}

6.17 Serving Static Resources

Static resource resolution is enabled by default. The Micronaut framework supports resolving resources from the classpath or the file system.

See the information below for available configuration options:

🔗
Table 1. Configuration Properties for StaticResourceConfiguration
Property Type Description

micronaut.router.static-resources.*.enabled

boolean

micronaut.router.static-resources.*.mapping

java.lang.String

The static resource mapping.

micronaut.router.static-resources.*.paths

java.util.List

A list of paths either starting with classpath: or file:. You can serve files from anywhere on disk or the classpath. For example to serve static resources from src/main/resources/public, you would use classpath:public as the path.

Read the Serving static resources in a Micronaut Application guide, a step-by-step tutorial, to learn how to expose static resources such as CSS or images in a Micronaut Framework application.

6.18 Error Handling

Sometimes with distributed applications, bad things happen. Having a good way to handle errors is important.

6.18.1 Status Handlers

The @Error annotation supports defining either an exception class or an HTTP status. Methods annotated with @Error must be defined within a class annotated with @Controller. The annotation also supports the notion of global and local, local being the default.

Local error handlers only respond to exceptions thrown as a result of the route being matched to another method in the same controller. Global error handlers can be invoked as a result of any thrown exception. A local error handler is always searched for first when resolving which handler to execute.

When defining an error handler for an exception, you can specify the exception instance as an argument to the method and omit the exception property of the annotation.
See the guide for Error Handling to learn more.

6.18.2 Local Error Handling

For example, the following method handles JSON parse exceptions from Jackson for the scope of the declaring controller:

Local exception handler
@Error
public HttpResponse<JsonError> jsonError(HttpRequest request, JsonSyntaxException e) { // (1)
    JsonError error = new JsonError("Invalid JSON: " + e.getMessage()) // (2)
            .link(Link.SELF, Link.of(request.getUri()));

    return HttpResponse.<JsonError>status(HttpStatus.BAD_REQUEST, "Fix Your JSON")
            .body(error); // (3)
}
Local exception handler
@Error
HttpResponse<JsonError> jsonError(HttpRequest request, JsonSyntaxException e) { // (1)
    JsonError error = new JsonError("Invalid JSON: " + e.message) // (2)
            .link(Link.SELF, Link.of(request.uri))

    HttpResponse.<JsonError>status(HttpStatus.BAD_REQUEST, "Fix Your JSON")
            .body(error) // (3)
}
Local exception handler
@Error
fun jsonError(request: HttpRequest<*>, e: JsonSyntaxException): HttpResponse<JsonError> { // (1)
    val error = JsonError("Invalid JSON: ${e.message}") // (2)
            .link(Link.SELF, Link.of(request.uri))

    return HttpResponse.status<JsonError>(HttpStatus.BAD_REQUEST, "Fix Your JSON")
            .body(error) // (3)
}
1 A method that explicitly handles JsonSyntaxException is declared
2 An instance of JsonError is returned.
3 A custom response is returned to handle the error
Local status handler
@Error(status = HttpStatus.NOT_FOUND)
public HttpResponse<JsonError> notFound(HttpRequest request) { // (1)
    JsonError error = new JsonError("Person Not Found") // (2)
            .link(Link.SELF, Link.of(request.getUri()));

    return HttpResponse.<JsonError>notFound()
            .body(error); // (3)
}
Local status handler
@Error(status = HttpStatus.NOT_FOUND)
HttpResponse<JsonError> notFound(HttpRequest request) { // (1)
    JsonError error = new JsonError("Person Not Found") // (2)
            .link(Link.SELF, Link.of(request.uri))

    HttpResponse.<JsonError>notFound()
            .body(error) // (3)
}
Local status handler
@Error(status = HttpStatus.NOT_FOUND)
fun notFound(request: HttpRequest<*>): HttpResponse<JsonError> { // (1)
    val error = JsonError("Person Not Found") // (2)
            .link(Link.SELF, Link.of(request.uri))

    return HttpResponse.notFound<JsonError>()
            .body(error) // (3)
}
1 The Error declares which HttpStatus error code to handle (in this case 404)
2 A JsonError instance is returned for all 404 responses
3 An NOT_FOUND response is returned

Similar to other controller methods, error handlers can use request binding annotations on parameters e.g. to access header values. However, binding the request body comes with additional restrictions depending on the HTTP server implementation used. If the body has already been bound to a parameter of the original controller method, it may not be possible to bind the body to a different type, as the original bytes may already have been discarded.

6.18.3 Global Error Handling

Global error handler
@Error(global = true) // (1)
public HttpResponse<JsonError> error(HttpRequest request, Throwable e) {
    JsonError error = new JsonError("Bad Things Happened: " + e.getMessage()) // (2)
            .link(Link.SELF, Link.of(request.getUri()));

    return HttpResponse.<JsonError>serverError()
            .body(error); // (3)
}
Global error handler
@Error(global = true) // (1)
HttpResponse<JsonError> error(HttpRequest request, Throwable e) {
    JsonError error = new JsonError("Bad Things Happened: " + e.message) // (2)
            .link(Link.SELF, Link.of(request.uri))

    HttpResponse.<JsonError>serverError()
            .body(error) // (3)
}
Global error handler
@Error(global = true) // (1)
fun error(request: HttpRequest<*>, e: Throwable): HttpResponse<JsonError> {
    val error = JsonError("Bad Things Happened: ${e.message}") // (2)
            .link(Link.SELF, Link.of(request.uri))

    return HttpResponse.serverError<JsonError>()
            .body(error) // (3)
}
1 The @Error declares the method a global error handler
2 A JsonError instance is returned for all errors
3 An INTERNAL_SERVER_ERROR response is returned
Global status handler
@Error(status = HttpStatus.NOT_FOUND)
public HttpResponse<JsonError> notFound(HttpRequest request) { // (1)
    JsonError error = new JsonError("Person Not Found") // (2)
            .link(Link.SELF, Link.of(request.getUri()));

    return HttpResponse.<JsonError>notFound()
            .body(error); // (3)
}
Global status handler
@Error(status = HttpStatus.NOT_FOUND)
HttpResponse<JsonError> notFound(HttpRequest request) { // (1)
    JsonError error = new JsonError("Person Not Found") // (2)
            .link(Link.SELF, Link.of(request.uri))

    HttpResponse.<JsonError>notFound()
            .body(error) // (3)
}
Global status handler
@Error(status = HttpStatus.NOT_FOUND)
fun notFound(request: HttpRequest<*>): HttpResponse<JsonError> { // (1)
    val error = JsonError("Person Not Found") // (2)
            .link(Link.SELF, Link.of(request.uri))

    return HttpResponse.notFound<JsonError>()
            .body(error) // (3)
}
1 The @Error declares which HttpStatus error code to handle (in this case 404)
2 A JsonError instance is returned for all 404 responses
3 An NOT_FOUND response is returned
A few things to note about the @Error annotation. You cannot declare identical global @Error annotations. Identical non-global @Error annotations cannot be declared in the same controller. If an @Error annotation with the same parameter exists as global and another as local, the local one takes precedence.

6.18.4 ExceptionHandler

Alternatively, you can implement an ExceptionHandler, a generic hook for handling exceptions that occur during execution of an HTTP request.

An @Error annotation capturing an exception has precedence over an implementation of ExceptionHandler capturing the same exception.

6.18.4.1 Built-In Exception Handlers

The Micronaut framework ships with several built-in handlers:

Exception

Handler

jakarta.validation.ConstraintViolationException

ConstraintExceptionHandler

ContentLengthExceededException

ContentLengthExceededHandler

ConversionErrorException

ConversionErrorHandler

DuplicateRouteException

DuplicateRouteHandler

HttpStatusException

HttpStatusHandler

UnsupportedMediaException

HttpStatusHandler

NotFoundException

HttpStatusHandler

NotAcceptableException

HttpStatusHandler

NotAllowedException

NotAllowedExceptionHandler

com.fasterxml.jackson.core.JsonProcessingException

JsonExceptionHandler

java.net.URISyntaxException

URISyntaxHandler

UnsatisfiedArgumentException

UnsatisfiedArgumentHandler

UnsatisfiedRouteException

UnsatisfiedRouteHandler

6.18.4.2 Custom Exception Handler

Imagine your e-commerce app throws an OutOfStockException when a book is out of stock:

public class OutOfStockException extends RuntimeException {
}
class OutOfStockException extends RuntimeException {
}
class OutOfStockException : RuntimeException()

Along with BookController:

@Controller("/books")
public class BookController {

    @Produces(MediaType.TEXT_PLAIN)
    @Get("/stock/{isbn}")
    Integer stock(String isbn) {
        throw new OutOfStockException();
    }
}
@Controller("/books")
class BookController {

    @Produces(MediaType.TEXT_PLAIN)
    @Get("/stock/{isbn}")
    Integer stock(String isbn) {
        throw new OutOfStockException()
    }
}
@Controller("/books")
class BookController {

    @Produces(MediaType.TEXT_PLAIN)
    @Get("/stock/{isbn}")
    internal fun stock(isbn: String): Int? {
        throw OutOfStockException()
    }
}

The server returns a 500 (Internal Server Error) status code if you don’t handle the exception.

To respond with 400 Bad Request as the response when the OutOfStockException is thrown, you can register a ExceptionHandler:

@Produces
@Singleton
@Requires(classes = {OutOfStockException.class, ExceptionHandler.class})
public class OutOfStockExceptionHandler implements ExceptionHandler<OutOfStockException, HttpResponse> {

    private final ErrorResponseProcessor<?> errorResponseProcessor;

    public OutOfStockExceptionHandler(ErrorResponseProcessor<?> errorResponseProcessor) {
        this.errorResponseProcessor = errorResponseProcessor;
    }

    @Override
    public HttpResponse handle(HttpRequest request, OutOfStockException e) {
        return errorResponseProcessor.processResponse(ErrorContext.builder(request)
                .cause(e)
                .errorMessage("No stock available")
                .build(), HttpResponse.badRequest()); // (1)
    }
}
@Produces
@Singleton
@Requires(classes = [OutOfStockException, ExceptionHandler])
class OutOfStockExceptionHandler implements ExceptionHandler<OutOfStockException, HttpResponse> {

    private final ErrorResponseProcessor<?> errorResponseProcessor

    OutOfStockExceptionHandler(ErrorResponseProcessor<?> errorResponseProcessor) {
        this.errorResponseProcessor = errorResponseProcessor
    }

    @Override
    HttpResponse handle(HttpRequest request, OutOfStockException e) {
        errorResponseProcessor.processResponse(ErrorContext.builder(request)
                .cause(e)
                .errorMessage("No stock available")
                .build(), HttpResponse.badRequest()) // (1)
    }
}
@Produces
@Singleton
@Requirements(
    Requires(classes = [OutOfStockException::class, ExceptionHandler::class])
)
class OutOfStockExceptionHandler(private val errorResponseProcessor: ErrorResponseProcessor<Any>) :
    ExceptionHandler<OutOfStockException, HttpResponse<*>> {

    override fun handle(request: HttpRequest<*>, exception: OutOfStockException): HttpResponse<*> {
        return errorResponseProcessor.processResponse(
                ErrorContext.builder(request)
                    .cause(exception)
                    .errorMessage("No stock available")
                    .build(), HttpResponse.badRequest<Any>()) // (1)
    }
}
1 The default ErrorResponseProcessor is used to create the body of the response

6.18.5 Error Formatting

The Micronaut framework produces error responses via a bean of type ErrorResponseProcessor.

JSON error responses are provided with a bean of type JsonErrorResponseBodyProvider. The default implementation outputs vnd.error responses.

HTML error responses are provided via a bean of type HtmlErrorResponseBodyProvider. The default implementation outputs HTML which can be localized with codes such as: <status>.error.bold, <status>.error.title, <status>.error. For example, you could localize the default 404 error page into Spanish:

404.error.bold=La página que buscabas no existe
404.error.title=No encontrado
404.error=Es posible que haya escrito mal la dirección o que la página se haya movido.

If customization of the response other than items related to the errors is desired, the exception handler that is handling the exception needs to be overridden.

6.19 API Versioning

Since 1.1.x, the Micronaut framework supports API versioning via a dedicated @Version annotation.

The following example demonstrates how to version an API:

Versioning an API
import io.micronaut.core.version.annotation.Version;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;

@Controller("/versioned")
class VersionedController {

    @Version("1") // (1)
    @Get("/hello")
    String helloV1() {
        return "helloV1";
    }

    @Version("2") // (2)
    @Get("/hello")
    String helloV2() {
        return "helloV2";
    }
Versioning an API
import io.micronaut.core.version.annotation.Version
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/versioned")
class VersionedController {

    @Version("1") // (1)
    @Get("/hello")
    String helloV1() {
        "helloV1"
    }

    @Version("2") // (2)
    @Get("/hello")
    String helloV2() {
        "helloV2"
    }
Versioning an API
import io.micronaut.core.version.annotation.Version
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get

@Controller("/versioned")
internal class VersionedController {

    @Version("1") // (1)
    @Get("/hello")
    fun helloV1(): String {
        return "helloV1"
    }

    @Version("2") // (2)
    @Get("/hello")
    fun helloV2(): String {
        return "helloV2"
    }
1 The helloV1 method is declared as version 1
2 The helloV2 method is declared as version 2

Then enable versioning by setting micronaut.router.versioning.enabled to true in your configuration file (e.g application.yml):

Enabling Versioning
micronaut.router.versioning.enabled=true
micronaut:
  router:
    versioning:
      enabled: true
[micronaut]
  [micronaut.router]
    [micronaut.router.versioning]
      enabled=true
micronaut {
  router {
    versioning {
      enabled = true
    }
  }
}
{
  micronaut {
    router {
      versioning {
        enabled = true
      }
    }
  }
}
{
  "micronaut": {
    "router": {
      "versioning": {
        "enabled": true
      }
    }
  }
}

By default, the Micronaut framework has two strategies for resolving the version based on an HTTP header named X-API-VERSION or a request parameter named api-version, however this is configurable. A full configuration example can be seen below:

Configuring Versioning
micronaut.router.versioning.enabled=true
micronaut.router.versioning.parameter.enabled=false
micronaut.router.versioning.parameter.names=v,api-version
micronaut.router.versioning.header.enabled=true
micronaut.router.versioning.header.names[0]=X-API-VERSION
micronaut.router.versioning.header.names[1]=Accept-Version
micronaut:
  router:
    versioning:
      enabled: true
      parameter:
        enabled: false
        names: 'v,api-version'
      header:
        enabled: true
        names:
          - 'X-API-VERSION'
          - 'Accept-Version'
[micronaut]
  [micronaut.router]
    [micronaut.router.versioning]
      enabled=true
      [micronaut.router.versioning.parameter]
        enabled=false
        names="v,api-version"
      [micronaut.router.versioning.header]
        enabled=true
        names=[
          "X-API-VERSION",
          "Accept-Version"
        ]
micronaut {
  router {
    versioning {
      enabled = true
      parameter {
        enabled = false
        names = "v,api-version"
      }
      header {
        enabled = true
        names = ["X-API-VERSION", "Accept-Version"]
      }
    }
  }
}
{
  micronaut {
    router {
      versioning {
        enabled = true
        parameter {
          enabled = false
          names = "v,api-version"
        }
        header {
          enabled = true
          names = ["X-API-VERSION", "Accept-Version"]
        }
      }
    }
  }
}
{
  "micronaut": {
    "router": {
      "versioning": {
        "enabled": true,
        "parameter": {
          "enabled": false,
          "names": "v,api-version"
        },
        "header": {
          "enabled": true,
          "names": ["X-API-VERSION", "Accept-Version"]
        }
      }
    }
  }
}
  • This example enables versioning

  • parameter.enabled enables or disables parameter-based versioning

  • parameter.names specifies the parameter names as a comma-separated list

  • header.enabled enables or disables header-based versioning

  • header.names specifies the header names as a list

If this is not enough you can also implement the RequestVersionResolver interface which receives the HttpRequest and can implement any strategy you choose.

Default Version

It is possible to supply a default version through configuration.

Configuring Default Version
micronaut.router.versioning.enabled=true
micronaut.router.versioning.default-version=3
micronaut:
  router:
    versioning:
      enabled: true
      default-version: 3
[micronaut]
  [micronaut.router]
    [micronaut.router.versioning]
      enabled=true
      default-version=3
micronaut {
  router {
    versioning {
      enabled = true
      defaultVersion = 3
    }
  }
}
{
  micronaut {
    router {
      versioning {
        enabled = true
        default-version = 3
      }
    }
  }
}
{
  "micronaut": {
    "router": {
      "versioning": {
        "enabled": true,
        "default-version": 3
      }
    }
  }
}
  • This example enables versioning and sets the default version

A route is not matched if the following conditions are met:

  • The default version is configured

  • No version is found in the request

  • The route defines a version

  • The route version does not match the default version

If the incoming request specifies a version, the default version has no effect.

Versioning Client Requests

Micronaut’s Declarative HTTP client also supports automatic versioning of outgoing requests via the @Version annotation.

By default, if you annotate a client interface with @Version, the value supplied to the annotation is included using the X-API-VERSION header.

For example:

import io.micronaut.core.version.annotation.Version;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.client.annotation.Client;
import org.reactivestreams.Publisher;
import io.micronaut.core.async.annotation.SingleResult;

@Client("/hello")
@Version("1") // (1)
public  interface HelloClient {

    @Get("/greeting/{name}")
    String sayHello(String name);

    @Version("2")
    @Get("/greeting/{name}")
    @SingleResult
    Publisher<String> sayHelloTwo(String name); // (2)
}
import io.micronaut.core.version.annotation.Version
import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import reactor.core.publisher.Mono


@Client("/hello")
@Version("1") // (1)
interface HelloClient {

    @Get("/greeting/{name}")
    String sayHello(String name)

    @Version("2")
    @Get("/greeting/{name}")
    Mono<String> sayHelloTwo(String name) // (2)
}
import io.micronaut.core.version.annotation.Version
import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import reactor.core.publisher.Mono

@Client("/hello")
@Version("1") // (1)
interface HelloClient {

    @Get("/greeting/{name}")
    fun sayHello(name : String) : String

    @Version("2")
    @Get("/greeting/{name}")
    fun sayHelloTwo(name : String) : Mono<String>  // (2)
}
1 The @Version annotation can be used at the type level to specify the version to use for all methods
2 When defined at the method level it is used only for that method

The default behaviour for how the version is sent for each call can be configured with DefaultClientVersioningConfiguration:

🔗
Table 1. Configuration Properties for DefaultClientVersioningConfiguration
Property Type Description

micronaut.http.client.versioning.default.headers

java.util.List

micronaut.http.client.versioning.default.parameters

java.util.List

For example to use Accept-Version as the header name:

Configuring Client Versioning
micronaut.http.client.versioning.default.headers[0]=Accept-Version
micronaut.http.client.versioning.default.headers[1]=X-API-VERSION
micronaut:
  http:
    client:
      versioning:
        default:
          headers:
            - 'Accept-Version'
            - 'X-API-VERSION'
[micronaut]
  [micronaut.http]
    [micronaut.http.client]
      [micronaut.http.client.versioning]
        [micronaut.http.client.versioning.default]
          headers=[
            "Accept-Version",
            "X-API-VERSION"
          ]
micronaut {
  http {
    client {
      versioning {
        'default' {
          headers = ["Accept-Version", "X-API-VERSION"]
        }
      }
    }
  }
}
{
  micronaut {
    http {
      client {
        versioning {
          default {
            headers = ["Accept-Version", "X-API-VERSION"]
          }
        }
      }
    }
  }
}
{
  "micronaut": {
    "http": {
      "client": {
        "versioning": {
          "default": {
            "headers": ["Accept-Version", "X-API-VERSION"]
          }
        }
      }
    }
  }
}

The default key refers to the default configuration. You can specify client-specific configuration by using the value passed to @Client (typically the service ID). For example:

Configuring Versioning
micronaut.http.client.versioning.greeting-service.headers[0]=Accept-Version
micronaut.http.client.versioning.greeting-service.headers[1]=X-API-VERSION
micronaut:
  http:
    client:
      versioning:
        greeting-service:
          headers:
            - 'Accept-Version'
            - 'X-API-VERSION'
[micronaut]
  [micronaut.http]
    [micronaut.http.client]
      [micronaut.http.client.versioning]
        [micronaut.http.client.versioning.greeting-service]
          headers=[
            "Accept-Version",
            "X-API-VERSION"
          ]
micronaut {
  http {
    client {
      versioning {
        greetingService {
          headers = ["Accept-Version", "X-API-VERSION"]
        }
      }
    }
  }
}
{
  micronaut {
    http {
      client {
        versioning {
          greeting-service {
            headers = ["Accept-Version", "X-API-VERSION"]
          }
        }
      }
    }
  }
}
{
  "micronaut": {
    "http": {
      "client": {
        "versioning": {
          "greeting-service": {
            "headers": ["Accept-Version", "X-API-VERSION"]
          }
        }
      }
    }
  }
}

The above uses a key named greeting-service which can be used to configure a client annotated with @Client('greeting-service').

6.20 Handling Form Data

To make data binding model customizations consistent between form data and JSON, the Micronaut framework uses Jackson to implement binding data from form submissions.

The advantage of this approach is that the same Jackson annotations used for customizing JSON binding can be used for form submissions.

In practice this means that to bind regular form data, the only change required to the previous JSON binding code is updating the MediaType consumed:

Binding Form Data to POJOs
@Controller("/people")
public class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>();

@Post
public HttpResponse<Person> save(@Body Person person) {
    inMemoryDatastore.put(person.getFirstName(), person);
    return HttpResponse.created(person);
}

}
Binding Form Data to POJOs
@Controller("/people")
class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>()

@Post
HttpResponse<Person> save(@Body Person person) {
    inMemoryDatastore.put(person.getFirstName(), person)
    HttpResponse.created(person)
}

}
Binding Form Data to POJOs
@Controller("/people")
class PersonController {

    internal var inMemoryDatastore: MutableMap<String, Person> = ConcurrentHashMap()

@Post
fun save(@Body person: Person): HttpResponse<Person> {
    inMemoryDatastore[person.firstName] = person
    return HttpResponse.created(person)
}

}
To avoid denial of service attacks, collection types and arrays created during binding are limited by the setting jackson.arraySizeThreshold in your configuration file (e.g application.yml)

Alternatively, instead of using a POJO you can bind form data directly to method parameters (which works with JSON too!):

Binding Form Data to Parameters
@Controller("/people")
public class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>();

@Post("/saveWithArgs")
public HttpResponse<Person> save(String firstName, String lastName, Optional<Integer> age) {
    Person p = new Person(firstName, lastName);
    age.ifPresent(p::setAge);
    inMemoryDatastore.put(p.getFirstName(), p);
    return HttpResponse.created(p);
}

}
Binding Form Data to Parameters
@Controller("/people")
class PersonController {

    Map<String, Person> inMemoryDatastore = new ConcurrentHashMap<>()

@Post("/saveWithArgs")
HttpResponse<Person> save(String firstName, String lastName, Optional<Integer> age) {
    Person p = new Person(firstName, lastName)
    age.ifPresent({ a -> p.setAge(a)})
    inMemoryDatastore.put(p.getFirstName(), p)
    HttpResponse.created(p)
}

}
Binding Form Data to Parameters
@Controller("/people")
class PersonController {

    internal var inMemoryDatastore: MutableMap<String, Person> = ConcurrentHashMap()

@Post("/saveWithArgs")
fun save(firstName: String, lastName: String, age: Optional<Int>): HttpResponse<Person> {
    val p = Person(firstName, lastName)
    age.ifPresent { p.age = it }
    inMemoryDatastore[p.firstName] = p
    return HttpResponse.created(p)
}

}

As you can see from the example above, this approach lets you use features such as support for Optional types and restrict the parameters to be bound. When using POJOs you must be careful to use Jackson annotations to exclude properties that should not be bound.

6.21 Writing Response Data

Reactively Writing Response Data

Micronaut’s HTTP server supports writing chunks of response data by returning a Publisher that emits objects that can be encoded to the HTTP response.

The following table summarizes example return type signatures and the behaviour the server exhibits to handle them:

Return Type Description

Publisher<String>

A Publisher that emits each chunk of content as a String

Flux<byte[]>

A Flux that emits each chunk of content as a byte[] without blocking

Flux<ByteBuf>

A Reactor Flux that emits each chunk as a Netty ByteBuf

Flux<Book>

When emitting a POJO, each emitted object is encoded as JSON by default without blocking

Flowable<byte[]>

A Flux that emits each chunk of content as a byte[] without blocking

Flowable<ByteBuf>

A Reactor Flux that emits each chunk as a Netty ByteBuf

Flowable<Book>

When emitting a POJO, each emitted object is encoded as JSON by default without blocking

When returning a reactive type, the server uses a Transfer-Encoding of chunked and keeps writing data until the Publisher onComplete method is called.

The server requests a single item from the Publisher, writes it, and requests the next, controlling back pressure.

It is up to the implementation of the Publisher to schedule any blocking I/O work that may be done as a result of subscribing to the publisher.
To use Project Reactor's Flux or Mono you need to add the Micronaut Reactor dependency to your project to include the necessary converters.
To use RxJava's Flowable, Single or Maybe you need to add the Micronaut RxJava dependency to your project to include the necessary converters.

Performing Blocking I/O

In some cases you may wish to integrate a library that does not support non-blocking I/O.

Writable

In this case you can return a Writable object from any controller method. The Writable interface has various signatures that allow writing to traditional blocking streams like Writer or OutputStream.

When returning a Writable, the blocking I/O operation is shifted to the I/O thread pool so the Netty event loop is not blocked.

See the section on configuring Server Thread Pools for details on how to configure the I/O thread pool to meet your application requirements.

The following example demonstrates how to use this API with Groovy’s SimpleTemplateEngine to write a server side template:

Performing Blocking I/O With Writable
import groovy.text.SimpleTemplateEngine;
import groovy.text.Template;
import io.micronaut.core.io.Writable;
import io.micronaut.core.util.CollectionUtils;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.server.exceptions.HttpServerException;

@Controller("/template")
public class TemplateController {

    private final SimpleTemplateEngine templateEngine = new SimpleTemplateEngine();
    private final Template template = initTemplate(); // (1)

    @Get(value = "/welcome", produces = MediaType.TEXT_PLAIN)
    Writable render() { // (2)
        return writer -> template.make( // (3)
            CollectionUtils.mapOf(
                    "firstName", "Fred",
                    "lastName", "Flintstone"
            )
        ).writeTo(writer);
    }

    private Template initTemplate() {
        try {
            return templateEngine.createTemplate(
                    "Dear $firstName $lastName. Nice to meet you."
            );
        } catch (Exception e) {
            throw new HttpServerException("Cannot create template");
        }
    }
}
Performing Blocking I/O With Writable
import groovy.text.SimpleTemplateEngine
import groovy.text.Template
import io.micronaut.core.io.Writable
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.server.exceptions.HttpServerException

@Controller("/template")
class TemplateController {

    private final SimpleTemplateEngine templateEngine = new SimpleTemplateEngine()
    private final Template template = initTemplate() // (1)

    @Get(value = "/welcome", produces = MediaType.TEXT_PLAIN)
    Writable render() { // (2)
        { writer ->
            template.make( // (3)
                    firstName: "Fred",
                    lastName: "Flintstone"
            ).writeTo(writer)
        }
    }

    private Template initTemplate() {
        try {
            return templateEngine.createTemplate(
                    'Dear $firstName $lastName. Nice to meet you.'
            )
        } catch (Exception e) {
            throw new HttpServerException("Cannot create template")
        }
    }
}
Performing Blocking I/O With Writable
import groovy.text.SimpleTemplateEngine
import groovy.text.Template
import io.micronaut.core.io.Writable
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.server.exceptions.HttpServerException
import java.io.Writer

@Controller("/template")
class TemplateController {

    private val templateEngine = SimpleTemplateEngine()
    private val template = initTemplate() // (1)

    @Get(value = "/welcome", produces = [MediaType.TEXT_PLAIN])
    internal fun render(): Writable { // (2)
        return { writer: Writer ->
            template.make( // (3)
                    mapOf(
                        "firstName" to "Fred",
                        "lastName" to "Flintstone"
                    )
            ).writeTo(writer)
        } as Writable
    }

    private fun initTemplate(): Template {
        return try {
            templateEngine.createTemplate(
                "Dear \$firstName \$lastName. Nice to meet you."
            )
        } catch (e: Exception) {
            throw HttpServerException("Cannot create template")
        }
    }
}
1 The controller creates a simple template
2 The controller method returns a Writable
3 The returned function receives a Writer and calls writeTo on the template.

InputStream

Another option is to return an input stream. This is useful for many scenarios that interact with other APIs that expose a stream.

Performing Blocking I/O With InputStream
@Get(value = "/write", produces = MediaType.TEXT_PLAIN)
InputStream write() {
    byte[] bytes = "test".getBytes(StandardCharsets.UTF_8);
    return new ByteArrayInputStream(bytes); // (1)
}
Performing Blocking I/O With InputStream
@Get(value = "/write", produces = MediaType.TEXT_PLAIN)
InputStream write() {
    byte[] bytes = "test".getBytes(StandardCharsets.UTF_8);
    new ByteArrayInputStream(bytes) // (1)
}
Performing Blocking I/O With InputStream
@Get(value = "/write", produces = [MediaType.TEXT_PLAIN])
fun write(): InputStream {
    val bytes = "test".toByteArray(StandardCharsets.UTF_8)
    return ByteArrayInputStream(bytes) // (1)
}
1 The input stream is returned and its contents will be the response body
The reading of the stream will be offloaded to the IO thread pool if the controller method is executed on the event loop.

404 Responses

Often, you want to respond 404 (Not Found) when you don’t find an item in your persistence layer or in similar scenarios.

See the following example:

@Controller("/books")
public class BooksController {

    @Get("/stock/{isbn}")
    public Map stock(String isbn) {
        return null; //(1)
    }

    @Get("/maybestock/{isbn}")
    @SingleResult
    public Publisher<Map> maybestock(String isbn) {
        return Mono.empty(); //(2)
    }
}
@Controller("/books")
class BooksController {

    @Get("/stock/{isbn}")
    Map stock(String isbn) {
        null //(1)
    }

    @Get("/maybestock/{isbn}")
    Mono<Map> maybestock(String isbn) {
        Mono.empty() //(2)
    }
}
@Controller("/books")
class BooksController {

    @Get("/stock/{isbn}")
    fun stock(isbn: String): Map<*, *>? {
        return null //(1)
    }

    @Get("/maybestock/{isbn}")
    fun maybestock(isbn: String): Mono<Map<*, *>> {
        return Mono.empty() //(2)
    }
}
1 Returning null triggers a 404 (Not Found) response.
2 Returning an empty Mono triggers a 404 (Not Found) response.
Responding with an empty Publisher or Flux is a streaming response, which results in an empty array being returned if the content type is JSON. Annotate the method with SingleResult to disable streaming.

To disable 404 on a null body or an empty publisher set micronaut.server.not-found-on-missing-body to false.

6.22 File Uploads

Handling of file uploads has special treatment in Micronaut. Support is provided for streaming of uploads in a non-blocking manner through streaming uploads or completed uploads.

To receive data from a multipart request, set the consumes argument of the method annotation to MULTIPART_FORM_DATA. For example:

@Post(consumes = MediaType.MULTIPART_FORM_DATA)
HttpResponse upload( ... )

Route Arguments

Method argument types determine how files are received. Data can be received a chunk at a time or when an upload is completed.

If the route argument name cannot or should not match the name of the part in the request, add the Part annotation to the argument and specify the expected name in the request.

Chunk Data Types

PartData represents a chunk of data received in a multipart request. PartData interface methods convert the data to a byte[], InputStream, or a ByteBuffer.

Data can only be retrieved from a PartData once. The underlying buffer is released, causing further attempts to fail.

Route arguments of type Publisher are treated as intended to receive a single file, and each chunk of the received file will be sent downstream. If the generic type is other than PartData, conversion will be attempted using Micronaut’s conversion service. Conversions to String and byte[] are supported by default.

If you need knowledge about the metadata of an uploaded file, the StreamingFileUpload class is a Publisher that also has file information such as the content type and file name.

Streaming file upload
import io.micronaut.core.async.annotation.SingleResult;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import io.micronaut.http.multipart.StreamingFileUpload;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;

import static io.micronaut.http.HttpStatus.CONFLICT;
import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA;
import static io.micronaut.http.MediaType.TEXT_PLAIN;

@Controller("/upload")
public class UploadController {

    @Post(value = "/", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    @SingleResult
    public Publisher<HttpResponse<String>> upload(StreamingFileUpload file) { // (2)

        File tempFile;
        try {
            tempFile = File.createTempFile(file.getFilename(), "temp");
        } catch (IOException e) {
            return Mono.error(e);
        }
        Publisher<Boolean> uploadPublisher = file.transferTo(tempFile); // (3)

        return Mono.from(uploadPublisher)  // (4)
            .map(success -> {
                if (success) {
                    return HttpResponse.ok("Uploaded");
                } else {
                    return HttpResponse.<String>status(CONFLICT)
                                       .body("Upload Failed");
                }
            });
    }

}
Streaming file upload
import io.micronaut.core.async.annotation.SingleResult
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.StreamingFileUpload
import org.reactivestreams.Publisher
import reactor.core.publisher.Mono

import static io.micronaut.http.HttpStatus.CONFLICT
import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import static io.micronaut.http.MediaType.TEXT_PLAIN

@Controller("/upload")
class UploadController {

    @Post(value = "/", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    Mono<HttpResponse<String>> upload(StreamingFileUpload file) { // (2)

        File tempFile = File.createTempFile(file.filename, "temp")
        Publisher<Boolean> uploadPublisher = file.transferTo(tempFile) // (3)

        Mono.from(uploadPublisher)  // (4)
            .map({ success ->
                if (success) {
                    HttpResponse.ok("Uploaded")
                } else {
                    HttpResponse.<String>status(CONFLICT)
                            .body("Upload Failed")
                }
            })
    }

}
Streaming file upload
import io.micronaut.core.async.annotation.SingleResult
import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus.CONFLICT
import io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import io.micronaut.http.MediaType.TEXT_PLAIN
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.StreamingFileUpload
import org.reactivestreams.Publisher
import reactor.core.publisher.Mono
import java.io.ByteArrayOutputStream
import java.io.File
import java.io.OutputStream

@Controller("/upload")
class UploadController {

    @Post(value = "/", consumes = [MULTIPART_FORM_DATA], produces = [TEXT_PLAIN]) // (1)
    fun upload(file: StreamingFileUpload): Mono<HttpResponse<String>> { // (2)

        val tempFile = File.createTempFile(file.filename, "temp")
        val uploadPublisher = file.transferTo(tempFile) // (3)

        return Mono.from(uploadPublisher)  // (4)
            .map { success ->
                if (success) {
                    HttpResponse.ok("Uploaded")
                } else {
                    HttpResponse.status<String>(CONFLICT)
                        .body("Upload Failed")
                }
            }
    }

}
1 The method consumes MULTIPART_FORM_DATA
2 The method parameters match form attribute names. In this case file will match for example an <input type="file" name="file">
3 The StreamingFileUpload.transferTo(File) method transfers the file to the server. The method returns a Publisher
4 The returned Mono subscribes to the Publisher and outputs a response once the upload is complete, without blocking.

It is also possible to pass an output stream with the transferTo method.

The reading of the file or stream will be offloaded to the IO thread pool to prevent the possibility of blocking the event loop.
Streaming file upload
import io.micronaut.core.async.annotation.SingleResult;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import io.micronaut.http.multipart.StreamingFileUpload;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;

import static io.micronaut.http.HttpStatus.CONFLICT;
import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA;
import static io.micronaut.http.MediaType.TEXT_PLAIN;

@Controller("/upload")
public class UploadController {

    @Post(value = "/outputStream", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    @SingleResult
    public Mono<HttpResponse<String>> uploadOutputStream(StreamingFileUpload file) { // (2)

        OutputStream outputStream = new ByteArrayOutputStream(); // (3)

        Publisher<Boolean> uploadPublisher = file.transferTo(outputStream); // (4)

        return Mono.from(uploadPublisher)  // (5)
                .map(success -> {
                    if (success) {
                        return HttpResponse.ok("Uploaded");
                    } else {
                        return HttpResponse.<String>status(CONFLICT)
                                .body("Upload Failed");
                    }
                });
    }

}
Streaming file upload
import io.micronaut.core.async.annotation.SingleResult
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.StreamingFileUpload
import org.reactivestreams.Publisher
import reactor.core.publisher.Mono

import static io.micronaut.http.HttpStatus.CONFLICT
import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import static io.micronaut.http.MediaType.TEXT_PLAIN

@Controller("/upload")
class UploadController {

    @Post(value = "/outputStream", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    @SingleResult
    Mono<HttpResponse<String>> uploadOutputStream(StreamingFileUpload file) { // (2)

        OutputStream outputStream = new ByteArrayOutputStream() // (3)

        Publisher<Boolean> uploadPublisher = file.transferTo(outputStream) // (4)

        Mono.from(uploadPublisher)  // (5)
                .map({ success ->
                    if (success) {
                        HttpResponse.ok("Uploaded")
                    } else {
                        HttpResponse.<String>status(CONFLICT)
                                .body("Upload Failed")
                    }
                })
    }

}
Streaming file upload
import io.micronaut.core.async.annotation.SingleResult
import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus.CONFLICT
import io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import io.micronaut.http.MediaType.TEXT_PLAIN
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.StreamingFileUpload
import org.reactivestreams.Publisher
import reactor.core.publisher.Mono
import java.io.ByteArrayOutputStream
import java.io.File
import java.io.OutputStream

@Controller("/upload")
class UploadController {

    @Post(value = "/outputStream", consumes = [MULTIPART_FORM_DATA], produces = [TEXT_PLAIN]) // (1)
    @SingleResult
    fun uploadOutputStream(file: StreamingFileUpload): Mono<HttpResponse<String>> { // (2)
        val outputStream  = ByteArrayOutputStream() // (3)
        val uploadPublisher = file.transferTo(outputStream) // (4)

        return Mono.from(uploadPublisher) // (5)
            .map { success: Boolean ->
                return@map if (success) {
                    HttpResponse.ok("Uploaded")
                } else {
                    HttpResponse.status<String>(CONFLICT)
                        .body("Upload Failed")
                }
            }
    }

}
1 The method consumes MULTIPART_FORM_DATA
2 The method parameters match form attribute names. In this case file will match for example an <input type="file" name="file">
3 A stream is created to output the data to. In real world scenarios this would come from some other source.
4 The StreamingFileUpload.transferTo(OutputStream) method transfers the file to the server. The method returns a Publisher
5 The returned Mono subscribes to the Publisher and outputs a response once the upload is complete, without blocking.

Whole Data Types

Route arguments that are not publishers cause route execution to be delayed until the upload has finished. The received data will attempt to be converted to the requested type. Conversions to a String or byte[] are supported by default. In addition, the file can be converted to a POJO if a media type codec is registered that supports the media type of the file. A media type codec is included by default that allows conversion of JSON files to POJOs.

Receiving a byte array
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA;
import static io.micronaut.http.MediaType.TEXT_PLAIN;

@Controller("/upload")
public class BytesUploadController {

    @Post(value = "/bytes", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    public HttpResponse<String> uploadBytes(byte[] file, String fileName) { // (2)
        try {
            File tempFile = File.createTempFile(fileName, "temp");
            Path path = Paths.get(tempFile.getAbsolutePath());
            Files.write(path, file); // (3)
            return HttpResponse.ok("Uploaded");
        } catch (IOException e) {
            return HttpResponse.badRequest("Upload Failed");
        }
    }
}
Receiving a byte array
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post

import java.nio.file.Files
import java.nio.file.Path
import java.nio.file.Paths

import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import static io.micronaut.http.MediaType.TEXT_PLAIN

@Controller("/upload")
class BytesUploadController {

    @Post(value = "/bytes", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    HttpResponse<String> uploadBytes(byte[] file, String fileName) { // (2)
        try {
            File tempFile = File.createTempFile(fileName, "temp")
            Path path = Paths.get(tempFile.absolutePath)
            Files.write(path, file) // (3)
            HttpResponse.ok("Uploaded")
        } catch (IOException e) {
            HttpResponse.badRequest("Upload Failed")
        }
    }
}
Receiving a byte array
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType
import io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import io.micronaut.http.MediaType.TEXT_PLAIN
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import java.io.File
import java.io.IOException
import java.nio.file.Files
import java.nio.file.Paths

@Controller("/upload")
class BytesUploadController {

    @Post(value = "/bytes", consumes = [MULTIPART_FORM_DATA], produces = [TEXT_PLAIN]) // (1)
    fun uploadBytes(file: ByteArray, fileName: String): HttpResponse<String> { // (2)
        return try {
            val tempFile = File.createTempFile(fileName, "temp")
            val path = Paths.get(tempFile.absolutePath)
            Files.write(path, file) // (3)
            HttpResponse.ok("Uploaded")
        } catch (e: IOException) {
            HttpResponse.badRequest("Upload Failed")
        }
    }
}

If you need knowledge about the metadata of an uploaded file, the CompletedFileUpload class has methods to retrieve the data of the file, and also file information such as the content type and file name.

File upload with metadata
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import io.micronaut.http.multipart.CompletedFileUpload;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA;
import static io.micronaut.http.MediaType.TEXT_PLAIN;

@Controller("/upload")
public class CompletedUploadController {

    @Post(value = "/completed", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    public HttpResponse<String> uploadCompleted(CompletedFileUpload file) { // (2)
        try {
            File tempFile = File.createTempFile(file.getFilename(), "temp"); //(3)
            Path path = Paths.get(tempFile.getAbsolutePath());
            Files.write(path, file.getBytes()); //(3)
            return HttpResponse.ok("Uploaded");
        } catch (IOException e) {
            return HttpResponse.badRequest("Upload Failed");
        }
    }
}
File upload with metadata
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.CompletedFileUpload

import java.nio.file.Files
import java.nio.file.Path
import java.nio.file.Paths

import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import static io.micronaut.http.MediaType.TEXT_PLAIN

@Controller("/upload")
class CompletedUploadController {

    @Post(value = "/completed", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    HttpResponse<String> uploadCompleted(CompletedFileUpload file) { // (2)
        try {
            File tempFile = File.createTempFile(file.filename, "temp") //(3)
            Path path = Paths.get(tempFile.absolutePath)
            Files.write(path, file.bytes) //(3)
            HttpResponse.ok("Uploaded")
        } catch (IOException e) {
            HttpResponse.badRequest("Upload Failed")
        }
    }
}
File upload with metadata
import io.micronaut.http.HttpResponse
import io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import io.micronaut.http.MediaType.TEXT_PLAIN
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.CompletedFileUpload
import java.io.File
import java.io.IOException
import java.nio.file.Files
import java.nio.file.Paths

@Controller("/upload")
class CompletedUploadController {

    @Post(value = "/completed", consumes = [MULTIPART_FORM_DATA], produces = [TEXT_PLAIN]) // (1)
    fun uploadCompleted(file: CompletedFileUpload): HttpResponse<String> { // (2)
        return try {
            val tempFile = File.createTempFile(file.filename, "temp") //(3)
            val path = Paths.get(tempFile.absolutePath)
            Files.write(path, file.bytes) //(3)
            HttpResponse.ok("Uploaded")
        } catch (e: IOException) {
            HttpResponse.badRequest("Upload Failed")
        }
    }
}
1 The method consumes MULTIPART_FORM_DATA
2 The method parameters match form attribute names. In this case the file will match for example an <input type="file" name="file">
3 The CompletedFileUpload instance gives access to metadata about the upload as well as access to the file contents.
If a file will not be read, the discard method on the file object must be called to prevent memory leaks.

Multiple Uploads

Different Names

If a multipart request has multiple uploads that have different part names, create an argument to your route that receives each part. For example:

HttpResponse upload(String title, String name)

A route method signature like the above expects two different parts, one named "title" and the other "name".

Same Name

To receive multiple parts with the same part name, the argument must be a Publisher. When used in one of the following ways, the publisher emits one item per part found with the specified name. The publisher must accept one of the following types:

For example:

HttpResponse upload(Publisher<StreamingFileUpload> files)
HttpResponse upload(Publisher<CompletedFileUpload> files)
HttpResponse upload(Publisher<MyObject> files)
HttpResponse upload(Publisher<Publisher<PartData>> files)
HttpResponse upload(Publisher<CompletedPart> attributes)

Whole Body Binding

When request part names aren’t known ahead of time, or to read the entire body, a special type can be used to indicate the entire body is desired.

If a route has an argument of type MultipartBody (not to be confused with the class for the client) annotated with @Body, each part of the request will be emitted through the argument. A MultipartBody is a publisher of CompletedPart instances.

For example:

Binding to the entire multipart body
import io.micronaut.http.annotation.Body;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Post;
import io.micronaut.http.multipart.CompletedFileUpload;
import io.micronaut.http.multipart.CompletedPart;
import io.micronaut.http.server.multipart.MultipartBody;
import org.reactivestreams.Publisher;
import org.reactivestreams.Subscriber;
import org.reactivestreams.Subscription;
import reactor.core.publisher.Mono;
import io.micronaut.core.async.annotation.SingleResult;
import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA;
import static io.micronaut.http.MediaType.TEXT_PLAIN;

@Controller("/upload")
public class WholeBodyUploadController {

    @Post(value = "/whole-body", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    @SingleResult
    public Publisher<String> uploadBytes(@Body MultipartBody body) { // (2)

        return Mono.create(emitter -> {
            body.subscribe(new Subscriber<CompletedPart>() {
                private Subscription s;

                @Override
                public void onSubscribe(Subscription s) {
                    this.s = s;
                    s.request(1);
                }

                @Override
                public void onNext(CompletedPart completedPart) {
                    String partName = completedPart.getName();
                    if (completedPart instanceof CompletedFileUpload upload) {
                        String originalFileName = upload.getFilename();
                    }
                }

                @Override
                public void onError(Throwable t) {
                    emitter.error(t);
                }

                @Override
                public void onComplete() {
                    emitter.success("Uploaded");
                }
            });
        });
    }
}
Binding to the entire multipart body
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.CompletedFileUpload
import io.micronaut.http.multipart.CompletedPart
import io.micronaut.http.server.multipart.MultipartBody
import org.reactivestreams.Subscriber
import org.reactivestreams.Subscription
import reactor.core.publisher.Mono

import static io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import static io.micronaut.http.MediaType.TEXT_PLAIN

@Controller("/upload")
class WholeBodyUploadController {

    @Post(value = "/whole-body", consumes = MULTIPART_FORM_DATA, produces = TEXT_PLAIN) // (1)
    Mono<String> uploadBytes(@Body MultipartBody body) { // (2)

        Mono.<String>create({ emitter ->
            body.subscribe(new Subscriber<CompletedPart>() {
                private Subscription s

                @Override
                void onSubscribe(Subscription s) {
                    this.s = s
                    s.request(1)
                }

                @Override
                void onNext(CompletedPart completedPart) {
                    String partName = completedPart.name
                    if (completedPart instanceof CompletedFileUpload) {
                        String originalFileName = completedPart.filename
                    }
                }

                @Override
                void onError(Throwable t) {
                    emitter.error(t)
                }

                @Override
                void onComplete() {
                    emitter.success("Uploaded")
                }
            })
        })
    }
}
Binding to the entire multipart body
import io.micronaut.http.MediaType.MULTIPART_FORM_DATA
import io.micronaut.http.MediaType.TEXT_PLAIN
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import io.micronaut.http.multipart.CompletedFileUpload
import io.micronaut.http.multipart.CompletedPart
import io.micronaut.http.server.multipart.MultipartBody
import org.reactivestreams.Subscriber
import org.reactivestreams.Subscription
import reactor.core.publisher.Mono

@Controller("/upload")
class WholeBodyUploadController {

    @Post(value = "/whole-body", consumes = [MULTIPART_FORM_DATA], produces = [TEXT_PLAIN]) // (1)
    fun uploadBytes(@Body body: MultipartBody): Mono<String> { // (2)
        return Mono.create { emitter ->
            body.subscribe(object : Subscriber<CompletedPart> {
                private var s: Subscription? = null

                override fun onSubscribe(s: Subscription) {
                    this.s = s
                    s.request(1)
                }

                override fun onNext(completedPart: CompletedPart) {
                    val partName = completedPart.name
                    if (completedPart is CompletedFileUpload) {
                        val originalFileName = completedPart.filename
                    }
                }

                override fun onError(t: Throwable) {
                    emitter.error(t)
                }

                override fun onComplete() {
                    emitter.success("Uploaded")
                }
            })
        }
    }
}

6.23 File Transfers

The Micronaut framework supports sending files to the client in a couple of easy ways.

Sending File Objects

It is possible to return a File object from your controller method, and the data will be returned to the client. The Content-Type header of file responses is calculated based on the name of the file.

To control either the media type of the file being sent, or to set the file to be downloaded (i.e. using the Content-Disposition header), instead construct a SystemFile with the file to use. For example:

Sending a SystemFile
@Get
public SystemFile download() {
    File file = ...
    return new SystemFile(file).attach("myfile.txt");
    // or new SystemFile(file, MediaType.TEXT_HTML_TYPE)
}

Sending an InputStream

For cases where a reference to a File object is not possible (for example resources in JAR files), the Micronaut framework supports transferring input streams. To return a stream of data from the controller method, construct a StreamedFile.

The constructor for StreamedFile also accepts a java.net.URL for your convenience.
Sending a StreamedFile
@Get
public StreamedFile download() {
    InputStream inputStream = ...
    return new StreamedFile(inputStream, MediaType.TEXT_PLAIN_TYPE)
    // An attach(String filename) method is also available to set the Content-Disposition
}

The server supports returning 304 (Not Modified) responses if the files being transferred have not changed, and the request contains the appropriate header. In addition, if the client accepts encoded responses, the Micronaut framework encodes the file if appropriate. Encoding happens if the file is text-based and larger than 1KB by default. The threshold at which data is encoded is configurable. See the server configuration reference for details.

To use a custom data source to send data through an input stream, construct a PipedInputStream and PipedOutputStream to write data from the output stream to the input. Make sure to do the work on a separate thread so the file can be returned immediately.

Cache Configuration

By default, file responses include caching headers. The following options determine how the Cache-Control header is built.

🔗
Table 1. Configuration Properties for NettyHttpServerConfiguration$FileTypeHandlerConfiguration
Property Type Description

micronaut.server.netty.responses.file.cache-seconds

int

🔗
Table 2. Configuration Properties for NettyHttpServerConfiguration$FileTypeHandlerConfiguration$CacheControlConfiguration
Property Type Description

micronaut.server.netty.responses.file.cache-control.public

boolean

6.24 HTTP Filters

The Micronaut HTTP server supports applying filters to request/response processing in a similar (but reactive) way to Servlet filters in traditional Java applications.

Filters support the following use cases: