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: 3.7.0

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin 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 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 \
    --build=gradle --lang=kotlin
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.

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 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.

4.1. Enable annotation Processing

If you use Java or Kotlin and IntelliJ IDEA, make sure to enable annotation processing.


4.2. GitHub API

In this guide, you will 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 io.micronaut.context.annotation.ConfigurationProperties
import io.micronaut.context.annotation.Requires

@Requires(property = GithubConfiguration.PREFIX)
class GithubConfiguration {
    var organization: String? = null
    var repo: String? = null
    var username: String? = null
    var token: String? = null

    companion object {
        const val PREFIX = "github"
        const val GITHUB_API_URL = "https://api.github.com"

In this guide, you will fetch Micronaut Core releases.

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

4.3. 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 io.micronaut.core.annotation.Introspected

data class GithubRelease(val name: String, val url: String)

Create GithubLowLevelClient:

package example.micronaut

import io.micronaut.core.type.Argument
import io.micronaut.http.HttpHeaders.ACCEPT
import io.micronaut.http.HttpHeaders.USER_AGENT
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 reactor.core.publisher.Mono
import java.net.URI

@Singleton (1)
class GithubLowLevelClient(@param:Client(GithubConfiguration.GITHUB_API_URL) private val httpClient: HttpClient,  (2)
                           configuration: GithubConfiguration) {  (3)
    private val uri: URI = UriBuilder.of("/repos")

    fun fetchReleases(): Mono<List<GithubRelease>> {
        val req: HttpRequest<*> = HttpRequest.GET<Any>(uri) (4)
            .header(USER_AGENT, "Micronaut HTTP Client") (5)
            .header(ACCEPT, "application/vnd.github.v3+json, application/json") (6)
        return Mono.from(httpClient.retrieve(req, Argument.listOf(GithubRelease::class.java))) (7)
1 Use jakarta.inject.Singleton to designate a class as a singleton.
2 Inject HttpClient via constructor injection.
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>.
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.

4.4. 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.

package example.micronaut

import io.micronaut.http.HttpHeaders.ACCEPT
import io.micronaut.http.HttpHeaders.USER_AGENT
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Header
import io.micronaut.http.annotation.Headers
import io.micronaut.http.client.annotation.Client
import org.reactivestreams.Publisher;

@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)
    fun fetchReleases(): Publisher<GithubRelease?>? (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 any reactive type of any implementation (RxJava, Reactor…​), but it’s better to use the Reactive Streams public interfaces like Publisher.

4.5. Controller

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

  • The Micronaut framework 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).

  • The Framework enables you to consume/produce JSON Streams.

package example.micronaut

import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import org.reactivestreams.Publisher
import reactor.core.publisher.Mono

@Controller("/github") (1)
class GithubController(private val githubLowLevelClient: GithubLowLevelClient, (2)
                       private val githubApiClient: GithubApiClient) {

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

    @Get(uri = "/releases", produces = [MediaType.APPLICATION_JSON_STREAM]) (5)
    fun fetchReleases(): Publisher<GithubRelease?>? { (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 maps 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 any reactive type of any implementation (RxJava, Reactor…​), but it’s better to use the Reactive Streams public interfaces like Publisher.

4.6. Tests

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.

package example.micronaut

import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpStatus
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.Assertions.assertEquals
import org.junit.jupiter.api.Assertions.assertNotNull
import org.junit.jupiter.api.Assertions.assertTrue
import org.junit.jupiter.api.Test

@MicronautTest (1)
class GithubControllerTest {

    lateinit var client: HttpClient (2)

    fun verifyGithubReleasesCanBeFetchedWithLowLevelHttpClient() {
        val request: HttpRequest<Any> = HttpRequest.GET("/github/releases-lowlevel")
        val rsp = client.toBlocking().exchange(request, (3)
            Argument.listOf(GithubRelease::class.java)) (4)

        //then: 'the endpoint can be accessed'
        assertEquals(HttpStatus.OK, rsp.status) (5)
        assertNotNull(rsp.body()) (6)

        val releases = rsp.body()

        val regex = Regex("Micronaut( Framework)? [0-9].[0-9].[0-9]([0-9])?( (RC|M)[0-9])?")
        for (release in releases) {

    fun verifyGithubReleasesCanBeFetchedWithCompileTimeAutoGeneratedAtClient() {
        val request: HttpRequest<Any> = HttpRequest.GET("/github/releases-lowlevel")
        val githubReleases = client.toBlocking().retrieve(request, Argument.listOf(GithubRelease::class.java)) (7)

        val regex = Regex("Micronaut( Framework)? [0-9].[0-9].[0-9]([0-9])?( (RC|M)[0-9])?")
        for (release in githubReleases) {
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 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 The Micronaut framework 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 Micronaut HTTP Client simplifies binding a JSON array to a list of POJOs by using Argument::listOf.

5. Testing the Application

To run the tests:

./gradlew test

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

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, 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:

package example.micronaut

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(condition = GithubFilterCondition::class) (2)
class GithubFilter(val configuration: GithubConfiguration) : HttpClientFilter { (3)

    override fun doFilter(request: MutableHttpRequest<*>, chain: ClientFilterChain): Publisher<out HttpResponse<*>?> {
        return chain.proceed(request.basicAuth(configuration.username, configuration.token)) (4)
1 Supply the pattern you want to match to the @Filter annotation.
2 Kotlin doesn’t support runtime repeatable annotations (see KT-12794. We use a custom condition to enable the bean where appropriate.
3 Constructor injection of the configuration parameters.
4 Enhance every request sent to GitHub API providing Basic Authentication.

Create a custom Condition:

package example.micronaut

import io.micronaut.context.condition.Condition
import io.micronaut.context.condition.ConditionContext
import io.micronaut.context.exceptions.NoSuchBeanException

class GithubFilterCondition : Condition { (1)

    override fun matches(context: ConditionContext<*>?): Boolean {
        if (context != null && context.beanContext != null) {
            try {
                val githubConfiguration: GithubConfiguration =
                    context.beanContext.getBean(GithubConfiguration::class.java) (2)
                if (githubConfiguration.token != null && githubConfiguration.username != null) {
                    return true (3)
            } catch (e: NoSuchBeanException) {
        return false

1 Implement Micronaut Condition.
2 Get the GithubConfiguration bean from the application context.
3 Only return true when the token and the username exist.

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 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: Basic xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
13:09:56.664 [default-nioEventLoopGroup-1-4] TRACE i.m.h.client.netty.DefaultHttpClient - host: api.github.com

7. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, the polyglot embeddable virtual machine, to generate a native executable of our Micronaut application.

Compiling native executables ahead of time with GraalVM improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

7.1. Native executable generation

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 11
sdk install java 22.1.0.r11-grl
If you still use Java 8, use the JDK11 version of GraalVM.
Java 17
sdk install java 22.1.0.r17-grl

For installation on Windows, or for manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

After installing GraalVM, install the native-image component, which is not installed by default:

gu install native-image

To generate a native executable using Gradle, run:

./gradlew nativeCompile

The native executable is created in build/native/nativeCompile directory and can be run with build/native/nativeCompile/micronautguide.

It is possible to customize the name of the native executable or pass additional parameters to GraalVM:

graalvmNative {
    binaries {
        main {
            imageName.set('mn-graalvm-application') (1)
            buildArgs.add('--verbose') (2)
1 The native executable name will now be mn-graalvm-application
2 It is possible to pass extra arguments to build the native executable

8. Next steps

9. Help with the Micronaut Framework

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