Micronaut HTTP Client

Learn how to use Micronaut low-level HTTP Client. Simplify your code with the declarative HTTP client.

Authors: Sergio del Amo, Iván López

Micronaut Version: 2.5.0

1. Getting Started

In this guide we are going to create a Micronaut app written in Groovy to consume the GitHub API with the Micronaut HTTP Client.

2. 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

3. Solution

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

4. Writing the App

Create an app using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide --build=gradle --lang=groovy
If you don’t specify the --build argument, Gradle is used as a build tool.
If you don’t specify the --lang argument, Java is used as a language.

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

4.1. GitHub API

In this guide, you are going to consume the GitHub API from a Micronaut application.

In particular, we consume the List releases endpoint.

This returns a list of releases, which does not include regular Git tags that have not been associated with a release.

This API resource can be consumed by both authenticated and anonymous clients.

Initially, you will consume it anonymously, later we will discuss authentication.

Modify src/main/resources/application.yml to create some configuration parameters.

  organization: micronaut-projects
  repo: micronaut-core

To encapsulate type-safe configuration retrieval, we use a @ConfigurationProperties object:

package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.context.annotation.Requires

@Requires(property = GithubConfiguration.PREFIX)
class GithubConfiguration {

    public static final String PREFIX = "github"
    public static final String GITHUB_API_URL = "https://api.github.com"

    String organization
    String repo
    String username
    String token

In this guide, you are going to fetch Micronaut Core releases.

To consume the GitHub API, you will use Micronaut HTTP Client.

4.2. Low Level Client

Initially, you will create a Bean which uses the low-level Client API.

Create a POJO to parse the JSON response into an object:

package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.core.annotation.Introspected

class GithubRelease {
    String name
    String url

Create GithubLowLevelClient:

package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.client.RxHttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.http.uri.UriBuilder
import io.reactivex.Flowable
import io.reactivex.Maybe

import javax.inject.Singleton

import static io.micronaut.http.HttpHeaders.ACCEPT
import static io.micronaut.http.HttpHeaders.USER_AGENT

@Singleton (1)
class GithubLowLevelClient {

    private final RxHttpClient httpClient
    private final URI uri

    GithubLowLevelClient(@Client(GithubConfiguration.GITHUB_API_URL) RxHttpClient httpClient,  (2)
                         GithubConfiguration configuration) {  (3)
        this.httpClient = httpClient
        this.uri = UriBuilder.of("/repos")

    Maybe<List<GithubRelease>> fetchReleases() {
        HttpRequest<?> req = HttpRequest.GET(uri) (4)
            .header(USER_AGENT, "Micronaut HTTP Client") (5)
            .header(ACCEPT, "application/vnd.github.v3+json, application/json") (6)
        Flowable<List<GithubRelease>> flowable = httpClient.retrieve(req, Argument.listOf(GithubRelease)) (7)
        return flowable.firstElement() (8)
1 Use javax.inject.Singleton to designate a class as a singleton.
2 Inject RxClient via constructor injection.
3 Inject the previously defined configuration parameters.
4 Creating HTTP Requests is easy thanks to Micronaut’s fluid API.
5 GitHub API requires to set the User-Agent header.
6 GitHub encourages to explicitly request the version 3 via the Accept header. With @Header, you add the Accept: application/vnd.github.v3+json HTTP header to every request.
7 Use retrieve to perform an HTTP request for the given request object and convert the full HTTP response’s body into the specified type. e.g. List<GithubRelease>.
8 The retrieve method returns a Flowable which has a firstElement method that returns the first emitted item or nothing
Instead of retrieve we could have used jsonStream. You can use jsonStream() to stream arrays of type application/json or JSON streams of type application/x-json-stream. If we use retrieve, such as in the previous code listing, the operation will not block. However, it will not return until all the data has been received from the server. In the case of a JSON array that would be the whole array. However, if you are interested in just the first element of the array, jsonStream provides a better alternative since it starts streaming data from the server without needing the whole response. For example, jsonStream().firstElement() will only parse the first item in a JSON array. Hence it is more efficient.

4.3. Declarative Client

It is time to take a look at Micronaut’s support for declarative clients via the Client annotation.

Create GithubApiClient which clearly illustrates how a declarative Micronaut HTTP Client, which is generated at compile-time, simplifies our code.

package example.micronaut

import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Header
import io.micronaut.http.client.annotation.Client
import io.reactivex.Flowable

import static io.micronaut.http.HttpHeaders.ACCEPT
import static io.micronaut.http.HttpHeaders.USER_AGENT

@Client(GithubConfiguration.GITHUB_API_URL) (1)
@Header(name = USER_AGENT, value = "Micronaut HTTP Client") (2)
@Header(name = ACCEPT, value = "application/vnd.github.v3+json, application/json") (3)
interface GithubApiClient {

    @Get('/repos/${github.organization}/${github.repo}/releases') (4)
    Flowable<GithubRelease> fetchReleases() (5)
1 URL of the remote service
2 GitHub API requires to set the User-Agent header.
3 GitHub encourages to explicitly request the version 3 via the Accept header. With @Header, you add the Accept: application/vnd.github.v3+json HTTP header to every request.
4 You can use configuration parameter interpolation when you define the path of the GET endpoint.
5 You can return reactive types, such as an RxJava Flowable.

4.4. Controller

Create a Controller. It uses both (low-level and declarative clients). It showcases several Micronaut’s capabilities.

  • Micronaut supports any framework that implements Reactive Streams, including RxJava, and Reactor. Thus, you can easily and efficiently compose multiple HTTP client calls without blocking (which will limit the throughput and scalability of your application).

  • Micronaut enables you to consume/produce JSON Streams.

package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.reactivex.Flowable
import io.reactivex.Maybe

@Controller("/github") (1)
class GithubController {

    private final GithubLowLevelClient githubLowLevelClient
    private final GithubApiClient githubApiClient

    GithubController(GithubLowLevelClient githubLowLevelClient,
                            GithubApiClient githubApiClient) { (2)
        this.githubLowLevelClient = githubLowLevelClient
        this.githubApiClient = githubApiClient

    @Get("/releases-lowlevel") (3)
    Maybe<List<GithubRelease>> releasesWithLowLevelClient() { (4)
        return githubLowLevelClient.fetchReleases()

    @Get(uri = "/releases", produces = MediaType.APPLICATION_JSON_STREAM) (5)
    Flowable<GithubRelease> fetchReleases() { (6)
        return githubApiClient.fetchReleases()
1 The class is defined as a controller with the @Controller annotation mapped to the path /github.
2 Inject beans via constructor injection.
3 The @Get annotation is used to map the index method to all requests that use an HTTP GET
4 The releasesWithLowLevelClient returns a Maybe which may or may not emit an item. If an item is not emitted a 404 is returned.
5 In order to do JSON streaming you can declare a controller method that returns a application/x-json-stream of JSON objects.
6 You can return reactive types, such as an RxJava Flowable.

4.5. Tests

Create a test which verifies both clients work as expected and the controller echoes the output of the GitHub API in a Reactive way.

package example.micronaut

import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus
import io.micronaut.http.client.RxStreamingHttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import io.reactivex.Flowable
import spock.lang.Shared
import spock.lang.Specification

import javax.inject.Inject

@MicronautTest (1)
class GithubControllerSpec extends Specification {

    RxStreamingHttpClient client (2)

    List<String> expectedReleases = ["Micronaut 2.5.0", "Micronaut 2.4.4", "Micronaut 2.4.3"]

    void 'verify github releases can be fetched with low level HttpClient'() {
        HttpRequest request = HttpRequest.GET('/github/releases-lowlevel')

        HttpResponse<List<GithubRelease>> rsp = client.toBlocking().exchange(request, (3)
                Argument.listOf(GithubRelease)) (4)

        then: 'the endpoint can be accessed'
        rsp.status == HttpStatus.OK (5)
        rsp.body() (6)

        List<GithubRelease> releases = rsp.body()

        for (String name : expectedReleases) {
            assert releases*.name.contains(name)

    void 'verify github releases can be fetched with compile-time autogenerated @Client'() {
        HttpRequest request = HttpRequest.GET('/github/releases-lowlevel')

        Flowable<GithubRelease> githubReleaseStream = client.jsonStream(request, GithubRelease) (7)
        Iterable<GithubRelease> githubReleases = githubReleaseStream.blockingIterable()

        for (String name : expectedReleases) {
            assert githubReleases*.name.contains(name)
1 Annotate the class with @MicronautTest to let Micronaut starts the embedded server and inject the beans. More info: https://micronaut-projects.github.io/micronaut-test/latest/guide/index.html.
2 Inject the RxStreamingHttpClient bean in the application context.
3 Sometimes, receiving just the object is not enough, and you need information about the response. In this case, instead of retrieve you should use the exchange method.
4 Micronaut makes it easy to parse JSON into Java objects.
5 Use status to check the HTTP status code.
6 Use .body() to retrieve the parsed payload.
7 Use the jsonStream method, which returns a Flowable, to consume the endpoint which generates a JSON Stream.

5. Testing the Application

To run the tests:

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

6. HTTP Client Filter

Often, you need to include the same HTTP headers or URL parameters in a set of requests against a third-party API or when calling another Microservice. To simplify this, Micronaut includes the ability to define HttpClientFilter classes that are applied to all matching HTTP clients.

For a real world example, let us provide GitHub Authentication via an HttpClientFilter. Follow the steps in to create you own Personal Token. Then you can use those credentials to access the GitHub API using Basic Auth.

Create a Filter:

package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.context.annotation.Requires
import io.micronaut.http.HttpResponse
import io.micronaut.http.MutableHttpRequest
import io.micronaut.http.annotation.Filter
import io.micronaut.http.filter.ClientFilterChain
import io.micronaut.http.filter.HttpClientFilter
import org.reactivestreams.Publisher

@Filter("/repos/**") (1)
@Requires(property = "github.username") (2)
@Requires(property = "github.token") (2)
class GithubFilter implements HttpClientFilter {

    private final GithubConfiguration configuration

    GithubFilter(GithubConfiguration configuration) { (3)
        this.configuration = configuration

    Publisher<? extends HttpResponse<?>> doFilter(MutableHttpRequest<?> request, ClientFilterChain chain) {
        chain.proceed(request.basicAuth(configuration.getUsername(), configuration.getToken())) (4)
1 Supply the pattern you want to match to the @Filter annotation.
2 Micronaut will not load the Bean unless configuration properties are set.
3 Constructor injection of the configuration parameters.
4 Enhance every request sent to GitHub API providing Basic Authentication.

6.1. Configuration Parameters

Add your GitHub username and token to src/main/resource/application.yml

  organization: micronaut-projects
  repo: micronaut-core
  username: yourgithubusername
  token: xxxxxxxxxxxx

Add to src/main/resources/logback.xml, a logger to see Micronaut’s HTTP client output.

<logger name="io.micronaut.http.client" level="TRACE"/>

If you run again the tests, you will see the that the Filter is invoked and HTTP Basic Auth is used against GitHub API.

13:09:56.662 [default-nioEventLoopGroup-1-4] DEBUG i.m.h.client.netty.DefaultHttpClient - Sending HTTP GET to https://api.github.com/repos/micronaut-projects/micronaut-core/releases
13:09:56.663 [default-nioEventLoopGroup-1-4] TRACE i.m.h.client.netty.DefaultHttpClient - User-Agent: Micronaut HTTP Client
13:09:56.663 [default-nioEventLoopGroup-1-4] TRACE i.m.h.client.netty.DefaultHttpClient - Accept: application/json
13:09:56.663 [default-nioEventLoopGroup-1-4] TRACE i.m.h.client.netty.DefaultHttpClient - Authorization: Basic xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
13:09:56.664 [default-nioEventLoopGroup-1-4] TRACE i.m.h.client.netty.DefaultHttpClient - host: api.github.com

7. Next steps

8. Help with Micronaut

Object Computing, Inc. (OCI) sponsored the creation of this Guide. A variety of consulting and support services are available.