//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;
    }
}

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

public class FeignClientWrapper implements IWebClient {
  private feign = something

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

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

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment =
SpringBootTest.WebEnvironment.DEFINED_PORT)
public class TestYourComponent {
    @Configuration
    @Import({YourConfiguration.class})
    public static class TestConfiguration {
    }

    @MockBean
    private UserClient userClient;

    @Test
    public void someTest()
    {
        //...
        mockSomeBehavior();
        //...
    }

    private void mockSomeBehavior() {
        Mockito.doReturn(someKindOfUser).when(userClient).getUser();
    }
}