Using Mockito doReturn always returns null

Question

I am using Mockito for my test class:

@RunWith(SpringRunner.class)
@Import({MyTestContextConfiguration.class})
public class MyWorkerTest extends BaseWorkerTest {

    @Spy
    protected static MyWorkerImpl externalTaskTopicServiceImpl;

    @Before
    @SneakyThrows
    public void setUp() {
        final Similarity similarity0 = Similarity.builder()
        .automatic(Boolean.TRUE)
        .score(0.555D)
        .build();

        final MyRequest mpr = MyRequest.builder()
        .reference(prod)
        .build();

        Mockito.doReturn(similarity0).when(externalTaskTopicServiceImpl).invokeMyService(mpr);
        //Mockito.when(ReflectionTestUtils.invokeMethod(externalTaskTopicServiceImpl, "invokeMyService", mpr)).thenReturn(similarity0);

    }

    @SneakyThrows
    @Test
    public void myTest() {
        ...
        externalTaskTopicServiceImpl.handle(externalTaskService, externalTask);
    }
}

MyRequest an Similarity are simple POJOs to be handled as JSON by ObjectMapper.

The worker impl class:

@Service
@Slf4j
public class MyWorkerImpl extends WorkerBase {
    @Override
    public void handle() {
        ...
        final MyRequest mpr = MyRequest.builder().reference(product).build();
        invokeMatchProductsService(mpr);
    }

    protected Similarity invokeMyService(final MyRequest req) throws MyServiceException {
        return httpService.matchPopResult(req);
    }
}

invokeMatchProductsService returns always null when I use Mockito.doReturn() notation. When I use Mockito.when() notation a Similarity object is return but in the end the test case fails with:

org.mockito.exceptions.misusing.WrongTypeOfReturnValue: 
Similarity cannot be returned by toString()
toString() should return String
***
If you're unsure why you're getting above error read on.
Due to the nature of the syntax above problem might occur because:
1. This exception *might* occur in wrongly written multi-threaded tests.
   Please refer to Mockito FAQ on limitations of concurrency testing.
2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies - 
   - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.

Can anybody tell me why I am always getting null when using doReturn() as suggested when a spy is stubbed?

du-it · Answer 1 · 2019-04-24T10:47:24.247

If you know how to do it correctly, it isn't difficult at all! ;-)

Instead of stubbing this way:

Mockito.doReturn(similarity0).when(externalTaskTopicServiceImpl).invokeMyService(mpr);

you have to use Matchers or, to be more precisely, ArgumentMatchers:

Mockito.doReturn(similarity0).when((MyWorkerImpl)externalTaskTopicServiceImpl).invokeMyService(ArgumentMatchers.any(MyRequest.class));

It's obvious that the wrong stubbing results in null, because when spying externalTaskTopicServiceImpl the mpr defined locally only for/in the test case will never be used during program execution. This avoids the null value in invokeMyService().

Concerning the org.mockito.exceptions.misusing.WrongTypeOfReturnValue it was just my design fault: I designed the method in the spied class to return a ConcurrentHashMap<String, Object> but in the testcase I returned a response object.

Using Mockito doReturn always returns null

1 Answers1