Table of Contents

Download a big file with StreamingHttpClient

Learn how to stream responses with ReactorStreamingHttpClient

Authors: 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:

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=reactor-http-client \
    --build=gradle \
    --lang=java \
    --test=junit
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.

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 reactor-http-client 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. Micronaut Reactor HTTP Client

To use an implementation of Micronaut HTTP Client based on Project Reactor, add the following dependency:

build.gradle
implementation("io.micronaut.reactor:micronaut-reactor-http-client")

6. Controller

Create a controller that streams a PNG downloaded from another URL to the response.

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

import io.micronaut.context.exceptions.ConfigurationException;
import io.micronaut.core.io.buffer.ByteBuffer;
import io.micronaut.core.io.buffer.ReferenceCounted;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.reactor.http.client.ReactorStreamingHttpClient;
import jakarta.annotation.PreDestroy;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Flux;

import java.net.MalformedURLException;
import java.net.URI;
import java.net.URL;

@Controller (1)
class HomeController implements AutoCloseable {
    private static final Logger LOG = LoggerFactory.getLogger(HomeController.class);
    private static final URI DEFAULT_URI = URI.create("https://guides.micronaut.io/micronaut5K.png");

    private final ReactorStreamingHttpClient reactorStreamingHttpClient;

    HomeController() {
        String urlStr = "https://guides.micronaut.io/";
        URL url;
        try {
            url = new URL(urlStr);
        } catch (MalformedURLException e) {
            throw new ConfigurationException("malformed URL" + urlStr);
        }
        this.reactorStreamingHttpClient = ReactorStreamingHttpClient.create(url); (2)
    }

    @Get (3)
    Flux<ByteBuffer<?>> download() {
        HttpRequest<?> request = HttpRequest.GET(DEFAULT_URI);
        return reactorStreamingHttpClient.dataStream(request).doOnNext(bb -> {
            if (bb instanceof ReferenceCounted rc) {
                rc.retain();
            }
        }); (4)
    }

    @PreDestroy (5)
    @Override
    public void close() {
        if (reactorStreamingHttpClient != null) {
            reactorStreamingHttpClient.close();
        }
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /.
2 ReactorStreamingHttpClient is a variation of the StreamingHttpClient interface for Project Reactor, which extends HttpClient to support responses streaming.
3 The @Get annotation maps the method to an HTTP GET request.
4 The dataStream method emits instances of ByteBuffer. These instances are garbage collected after each chunk is emitted. In order to propagate the chunks to the response stream they need to be retained by calling the retain() method. The framework will call release() on these garbage collected chunks once each one is written to the response.
5 To invoke a method when the bean is destroyed, use the jakarta.annotation.PreDestroy annotation.

7. Test

Create a test that verifies the controller’s download hash matches the same file hash, which is also stored in src/test/resources.

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

import io.micronaut.core.io.ResourceLoader;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
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 org.junit.jupiter.api.condition.DisabledInNativeImage;

import java.io.IOException;
import java.net.URISyntaxException;
import java.net.URL;
import java.nio.file.Files;
import java.nio.file.Path;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.HexFormat;
import java.util.Optional;

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

@MicronautTest (1)
class HomeControllerTest {

    private final HexFormat hexFormat = HexFormat.of();

    @DisabledInNativeImage
    @Test
    void downloadFile(@Client("/") HttpClient httpClient, (2)
                      ResourceLoader resourceLoader)
            throws URISyntaxException, IOException, NoSuchAlgorithmException {
        Optional<URL> resource = resourceLoader.getResource("micronaut5K.png");
        assertTrue(resource.isPresent());
        byte[] expectedByteArray = Files.readAllBytes(Path.of(resource.get().toURI()));

        MessageDigest digest = MessageDigest.getInstance("SHA-256");
        byte[] expectedEncodedHash = digest.digest(expectedByteArray);
        String expected = hexFormat.formatHex(expectedEncodedHash);

        BlockingHttpClient client = httpClient.toBlocking();
        HttpResponse<byte[]> resp = assertDoesNotThrow(() -> client.exchange(HttpRequest.GET("/"), byte[].class));
        byte[] responseBytes = resp.body();

        byte[] responseEncodedHash = digest.digest(responseBytes);
        String response = hexFormat.formatHex(responseEncodedHash);
        assertEquals(expected, response);
    }
}
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.

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 File Transfers and the Micronaut HTTP Client.

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…​).