Table of Contents

Content negotiation in a Micronaut Application

Learn how to respond HTML or JSON depending on the request Accept HTTP Header.

Authors: Sergio del Amo

Micronaut Version: 4.4.2

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=views-thymeleaf \
    --build=gradle \
    --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 views-thymeleaf 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.

5. Views

To use the Thymeleaf Java template engine to render views in a Micronaut application, add the following dependency on your classpath.

build.gradle
implementation("io.micronaut.views:micronaut-views-thymeleaf")

6. Controller

Create a controller that responds HTML or JSON depending on the request Accept HTTP Header.

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

import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.annotation.Produces;
import io.micronaut.views.ModelAndView;

import java.util.Collections;
import java.util.Map;

@Controller (1)
class MessageController {

    @Produces(value = {MediaType.TEXT_HTML, MediaType.APPLICATION_JSON}) (2)
    @Get (3)
    HttpResponse<?> index(HttpRequest<?> request) { (4)
        Map<String, Object> model = Collections.singletonMap("message", "Hello World");
        Object body = accepts(request, MediaType.TEXT_HTML_TYPE)
                ? new ModelAndView<>("message.html", model)
                : model;
        return HttpResponse.ok(body);
    }

    private static boolean accepts(HttpRequest<?> request, MediaType mediaType) {
        return request.getHeaders()
                .accept()
                .stream()
                .anyMatch(it -> it.getName().contains(mediaType));
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /.
2 Use the @Produces annotation to indicate the possible response’s content type. It matches the Accept header of the request, as you will see in the test.
3 The @Get annotation maps the method to an HTTP GET request.
4 You can bind the HTTP Request as a method parameter in a controller’s method.

In case of HTML, the previous controller uses a Thymeleaf view:

src/main/resources/views/message.html
<!DOCTYPE html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<body>
<h1 th:text="${message}"></h1>
</body>
</html>

7. Test

Write a test verifying the response content type depending on the request Accept HTTP Header.

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

import io.micronaut.http.HttpRequest;
import io.micronaut.http.MediaType;
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 org.junit.jupiter.api.Test;

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

@MicronautTest (1)
class MessageControllerTest {

    @Test
    void contentNegotiation(@Client("/") HttpClient httpClient) { (2)
        BlockingHttpClient client = httpClient.toBlocking();

        String expectedJson = """
                {"message":"Hello World"}""";
        String json = assertDoesNotThrow(() -> client.retrieve(HttpRequest.GET("/")
                .accept(MediaType.APPLICATION_JSON))); (3)
        assertEquals(expectedJson, json);

        String expectedHtml = """
                <!DOCTYPE html>
                <html lang="en">
                <body>
                <h1>Hello World</h1>
                </body>
                </html>
                """;
        String html = assertDoesNotThrow(() -> client.retrieve(HttpRequest.GET("/")
                .accept(MediaType.TEXT_HTML)));
        assertEquals(expectedHtml, html);
    }
}
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.

8. Testing the Application

To run the tests:

./gradlew test

Then open build/reports/tests/test/index.html in a browser to see the results.

9. Native Tests

The io.micronaut.application Micronaut Gradle Plugin automatically integrates with GraalVM by applying the Gradle plugin for GraalVM Native Image building.

This plugin supports running tests on the JUnit Platform as native images. This means that tests will be compiled and executed as native code.

To execute the tests, execute:

./gradlew nativeTest

Then open build/reports/tests/test/index.html in a browser to see the results.

INFO: A test may be disabled within a GraalVM native image via the @DisabledInNativeImage annotation.

10. Next steps

Explore more features with Micronaut Guides.

Read more about Micronaut Views.

11. Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.

12. 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…​).