单元测试：无法使用 ArgumentCaptor

Question

I am trying to create a unit test for the following method that calls another private method:

public CommandDTO create(final LabelRequest request) {
        return saveLabel(new Label(), request);
}

private CommandDTO saveLabel(final Label label, final LabelRequest request) {
        label.setName(request.getName());
        final Label saved = labelRepository.saveAndFlush(label);

        return CommandDTO.builder().uuid(saved.getUuid()).build();
}

It returns "nullpointer exception for the saved parameter as I did not mock in in my test.

Here is the Unit Test. I added 2 question as comment (Q1 and Q2). Could you please clarify me what is wrong?

@RunWith(MockitoJUnitRunner.class)
public class LabelServiceImplTest {

    @Mock
    private LabelRepository labelRepository;

    @InjectMocks
    private LabelServiceImpl labelService;
        
    @Captor
    ArgumentCaptor<Label> labelCaptor;

    @Test
    public void test_create {
            
        // Q1: I am not sure if the following parts are needed
        final LabelRequest request = new LabelRequest();
        request.setName("Label Name");
        final Label label = new Label();
        label.setName(request.getName());


        // Q2: I think there is no need to mock saveAndFlush method. 
        // But in this scene it returns "nullpointer exception"
        when(labelRepository.saveAndFlush(any())).thenReturn(label);


        CommandDTO result = labelService.create(request);

        Mockito.verify(labelRepository).saveAndFlush(labelCaptor.capture());
        final Label value = labelCaptor.getValue();
                
        // then make necessary assertions
    }
}

Answer 1

FI R ST

I think that you only declared your mocks but never defined/created them. There's basically three ways to do so:

1) mockito extension

The convenient way in JUnit 5 is to use the MockitoExtension:

@ExtendWith(MockitoExtension.class)
public class LabelServiceImplTest {

  // declare mocks, spies and captors ...

  // system under test
  @InjectMocks
  LabelService sut;

}

Nothing else should be needed now.

Mind that by @InjectMocks you instruct Mockito to create your system under test and inject the mocks. You may as well do that manually, eg by injecting mocks into the SUT's constructor. I prefer that way to have more control over the configuration of my SUT.

2) MockitoAnnotations.openMocks(testclass)

This method will inject mocks into the given object:

public class LabelServiceImplTest {

  // declare mocks, spies and captors ...

  // system under test
  @InjectMocks
  LabelService sut;

  @BeforeEach
  void setup() {
    MockitoAnnotations.openMock(this)
    
    // ...
  }

}

On JUnit 4 you'd use MockitoAnnotations.initMocks(…) instead.

3) the old school way

public class LabelServiceImplTest {

  LabelRepository labelRepositoryMock = Mockito.mock(LabelRepository.class);

  // ...

  // system under test
  LabelServiceImpl sut;


  @BeforeEach
  void setup() {
    sut = new LabelService(labelRepositoryMock);

    // ...
  }

}

---

SE C OND

Your questions …

1) Do you need to create the request and the label?

Yes. You call the service using the request and the mock returns the label. I would suggest you do not use the request's property to set up your label, use a String literal instead.

2) Do you need to mock the saveAndFlush() method?

Yes. This is the part where you configure your SUT's environment to behave in a predictable way.

---

You might consider postfixing your mocks, spies and captors by: "Mock", "Spy" and "Captor". You might also consider calling your system under test "sut".

@Mock
private LabelRepository labelRepositoryMock;

@Captor
ArgumentCaptor<Label> labelCaptor;

// system under test
@InjectMocks
private LabelServiceImpl sut;
    

This makes the test code more readably, I think.