FeignUnitTest.java

public interface PersonServiceClient {
  
  Person getPerson(String id);
  
}

@Component
public class MyComponent {
  
  private final PersonServiceClient personServiceClient;
  
  public MyComponent(PersonServiceClient personServiceClient) {
    this.personServiceClient = personServiceClient;
  }
  
  public boolean isAdult(String personId) {
    Person person = personServiceClient.getPerson(personId);
    return person.getAge() > 18;
  }
}

public class MyComponentTest {
  
  private MyComponent myComponent;
  private PersonServiceClient personServiceClient;
  
  @Before
  public void setup() throws Exception {
    personServiceClient = Mockito.mock(PersonServiceClient.class);
    myComponent = new MyComponent(personServiceClient);
  }
  
  @Test
  public void isAdultWhileMinor() throws Exception {
    //Given
    String personId = "123";
    
    Person person = new Person();
    person.setId(personid);
    person.setAge(14);
    
    Mockito.when(personServiceClient.getPerson(eq(personId))).thenReturn(person);
    
    //When
    boolean isAdult = myComponent.isAdult(personId);
    
    //Then
    AssertJ.assertThat(isAdult).isFalse();
  }
}