Table of Contents

Localize your application

Respond to a localized version of your application with LocalizedMessageSource.

Authors: Sergio del Amo

Micronaut Version: 4.4.0

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:

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 \
    --features=graalvm \
    --build=maven \
    --lang=java \
    --test=junit
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.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

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

If you use Micronaut Launch, select Micronaut Application as application type and add graalvm features.

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.

Setup IntelliJ IDEA to develop Micronaut Applications.

4.1. Properties

Create a default messages.properties file:

src/main/resources/i18n/messages.properties
hello.world=Hello World

Create a messages_es.properties file for Spanish locale:

src/main/resources/i18n/messages_es.properties
hello.world=Hola Mundo

4.2. Message Source

Create a MessageSource that uses the previous properties files:

src/main/java/example/micronaut/MessageSourceFactory.java
package example.micronaut;

import io.micronaut.context.MessageSource;
import io.micronaut.context.annotation.Factory;
import io.micronaut.context.i18n.ResourceBundleMessageSource;
import jakarta.inject.Singleton;

@Factory (1)
class MessageSourceFactory {
    @Singleton (2)
    MessageSource createMessageSource() {
        return new ResourceBundleMessageSource("i18n.messages");
    }
}
1 A class annotated with the @Factory annotated is a factory. It provides one or more methods annotated with a bean scope annotation (e.g. @Singleton). Read more about Bean factories.
2 Use jakarta.inject.Singleton to designate a class as a singleton.

4.3. Controller

src/main/java/example/micronaut/HelloWorldController.java
package example.micronaut;

import io.micronaut.context.LocalizedMessageSource;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.annotation.Produces;

import java.util.Optional;

@Controller (1)
public class HelloWorldController {

    private final LocalizedMessageSource messageSource;

    public HelloWorldController(LocalizedMessageSource messageSource) { (2)
        this.messageSource = messageSource;
    }

    @Produces(MediaType.TEXT_PLAIN) (3)
    @Get (4)
    Optional<String> index() { (5)
        return messageSource.getMessage("hello.world"); (6)
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /.
2 Use constructor injection to inject a bean of type LocalizedMessageSource.
3 Set the response content-type to text/plain with the @Produces annotation.
4 LocalizedMessageSource contains methods to fetch a message with a code, a default message, or variables.

4.4. Tests

src/test/java/example/micronaut/HelloWorldControllerTest.java
package example.micronaut;

import io.micronaut.http.HttpHeaders;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.client.BlockingHttpClient;
import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import jakarta.inject.Inject;
import org.junit.jupiter.api.Test;

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

@MicronautTest (1)
class HelloWorldControllerTest {

    @Inject
    @Client("/") (2)
    HttpClient httpClient;

    @Test
    void useOfLocalizedMessageSource() {
        BlockingHttpClient client = httpClient.toBlocking();
        HttpRequest<?> request = HttpRequest.GET("/")
                .header(HttpHeaders.ACCEPT_LANGUAGE, "es"); (3)
        assertEquals("Hola Mundo", client.retrieve(request));
        request = HttpRequest.GET("/");
        assertEquals("Hello World", client.retrieve(request));
    }

}
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. Learn More

The application changes the locale with the Accept-Language HTTP Header. Learn more about Locale Resolution.

7. 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…​).