7.3 Declarative HTTP Clients with @Client

Now that you have gathered an understanding of the workings of the lower level HTTP client, it is time to take a look at Micronaut’s support for declarative clients via the Client annotation.

Essentially, the @Client annotation can be declared on any interface or abstract class and through the use of Introduction Advice the abstract methods will be implemented for you at compile time, greatly simplifying the creation of HTTP clients.

Let’s start with a simple example. Given the following class:

Pet.java

public class Pet {
    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

Pet.java

class Pet {
    String name
    int age
}

Pet.java

class Pet {
    var name: String? = null
    var age: Int = 0
}

You can define a common interface for saving new Pet instances:

PetOperations.java

import io.micronaut.http.annotation.Post;
import io.micronaut.validation.Validated;
import io.reactivex.Single;
import javax.validation.constraints.Min;
import javax.validation.constraints.NotBlank;
@Validated
public interface PetOperations {
    @Post
    Single<Pet> save(@NotBlank String name, @Min(1L) int age);
}

PetOperations.java

import io.micronaut.http.annotation.Post
import io.micronaut.validation.Validated
import io.reactivex.Single
import javax.validation.constraints.Min
import javax.validation.constraints.NotBlank
@Validated
interface PetOperations {
    @Post
    Single<Pet> save(@NotBlank String name, @Min(1L) int age)
}

PetOperations.java

import io.micronaut.http.annotation.Post
import io.micronaut.validation.Validated
import io.reactivex.Single
import javax.validation.constraints.Min
import javax.validation.constraints.NotBlank
@Validated
interface PetOperations {
    @Post
    fun save(@NotBlank name: String, @Min(1L) age: Int): Single<Pet>
}

Note how the interface uses Micronaut’s HTTP annotations which are usable on both the server and client side. Also, as you can see you can use javax.validation constraints to validate arguments.

Additionally, to use the javax.validation features you should have the validation module on your classpath:

implementation("io.micronaut:micronaut-validator")

<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-validator</artifactId>
</dependency>

On the server-side of Micronaut you can implement the PetOperations interface:

PetController.java

import io.micronaut.http.annotation.Controller;
import io.reactivex.Single;
@Controller("/pets")
public class PetController implements PetOperations {
    @Override
    public Single<Pet> save(String name, int age) {
        Pet pet = new Pet();
        pet.setName(name);
        pet.setAge(age);
        // save to database or something
        return Single.just(pet);
    }
}

PetController.java

import io.micronaut.http.annotation.Controller
import io.reactivex.Single
@Controller("/pets")
class PetController implements PetOperations {
    @Override
    Single<Pet> save(String name, int age) {
        Pet pet = new Pet()
        pet.setName(name)
        pet.setAge(age)
        // save to database or something
        return Single.just(pet)
    }
}

PetController.java

import io.micronaut.http.annotation.Controller
import io.reactivex.Single
@Controller("/pets")
open class PetController : PetOperations {
    override fun save(name: String, age: Int): Single<Pet> {
        val pet = Pet()
        pet.name = name
        pet.age = age
        // save to database or something
        return Single.just(pet)
    }
}

You can then define a declarative client in src/test/java that uses @Client to automatically, at compile time, implement a client:

PetClient.java

import io.micronaut.http.client.annotation.Client;
import io.reactivex.Single;
@Client("/pets") (1)
public interface PetClient extends PetOperations { (2)
    @Override
    Single<Pet> save(String name, int age); (3)
}

PetClient.java

import io.micronaut.http.client.annotation.Client
import io.reactivex.Single
@Client("/pets") (1)
interface PetClient extends PetOperations { (2)
    @Override
    Single<Pet> save(String name, int age) (3)
}

PetClient.java

import io.micronaut.http.client.annotation.Client
import io.reactivex.Single
@Client("/pets") (1)
interface PetClient : PetOperations { (2)
    override fun save(name: String, age: Int): Single<Pet>  (3)
}

Once you have defined a client you can simply @Inject it wherever you may need it.

Recall that the value of @Client can be:

• An absolute URI. Example [https://api.twitter.com/1.1](https://api.twitter.com/1.1)

• A relative URI, in which case the server targeted will be the current server (useful for testing)

• A service identifier. See the section on Service Discovery for more information on this topic.

In a production deployment you would typically use a service ID and Service Discovery to discover services automatically.

Another important thing to notice regarding the save method in the example above is that is returns a Single type.

This is a non-blocking reactive type and typically you want your HTTP clients not to block. There are cases where you may want to write an HTTP client that does block (such as in unit test cases), but this are rare.

The following table illustrates common return types usable with @Client:

Generally, any reactive type that can be converted to the Publisher interface is supported as a return type including (but not limited to) the reactive types defined by RxJava 1.x, RxJava 2.x and Reactor 3.x.

Returning CompletableFuture instances is also supported. Note that returning any other type will result in a blocking request and is not recommended other than for testing.