Groovy / Maven

Micronaut HTTP Client

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

Sergio del Amo, Iván López

On this guide

In this section

Getting Started

In this guide, we will create a Micronaut application written in Groovy to consume the GitHub API with the Micronaut HTTP Client.

What you will need

To complete this guide, you will need the following:

Some time on your hands
A decent text editor or IDE (e.g. IntelliJ IDEA)
JDK 21 or greater installed with JAVA_HOME configured appropriately

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

Writing the Application

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

mn create-app example.micronaut.micronautguide \
    --features=http-client \
    --build=maven \
    --lang=groovy \
    --test=spock

Note	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 http-client features.

Note	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.

Diff

Setup IntelliJ IDEA to develop Micronaut Applications.

Dependency

To use the Micronaut HTTP Client based on Netty, add the following dependency:

pom.xml

<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-http-client</artifactId>
    <scope>compile</scope>
</dependency>

To use the Micronaut HTTP Client based on Java HTTP Client, add the following dependency:

pom.xml

<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-http-client-jdk</artifactId>
    <scope>compile</scope>
</dependency>

GitHub API

In this guide, you will consume the GitHub API from a Micronaut application.

In this guide, you will fetch Micronaut Core releases via 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.

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

groovy/src/main/groovy/example/micronaut/GithubRelease.groovy

package example.micronaut

import groovy.transform.CompileStatic
import io.micronaut.serde.annotation.Serdeable

@Serdeable
@CompileStatic
class GithubRelease {
    String name
    String url
}

Configuration

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

src/main/resources/application.properties

github.organization=micronaut-projects
github.repo=micronaut-core

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

groovy/src/main/groovy/example/micronaut/GithubConfiguration.groovy

package example.micronaut

import io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.context.annotation.Requires
import io.micronaut.core.annotation.Nullable

@ConfigurationProperties(GithubConfiguration.PREFIX)
@Requires(property = GithubConfiguration.PREFIX)
class GithubConfiguration {
    public static final String PREFIX = "github"

    String organization

    String repo

    @Nullable
    String username

    @Nullable
    String token
}

JSON Codec Configuration

Add configuration to treat application/vnd.github.v3+json as JSON.

src/main/resources/application.properties

micronaut.codec.json.additional-types[0]=application/vnd.github.v3+json

HTTP Client Service Configuration

Add configuration to associate a service identifier to the GitHub API URL.

src/main/resources/application.properties

micronaut.http.services.github.url=https://api.github.com

Low Level Client

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

Create GithubLowLevelClient:

groovy/src/main/groovy/example/micronaut/GithubLowLevelClient.groovy

package example.micronaut

import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.http.uri.UriBuilder
import jakarta.inject.Singleton
import org.reactivestreams.Publisher
import static io.micronaut.http.HttpHeaders.ACCEPT
import static io.micronaut.http.HttpHeaders.USER_AGENT

@Singleton // 
class GithubLowLevelClient {

    private final HttpClient httpClient
    private final URI uri

    GithubLowLevelClient(@Client(id = "github") HttpClient httpClient,  // 
                                GithubConfiguration configuration) {  // 
        this.httpClient = httpClient
        uri = UriBuilder.of("/repos")
                .path(configuration.organization)
                .path(configuration.repo)
                .path("releases")
                .build()
    }

    Publisher<List<GithubRelease>> fetchReleases() {
        HttpRequest<?> req = HttpRequest.GET(uri) // 
                .header(USER_AGENT, "Micronaut HTTP Client") // 
                .header(ACCEPT, "application/vnd.github.v3+json, application/json") // 
        httpClient.retrieve(req, Argument.listOf(GithubRelease.class)) // 
    }
}

1	Use `jakarta.inject.Singleton` to designate a class as a singleton.
2	Inject `HttpClient` via constructor injection. The `@Client` `id` member uses `github`; the service identifier set in the configuration.
3	Inject the previously defined configuration parameters.
4	Creating HTTP Requests is easy thanks to the Micronaut framework 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>`.

Declarative Client

It is time to take a look at 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.

groovy/src/main/groovy/example/micronaut/GithubApiClient.groovy

package example.micronaut

import io.micronaut.core.async.annotation.SingleResult
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Header
import io.micronaut.http.client.annotation.Client
import org.reactivestreams.Publisher
import static io.micronaut.http.HttpHeaders.ACCEPT
import static io.micronaut.http.HttpHeaders.USER_AGENT

@Client(id = "github") // 
@Header(name = USER_AGENT, value = "Micronaut HTTP Client") // 
@Header(name = ACCEPT, value = "application/vnd.github.v3+json, application/json") // 
public interface GithubApiClient {

    @Get('/repos/${github.organization}/${github.repo}/releases') // 
    @SingleResult // 
    Publisher<List<GithubRelease>> fetchReleases() // 
}

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	Annotation to describe that an API emits a single result even if the return type is an `org.reactivestreams.Publisher`.
6	You can return any reactive type of any implementation (RxJava, Reactor…), but it’s better to use the Reactive Streams public interfaces like `Publisher`.

Controller

Create a Controller. It uses both (low-level and declarative clients). The Micronaut framework supports Reactive Streams implementations such as RxJava or Project Reactor. Thus, you can efficiently compose multiple HTTP client calls without blocking (which will limit the throughput and scalability of your application).

groovy/src/main/groovy/example/micronaut/GithubController.groovy

package example.micronaut

import io.micronaut.core.async.annotation.SingleResult
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import org.reactivestreams.Publisher

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

    private final GithubLowLevelClient githubLowLevelClient
    private final GithubApiClient githubApiClient

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

    @Get("/releases-lowlevel") // 
    @SingleResult // 
    Publisher<List<GithubRelease>> releasesWithLowLevelClient() {
        githubLowLevelClient.fetchReleases()
    }

    @Get("/releases") // 
    @SingleResult // 
    Publisher<List<GithubRelease>> fetchReleases() {
        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 maps the `index` method to all requests that use an HTTP GET
4	Annotation to describe that an API emits a single result even if the return type is an `org.reactivestreams.Publisher`.
5	The @Get annotation maps the `fetchReleases` method to an HTTP GET request on `/releases`.

Tests

We will mock the GitHub API with some releases

Note	Here we only show a single release. You can add more entries to the file if you want to test with multiple releases.

src/test/resources/single-release.json

[{"url":"https://api.github.com/repos/micronaut-projects/micronaut-core/releases/91622014","assets_url":"https://api.github.com/repos/micronaut-projects/micronaut-core/releases/91622014/assets","upload_url":"https://uploads.github.com/repos/micronaut-projects/micronaut-core/releases/91622014/assets{?name,label}","html_url":"https://github.com/micronaut-projects/micronaut-core/releases/tag/v3.8.4","id":91622014,"author":{"login":"sdelamo","id":864788,"node_id":"MDQ6VXNlcjg2NDc4OA==","avatar_url":"https://avatars.githubusercontent.com/u/864788?v=4","gravatar_id":"","url":"https://api.github.com/users/sdelamo","html_url":"https://github.com/sdelamo","followers_url":"https://api.github.com/users/sdelamo/followers","following_url":"https://api.github.com/users/sdelamo/following{/other_user}","gists_url":"https://api.github.com/users/sdelamo/gists{/gist_id}","starred_url":"https://api.github.com/users/sdelamo/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/sdelamo/subscriptions","organizations_url":"https://api.github.com/users/sdelamo/orgs","repos_url":"https://api.github.com/users/sdelamo/repos","events_url":"https://api.github.com/users/sdelamo/events{/privacy}","received_events_url":"https://api.github.com/users/sdelamo/received_events","type":"User","site_admin":false},"node_id":"RE_kwDOB2eaPM4Fdgp-","tag_name":"v3.8.4","target_commitish":"3.8.x","name":"Micronaut Framework 3.8.4","draft":false,"prerelease":false,"created_at":"2023-02-07T15:53:03Z","published_at":"2023-02-07T15:53:05Z","assets":[{"url":"https://api.github.com/repos/micronaut-projects/micronaut-core/releases/assets/94667997","id":94667997,"node_id":"RA_kwDOB2eaPM4FpITd","name":"artifacts.zip","label":"","uploader":{"login":"github-actions[bot]","id":41898282,"node_id":"MDM6Qm90NDE4OTgyODI=","avatar_url":"https://avatars.githubusercontent.com/in/15368?v=4","gravatar_id":"","url":"https://api.github.com/users/github-actions%5Bbot%5D","html_url":"https://github.com/apps/github-actions","followers_url":"https://api.github.com/users/github-actions%5Bbot%5D/followers","following_url":"https://api.github.com/users/github-actions%5Bbot%5D/following{/other_user}","gists_url":"https://api.github.com/users/github-actions%5Bbot%5D/gists{/gist_id}","starred_url":"https://api.github.com/users/github-actions%5Bbot%5D/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/github-actions%5Bbot%5D/subscriptions","organizations_url":"https://api.github.com/users/github-actions%5Bbot%5D/orgs","repos_url":"https://api.github.com/users/github-actions%5Bbot%5D/repos","events_url":"https://api.github.com/users/github-actions%5Bbot%5D/events{/privacy}","received_events_url":"https://api.github.com/users/github-actions%5Bbot%5D/received_events","type":"Bot","site_admin":false},"content_type":"application/zip","state":"uploaded","size":28063979,"download_count":33,"created_at":"2023-02-07T16:18:05Z","updated_at":"2023-02-07T16:18:06Z","browser_download_url":"https://github.com/micronaut-projects/micronaut-core/releases/download/v3.8.4/artifacts.zip"},{"url":"https://api.github.com/repos/micronaut-projects/micronaut-core/releases/assets/94669478","id":94669478,"node_id":"RA_kwDOB2eaPM4FpIqm","name":"multiple.intoto.jsonl","label":"","uploader":{"login":"github-actions[bot]","id":41898282,"node_id":"MDM6Qm90NDE4OTgyODI=","avatar_url":"https://avatars.githubusercontent.com/in/15368?v=4","gravatar_id":"","url":"https://api.github.com/users/github-actions%5Bbot%5D","html_url":"https://github.com/apps/github-actions","followers_url":"https://api.github.com/users/github-actions%5Bbot%5D/followers","following_url":"https://api.github.com/users/github-actions%5Bbot%5D/following{/other_user}","gists_url":"https://api.github.com/users/github-actions%5Bbot%5D/gists{/gist_id}","starred_url":"https://api.github.com/users/github-actions%5Bbot%5D/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/github-actions%5Bbot%5D/subscriptions","organizations_url":"https://api.github.com/users/github-actions%5Bbot%5D/orgs","repos_url":"https://api.github.com/users/github-actions%5Bbot%5D/repos","events_url":"https://api.github.com/users/github-actions%5Bbot%5D/events{/privacy}","received_events_url":"https://api.github.com/users/github-actions%5Bbot%5D/received_events","type":"Bot","site_admin":false},"content_type":"application/octet-stream","state":"uploaded","size":65612,"download_count":34,"created_at":"2023-02-07T16:32:49Z","updated_at":"2023-02-07T16:32:49Z","browser_download_url":"https://github.com/micronaut-projects/micronaut-core/releases/download/v3.8.4/multiple.intoto.jsonl"}],"tarball_url":"https://api.github.com/repos/micronaut-projects/micronaut-core/tarball/v3.8.4","zipball_url":"https://api.github.com/repos/micronaut-projects/micronaut-core/zipball/v3.8.4","body":"<!-- Release notes generated using configuration in .github/release.yml at 3.8.x -->\r\n\r\n\r\n## What's Changed\r\n\r\n### Bugs 🐛\r\n* Allow programmatic logback config if xml config is absent (#8674)\r\n* Tweak uri to account for WebLogic/Windows URIs by @mattmoss in https://github.com/micronaut-projects/micronaut-core/pull/8704\r\n\r\n### Docs 📖\r\n* Docs on self-signed cert setup (#8684)\r\n\r\n### Dependency Upgrades 🚀\r\n\r\n* Micronaut Test to 3.8.2 (#8728)\r\n* Micronaut OpenAPI to 4.8.3 (#8724)\r\n* Micronaut Data to 3.9.6 (#8711)\r\n* Micronaut Rabbit to 3.4.1 (#8709)\r\n* Micronaut Azure to 3.7.1 (#8682)\r\n* Micronaut Micrometer to 4.7.2 (#8681)\r\n\r\n### Tests ✅\r\n\r\n* Require docker for test (#8698)\r\n* Add octet stream serialization to the TCK (#8712)\r\n\r\n**Full Changelog**: https://github.com/micronaut-projects/micronaut-core/compare/v3.8.3...v3.8.4","mentions_count":1}]

Create a test to verify that both clients work as expected, and the controller echoes the output of the GitHub API in a Reactive way.

groovy/src/test/groovy/example/micronaut/GithubControllerSpec.groovy

package example.micronaut

import io.micronaut.context.ApplicationContext
import io.micronaut.context.annotation.Requires
import io.micronaut.core.io.ResourceLoader
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.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Produces
import io.micronaut.http.client.BlockingHttpClient
import io.micronaut.http.client.HttpClient
import io.micronaut.runtime.server.EmbeddedServer
import java.util.regex.Pattern
import spock.lang.Specification

class GithubControllerSpec extends Specification {
    private static Pattern MICRONAUT_RELEASE =
            Pattern.compile("Micronaut (Core |Framework )?v?\\d+.\\d+.\\d+( (RC|M)\\d)?")

    void "verify GithubReleases can Be fetched With Low Level Http Client"(String path) {
        given:
        EmbeddedServer github = ApplicationContext.run(EmbeddedServer,
                Map.of("micronaut.codec.json.additional-types", "application/vnd.github.v3+json",
                        "spec.name", "GithubControllerTest")) // 
        String url = "http://localhost:${github.port}"
        EmbeddedServer embeddedServer = ApplicationContext.run(EmbeddedServer,
                Collections.singletonMap("micronaut.http.services.github.url", url)) // 

        when:
        GithubConfiguration configuration = embeddedServer.applicationContext.getBean(GithubConfiguration)

        then:
        configuration.organization
        configuration.repo

        when:
        HttpClient httpClient = embeddedServer.getApplicationContext()
                .createBean(HttpClient, embeddedServer.getURL())
        BlockingHttpClient client = httpClient.toBlocking()
        HttpRequest<?> request = HttpRequest.GET(path)
        HttpResponse<List<GithubRelease>> rsp = client.exchange(request, // 
                Argument.listOf(GithubRelease)) // 

        then:
        HttpStatus.OK == rsp.getStatus()   // 

        when:
        List<GithubRelease> releases = rsp.body() // 

        then:
        releases
        releases.stream()
                .map(GithubRelease::getName)
                .allMatch(name -> MICRONAUT_RELEASE.matcher(name)
                        .find())

        cleanup:
        httpClient.close()
        embeddedServer.close()
        github.close()

        where:
        path << ['/github/releases', '/github/releases-lowlevel']
    }

    @Requires(property = "spec.name", value = "GithubControllerTest") // 
    @Controller
    static class GithubReleases {
        private final ResourceLoader resourceLoader;
        GithubReleases(ResourceLoader resourceLoader) {
            this.resourceLoader = resourceLoader;
        }

        @Produces("application/vnd.github.v3+json")
        @Get("/repos/micronaut-projects/micronaut-core/releases")
        Optional<String> coreReleases() {
            resourceLoader.getResourceAsStream("single-release.json") // 
                    .map(inputStream -> inputStream.text)
        }
    }
}

1	Combine `@Requires` and properties (either via the `@Property` annotation or by passing properties when starting the context) to avoid bean pollution.
2	This test mocks an HTTP Server for `GitHub` with an extra Micronaut Embedded Server. This allows you to test how your application behaves with a specific JSON response or avoid issues such as rate limits which can make your tests flaky.
3	Create a sample `single-release.json` file in the `src/test/resources` directory. To get some test data, call the GitHub API with curl or provide a release entry yourself.
4	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.
5	Micronaut HTTP Client simplifies binding a JSON array to a list of POJOs by using `Argument::listOf`.
6	Use `status` to check the HTTP status code.
7	Use `.body()` to retrieve the parsed payload.

Testing the Application

To run the tests:

./mvnw test

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, the Micronaut framework 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 your own Personal Token. Then you can use those credentials to access the GitHub API using Basic Auth.

Create a Filter:

groovy/src/main/groovy/example/micronaut/GithubFilter.groovy

package example.micronaut

import io.micronaut.context.annotation.Requires
import io.micronaut.http.MutableHttpRequest
import io.micronaut.http.annotation.ClientFilter
import io.micronaut.http.annotation.RequestFilter

@ClientFilter("/repos/**") // 
@Requires(property = GithubConfiguration.PREFIX + ".username") // 
@Requires(property = GithubConfiguration.PREFIX + ".token") // 
class GithubFilter {

    private final GithubConfiguration configuration

    GithubFilter(GithubConfiguration configuration) { // 
        this.configuration = configuration
    }

    @RequestFilter // 
    void doFilter(MutableHttpRequest<?> request) {
        request.basicAuth(configuration.username, configuration.token) // 
    }
}

1	Annotate the class with `@ClientFilter` and define the ANT Matcher pattern to intercept all the calls to the desire URI.
2	The Micronaut framework will not load the bean unless configuration properties are set.
3	Constructor injection of the configuration parameters.
4	A request filter is called before the request is sent out.
5	Enhance every request sent to GitHub API providing Basic Authentication.

Configuration Parameters

Add your GitHub username and token to src/main/resources/application.properties

github.organization=micronaut-projects
github.repo=micronaut-core
github.username=yourgithubusername
github.token=xxxxxxxxxxxx

Add a logger to src/main/resources/logback.xml to see the 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: MASKED
13:09:56.664 [default-nioEventLoopGroup-1-4] TRACE i.m.h.client.netty.DefaultHttpClient - host: api.github.com

Next Steps

Visit Micronaut HTTP Client documentation to learn more.

Help with the Micronaut Framework

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

License

Note	All guides are released with an Apache License 2.0 for the code and a Creative Commons Attribution 4.0 license for the writing and media (images).