Creating your first Micronaut app

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

Authors: Sergio del Amo

Micronaut Version: 1.1.0

1 Getting Started

In this guide we are going to create a Micronaut app written in Java.

1.1 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

1.2 Solution

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

Download and unzip the source

Clone the Git repository: git clone https://github.com/micronaut-guides/creating-your-first-micronaut-app.git

Then, cd into the complete folder which you will find in the root project of the downloaded/cloned project.

2 Writing the App

Create an app using the Micronaut Command Line Interface.

mn create-app example.micronaut.complete

The previous command creates a micronaut app with the default package example.micronaut in a folder named complete.

By default, create-app generates a Java Micronaut app and it uses Gradle build system. However, you could use other build tool such as Maven or other programming languages such as Groovy or Kotlin.

If you are using Java or Kotlin and IntelliJ IDEA make sure you have enabled annotation processing.

3 Application

Application.java 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/java/example/micronaut/Application.java

package example.micronaut;

import io.micronaut.runtime.Micronaut;

public class Application {

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

4 Controller

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

Create a Controller:

src/main/java/example/micronaut/HelloController.java

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)
public class HelloController {
    @Get("/") (2)
    @Produces(MediaType.TEXT_PLAIN) (3)
    public String index() {
        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 is used to map the index method to all requests that use an HTTP GET
3	By default a Micronaut’s response uses `application/json` as `Content-Type`. We are returning a String not a JSON object. Because of that, we set it to `text/plain`.
4	A String "Hello World" is returned as the result

5 Test

Create a Junit test which verifies that when you do a GET request to /hello you get Hello World as a response:

src/test/java/example/micronaut/HelloControllerTest.java

package example.micronaut;

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

import io.micronaut.http.HttpRequest;
import io.micronaut.http.client.RxHttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.test.annotation.MicronautTest;
import org.junit.jupiter.api.Test;

import javax.inject.Inject;

@MicronautTest (1)
public class HelloControllerTest {

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

    @Test
    public void testHello() throws Exception {
        HttpRequest request = HttpRequest.GET("/hello"); (3)
        String body = client.toBlocking().retrieve(request);

        assertNotNull(body);
        assertEquals(body, "Hello World");
    }
}

1	Annotate the class with `@MicronautTest` so Micronaut will initialize the application context and the embedded server.
2	Inject the `RxHttpClient` bean. It is used the execute an HTTP call to the controller.
3	Creating HTTP Requests is easy thanks to Micronaut’s fluid API.

6 Testing the Application

To run the tests:

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

7 Running the Application

To run the application use the ./gradlew run command which will start the application on port 8080.