Creating your first Micronaut Graal application

Learn how to create a Hello World Micronaut GraalVM application.

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 with GraalVM support.

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 --features=graalvm 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.

If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features and apply those changes to your application.

4.1. Enable annotation Processing

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


4.2. Service

Create a POJO Conference:

package example.micronaut

import io.micronaut.core.annotation.Introspected

@Introspected (1)
data class Conference(val name: String)
1 Annotate the class with @Introspected

Create a Service:

package example.micronaut

import java.util.Random
import jakarta.inject.Singleton

@Singleton (1)
class ConferenceService {

    fun randomConf(): Conference = CONFERENCES[Random().nextInt(CONFERENCES.size)] (2)

    companion object {
        private val CONFERENCES = listOf(
                Conference("GR8Conf EU"),
                Conference("Micronaut Summit"),
                Conference("Devoxx Belgium"),
                Conference("Oracle Code One"),
                Conference("Codemotion Madrid")
1 Use jakarta.inject.Singleton to designate a class as a singleton.
2 Return a random conference.

4.3. Controller

Create a Controller with a method that returns a Conference. The Micronaut framework will convert it automatically to JSON in the response:

package example.micronaut

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

@Controller("/conferences") (1)
class ConferenceController(private val conferenceService: ConferenceService) { (2)

    @Get("/random") (3)
    fun randomConf(): Conference = conferenceService.randomConf() (4)
1 The class is defined as a controller with the @Controller annotation mapped to the path /conferences
2 Constructor injection
3 The @Get annotation maps the index method to an HTTP GET request on /random
4 Return a Conference.

5. 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.

5.1. Native image generation

The easiest way to install GraalVM is to use

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.

5.2. Creating native image inside Docker

The output following this approach is a Docker image that runs the native image of your application. You don’t need to install any additional dependencies.

Building GraalVM native image with Maven
$ ./mvnw package -Dpackaging=docker-native

5.3. Running the native image

Execute the application by either running the executable or starting the Docker container.

Executing the native image
10:29:46.845 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 12ms. Server Running: http://localhost:8080

We can see that the application starts in only 12ms.

5.4. Sending a request

Start the application either using Docker or the native executable. You can run a few cURL requests to test the application:

complete $ time curl localhost:8080/conferences/random
real    0m0.016s
user    0m0.005s
sys     0m0.004s

complete $ time curl localhost:8080/conferences/random
{"name":"GR8Conf EU"}
real    0m0.014s
user    0m0.005s
sys     0m0.004s

6. Next steps

Read more about GraalVM Support inside the Micronaut framework.

Take a look at the Micronaut Gradle plugin and Micronaut Maven Plugin documentation.

7. Help with the Micronaut Framework

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