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 Groovy.

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=gradle --lang=groovy
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. Application

Application.groovy 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/groovy/example/micronaut/Application.groovy
package example.micronaut

import io.micronaut.runtime.Micronaut
import groovy.transform.CompileStatic

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

4.2. Controller

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

Create a Controller:

src/main/groovy/example/micronaut/HelloController.groovy
package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Produces
import io.micronaut.http.MediaType

@CompileStatic
@Controller("/hello") (1)
class HelloController {
    @Get (2)
    @Produces(MediaType.TEXT_PLAIN) (3)
    String index() {
        "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

4.3. Test

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

src/test/groovy/example/micronaut/HelloControllerSpec.groovy
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.spock.annotation.MicronautTest
import spock.lang.Specification

import jakarta.inject.Inject

@MicronautTest (1)
class HelloControllerSpec extends Specification {

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

    void "test hello world response"() {
        when:
        HttpRequest request = HttpRequest.GET('/hello')  (3)
        String rsp = client.toBlocking().retrieve(request)

        then:
        rsp == "Hello World"
    }
}
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:

$ ./gradlew test
$ open build/reports/tests/test/index.html

6. Running the Application

To run the application, use the ./gradlew run command which starts the application on port 8080.

7. Next steps

Read more about Micronaut testing.

8. Help with the Micronaut Framework

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