How to mock a private dao variable?

Question

I have a dao.create() call that I want to mock when testing a method. But I am missing something as I'm still getting NPE. What is wrong here?

class MyService {
    @Inject
    private Dao dao;

    public void myMethod() {
        //..
        dao.create(object);
        //
    }
}

How can I mock out the dao.create() call?

@RunWith(PowerMockRunner.class)
@PrepareForTest(DAO.class)
public void MyServiceTest {

    @Test
    public void testMyMethod() {
        PowerMockito.mock(DAO.class);

        MyService service = new MyService();
        service.myMethod(); //NPE for dao.create()
    }
}

Answer 1

You are not injecting the DAO. With mockito you can change your test class to use @InjectMocks and use mockito runner.

@RunWith(MockitoJUnitRunner.class)
public void MyServiceTest {
    @Mock
    private Dao dao;
    @InjectMocks
    private MyService myService;
    ...
}

You can read more about InjectMocks at Inject Mocks API

Simpler way is changing your injection to injection by constructor. For example, you would change MyService to

class MyService {
    ...
    private final Dao dao;

    @Inject
    public MyService(Dao dao) {
        this.dao = dao;
    } 
    ...
}

then your test you could simple pass the mocked DAO in setup.

...
@Mock
private Dao dao;

@Before
public void setUp() {
    this.dao = mock(Dao.class);
    this.service = new MyService(dao);
}
...

now you can use verify to check if create was called, like:

...
   verify(dao).create(argThat(isExpectedObjectBeingCreated(object)));
}

private Matcher<?> isExpectedObjectBeingCreated(Object object) { ... }

Using injection by constructor will let your dependencies clearer to other developers and it will help when creating tests :)