mn create-app example.micronaut.micronautguide --build=maven --lang=java
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: 4.6.3
1. Getting Started
In this guide, we will create a Micronaut application written in Java.
2. What you will need
To complete this guide, you will need the following:
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Some time on your hands
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A decent text editor or IDE (e.g. IntelliJ IDEA)
-
JDK 21 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.
-
Download and unzip the source
4. Writing the Application
Create an application using the Micronaut Command Line Interface or with Micronaut Launch.
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool. If you don’t specify the --lang argument, Java is used as the language.If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.
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The previous command creates a Micronaut application with the default package example.micronaut
in a directory named micronautguide
.
4.1. 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.
package example.micronaut;
import io.micronaut.runtime.Micronaut;
public class Application {
public static void main(String[] args) {
Micronaut.run(Application.class, args);
}
}
4.2. Controller
In order to create a microservice that responds with "Hello World" you first need a controller.
Create a Controller:
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 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 with the @Produces annotation. |
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:
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.MediaType;
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.Test;
import jakarta.inject.Inject;
@MicronautTest (1)
public class HelloControllerTest {
@Inject
@Client("/") (2)
HttpClient client;
@Test
public void testHello() {
HttpRequest<?> request = HttpRequest.GET("/hello").accept(MediaType.TEXT_PLAIN); (3)
String 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 Executable with GraalVM
We will use GraalVM, the polyglot embeddable virtual machine, to generate a native executable of our Micronaut application.
Compiling native executables 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.
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7.1. GraalVM installation
sdk install java 21.0.5-graal
For installation on Windows, or for manual installation on Linux or Mac, see the GraalVM Getting Started documentation.
The previous command installs Oracle GraalVM, which is free to use in production and free to redistribute, at no cost, under the GraalVM Free Terms and Conditions.
Alternatively, you can use the GraalVM Community Edition:
sdk install java 21.0.2-graalce
7.2. Native executable generation
To generate a native executable using Maven, run:
./mvnw package -Dpackaging=native-image
The native executable is created in the target
directory and can be run with target/micronautguide
.
You can execute the endpoint exposed by the native executable:
curl localhost:8080/hello
Hello World
8. Next steps
Read more about Micronaut testing.
9. Help with the Micronaut Framework
The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.
10. License
All guides are released with an Apache license 2.0 license for the code and a Creative Commons Attribution 4.0 license for the writing and media (images…). |