As we all know Spring there is now a lightweigth and claimed more efficient framework called Micronaut. The key difference is it instruments the code AOT (ahead of time) ensuring less memory footprint (less caching, less dymanic proxy to be created) and so a quicker start time.

Cli Installation

Micronaut has a CLI to scaffold your application

  1. install micronaut-cli using a package manager as per Micronaut documentation

    2. Windows

    Using Chocolatey

    choco install micronaut

    Ubuntu

    With SDKMAN

    sdk install micronaut


  2. ensure $MICRONAUT_CLI_HOME/bin to PATH; it will be referred as mn
  3. check installation mn --version


Sample Application

You can use Mirconaut CLI to initiate your application

> mn create-app hello-world --build maven --features openapi

The project is scaffolded and the main class is generated

public class Application {

  public static void main(String[] args) {
    Micronaut.run(Application.class, args);
  }
}

You can start adding a Controller to spawn a server side application

@Controller("/service")
public class ServiceController {

  @Post("execute")
  @Status(HttpStatus.CREATED)
  public Single<Result> execute(@Body Request request) {
    Result result = new Result();
    result.setValue(request.getValue() * 2);
    return Single.just(result);
  }

}

To compile or build simply run mvn compile or mvn install, or use mn

mvnw mn:run

Maven Dependencies

In in your maven pom.xml

  • add compile dependencies
<dependency>
	<groupId>javax.annotation</groupId>
	<artifactId>javax.annotation-api</artifactId>
	<scope>compile</scope>
</dependency>
<dependency>
	<groupId>io.swagger.core.v3</groupId>
	<artifactId>swagger-annotations</artifactId>
	<scope>compile</scope>
</dependency>
  • configure annotation processor
<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-compiler-plugin</artifactId>
	<configuration>
		<annotationProcessorPaths combine.children="append">
			<path>
				<groupId>io.micronaut.openapi</groupId>
				<artifactId>micronaut-openapi</artifactId>
				<version>${micronaut.openapi.version}</version>
			</path>
		</annotationProcessorPaths>
		<compilerArgs>
			<arg>-Amicronaut.processing.group=fr.kdefombelle.app</arg>
			<arg>-Amicronaut.processing.module=app-service</arg>
		</compilerArgs>
	</configuration>
</plugin>

Swagger

Reference documentation is Micronaut Open API

In your application properties point swagger static resources

micronaut:
  application:
    name: sample
  router:
    static-resources:
      swagger:
        paths: classpath:META-INF/swagger
        mapping: /swagger/**
      swagger-ui:
        paths: classpath:META-INF/swagger/views/swagger-ui
        mapping: /swagger-ui/**

Document the API

@Operation(
    summary = "Double value",
    description = "Simple webservice which doubles a value provided by the requester")
  @ApiResponse(
    responseCode = "201",
    description = "Response object",
    content = @Content(mediaType = "application/json",
      schema = @Schema(type = "Response")
    )
  )
  @ApiResponse(
    responseCode = "400",
    description = "Response object",
    content = @Content(
      mediaType = "application/json",
      schema = @Schema(type = "Response")
    )
  )
  @Post("execute")
  @Status(HttpStatus.CREATED)
  public Single<Result> execute(@Body Request request) {
    Result result = new Result();
    result.setValue(request.getValue() * 2);
    return Single.just(result);
  }

Enable Swagger

Reference documentation is Micronaut Open API

Create an openapi.properties file at the root of your project containing

swagger-ui.enabled=true

In your application properties point swagger static resources, e.g.: in application.yml

micronaut:
  application:
    name: sample
  router:
    static-resources:
      swagger:
        paths: classpath:META-INF/swagger
        mapping: /swagger/**
      swagger-ui:
        paths: classpath:META-INF/swagger/views/swagger-ui
        mapping: /swagger-ui/**

Visualise

To access swagger you can run your app and go to URL http://localhost:8080/swagger-ui