unit testing - Mock an Eureka Feign Client for Unittesting

Question

i am using spring cloud's eureka and feign to communicate between some services (lets say A and B). Now id like to unittest my service layer of a single service (A). The problem is, that this service (A) is using a feign client to request some information of the other service (B).

Running the unittests without any special configuration throws the following exception: java.lang.RuntimeException: com.netflix.client.ClientException: Load balancer does not have available server for client: service-b => but i do not want any server to run.

My question is: Is there a way to mock the feign client, so i can unittest my service (A) without running an eureka instance and service (B)?

Edit: I ended up creating a stub for the feign client. The stub is marked as a primary component to force spring instantiating the stub within my tests.
This is the solution i came up with.

//the feign client
@FeignClient("user") 
public interface UserClient { 
    UserEntity getUser(); 
}

//the implementation i use for the tests 
@Component 
@Primary //mark as primary implementation
public class UserClientTestImpl implements UserClient { 
    @Override public UserEntity getUser() { 
        return someKindOfUser; 
    } 
}

Answer 1

The question is ... do you even need to mock? I often see that people mention "mock" as the first solution to anything that "should not be part of the unit test". Mocking is a technique, not the solution to everything. (see here).

If you are still at the early stages of your code, just refactor and use something else instead of depending on the concrete instance of the Feign Client. You might use an interface, an abstract class, a trait or whatever you want. Don't depend on the object itself, otherwise you have to "mock it".

public interface IWebClient {
  public String get(...);
  public String post(...);
} 

To the question: but I will have other code that will do exactly the same (except that it will be on the concrete instance of Feign), what do I do then? Well, you can write a functional test and call an instance of a web server that you can setup locally - or use Wiremock, as mentioned by Marcin Grzejszczak in one of the answers.

public class FeignClientWrapper implements IWebClient {
  private feign = something

  public String get() {
    feign.get( ... ) 
  }

  public String post() {
    feign.post( ... ) 
  }
} 

Unit tests are used to test algorithms, if/else, loops: how units work. Don't write code to make mocks fit - it must be the other way around: your code should have less dependencies, and you should mock only when you need to verify the behavior (otherwise you can use a stub or a fake object): do you need to verify the behavior? Do you need to test that a particular method gets called in your code? Or that a particular method gets called with X, Y, and Z for 3 times in a row? Well, then yes, mocking is ok.

Otherwise, use a fake object: what you want is to test just the call/response and maybe the status code. All you probably want is to test how your code reacts to different outputs (e.g., the field "error" is present or not in a JSON response), different status codes (assuming that the Client documentation is right: 200 OK when GET, 201 when POST, etc).