2.4 Creating a Client

As mentioned previously, Micronaut includes both an HTTP server and an HTTP client. A low-level HTTP client is provided which you can use to test the HelloController created in the previous section.

import io.micronaut.http.HttpRequest;
import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.runtime.server.EmbeddedServer;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;
import jakarta.inject.Inject;
import static org.junit.jupiter.api.Assertions.assertEquals;
@MicronautTest
public class HelloControllerSpec {
    @Inject
    EmbeddedServer server; (1)
    @Inject
    @Client("/")
    HttpClient client; (2)
    @Test
    void testHelloWorldResponse() {
        String response = client.toBlocking() (3)
                .retrieve(HttpRequest.GET("/hello"));
        assertEquals("Hello World", response); (4)
    }
}

import io.micronaut.http.HttpRequest
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import spock.lang.Specification
import jakarta.inject.Inject
@MicronautTest
class HelloControllerSpec extends Specification {
    @Inject
    EmbeddedServer embeddedServer (1)
    @Inject
    @Client("/")
    HttpClient client (2)
    void "test hello world response"() {
        expect:
            client.toBlocking() (3)
                    .retrieve(HttpRequest.GET('/hello')) == "Hello World" (4)
    }
}

import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.runtime.server.EmbeddedServer
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import jakarta.inject.Inject
@MicronautTest
class HelloControllerSpec {
    @Inject
    lateinit var server: EmbeddedServer (1)
    @Inject
    @field:Client("/")
    lateinit var client: HttpClient (2)
    @Test
    fun testHelloWorldResponse() {
        val rsp: String = client.toBlocking() (3)
                .retrieve("/hello")
        assertEquals("Hello World", rsp) (4)
    }
}

In addition to a low-level client, Micronaut features a declarative, compile-time HTTP client, powered by the Client annotation.

To create a client, create an interface annotated with @Client, for example:

import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.client.annotation.Client;
import org.reactivestreams.Publisher;
import io.micronaut.core.async.annotation.SingleResult;
@Client("/hello") (1)
public interface HelloClient {
    @Get(consumes = MediaType.TEXT_PLAIN) (2)
    @SingleResult
    Publisher<String> hello(); (3)
}

import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import org.reactivestreams.Publisher
import io.micronaut.core.async.annotation.SingleResult
@Client("/hello") (1)
interface HelloClient {
    @Get(consumes = MediaType.TEXT_PLAIN) (2)
    @SingleResult
    Publisher<String> hello() (3)
}

import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import io.micronaut.core.async.annotation.SingleResult
import org.reactivestreams.Publisher
@Client("/hello") (1)
interface HelloClient {
    @Get(consumes = [MediaType.TEXT_PLAIN]) (2)
    @SingleResult
    fun hello(): Publisher<String>  (3)
}

To test the HelloClient, retrieve it from the ApplicationContext associated with the server:

import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import org.junit.jupiter.api.Test;
import io.micronaut.core.async.annotation.SingleResult;
import jakarta.inject.Inject;
import reactor.core.publisher.Mono;
import static org.junit.jupiter.api.Assertions.assertEquals;
@MicronautTest (1)
public class HelloClientSpec  {
    @Inject
    HelloClient client; (2)
    @Test
    public void testHelloWorldResponse(){
        assertEquals("Hello World", Mono.from(client.hello()).block());(3)
    }
}

import io.micronaut.test.extensions.spock.annotation.MicronautTest
import reactor.core.publisher.Mono
import spock.lang.Specification
import jakarta.inject.Inject
@MicronautTest (1)
class HelloClientSpec extends Specification {
    @Inject HelloClient client (2)
    void "test hello world response"() {
        expect:
        Mono.from(client.hello()).block() == "Hello World" (3)
    }
}

import io.micronaut.context.annotation.Property
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import jakarta.inject.Inject
import reactor.core.publisher.Mono
@MicronautTest (1)
class HelloClientSpec {
    @Inject
    lateinit var client: HelloClient (2)
    @Test
    fun testHelloWorldResponse() {
        assertEquals("Hello World", Mono.from(client.hello()).block())(3)
    }
}

The Client annotation produces an implementation automatically for you at compile time without the using proxies or runtime reflection.

The Client annotation is very flexible. See the section on the Micronaut HTTP Client for more information.