Creating your first Micronaut application

Learn how to create a Hello World Micronaut application with a controller and a functional test.

Authors: Iván López, Sergio del Amo

Micronaut Version: 3.1.0

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin.

2. What you will need

To complete this guide, you will need the following:

  • Some time on your hands

  • A decent text editor or IDE

  • JDK 1.8 or greater installed with JAVA_HOME configured appropriately

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

4. Writing the Application

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide --build=maven --lang=kotlin
If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

4.1. Enable annotation Processing

If you use Java or Kotlin and IntelliJ IDEA, make sure to enable annotation processing.

annotationprocessorsintellij

4.2. Application

Application.kt is used when running the application via Gradle or via deployment. You can also run the main class directly within your IDE if it is configured correctly.

src/main/kotlin/example/micronaut/Application.kt
package example.micronaut

import io.micronaut.runtime.Micronaut.*
fun main(args: Array<String>) {
        build()
            .args(*args)
                .packages("example.micronaut")
                .start()
}

4.3. Controller

In order to create a microservice that responds with "Hello World" you first need a controller.

Create a Controller:

src/main/kotlin/example/micronaut/HelloController.kt
package example.micronaut

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

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

    @Get(2)
    @Produces(MediaType.TEXT_PLAIN) (3)
    fun index(): String {
        return "Hello World" (4)
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /hello
2 The @Get annotation maps the index method to an HTTP GET request on /hello
3 By default a Micronaut response uses application/json as Content-Type. We are returning a String not a JSON object, so we set it to text/plain.
4 A String "Hello World" is returned as the result

4.4. Test

Create a test to verify that when you make a GET request to /hello you get Hello World as a response:

src/test/kotlin/example/micronaut/HelloControllerTest.kt
package example.micronaut

import io.micronaut.http.HttpRequest
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Assertions.assertNotNull
import org.junit.jupiter.api.Test
import jakarta.inject.Inject

@MicronautTest (1)
class HelloControllerTest {
    @Inject
    @field:Client("/")  (2)
    lateinit var client : HttpClient

    @Test
    fun testHello() {
        val request: HttpRequest<Any> = HttpRequest.GET("/hello")  (3)
        val body = client.toBlocking().retrieve(request)
        assertNotNull(body)
        assertEquals("Hello World", body)
    }
}
1 Annotate the class with @MicronautTest so the Micronaut framework will initialize the application context and the embedded server. More info.
2 Inject the HttpClient bean and point it to the embedded server.
3 Creating HTTP Requests is easy thanks to the Micronaut framework fluid API.

5. Testing the Application

To run the tests:

$ ./mvnw test

6. Running the Application

To run the application, use the ./mvnw mn:run command which starts the application on port 8080.

7. Generate a Micronaut Application Native Image with GraalVM

We will use GraalVM, the polyglot embeddable virtual machine, to generate a native image of our Micronaut application.

Compiling native images ahead of time with GraalVM improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

7.1. Native image generation

The easiest way to install GraalVM is to use SDKMan.io.

Java 11
$ sdk install java 21.3.0.r11-grl
Java 17
$ sdk install java 21.3.0.r17-grl
If you still use Java 8 use the GraalVM JDK11 version.

You need to install the native-image component, which is not installed by default.

$ gu install native-image

To generate a native image using Maven run:

$ ./mvnw package -Dpackaging=native-image

The native image is created in target/application and can be run with ./target/application.

You can execute the endpoint exposed by the native image:

$ curl localhost:8080/hello
Hello World

8. Next steps

Read more about Micronaut testing.

9. Help with the Micronaut Framework

Object Computing, Inc. (OCI) sponsored the creation of this Guide. A variety of consulting and support services are available.