Mocking member variables of a class using Mockito

Question

I am a newbie to development and to unit tests in particular . I guess my requirement is pretty simple, but I am keen to know others thoughts on this.

Suppose I h

Answer 1

Lots of others have already advised you to rethink your code to make it more testable - good advice and usually simpler than what I'm about to suggest.

If you can't change the code to make it more testable, PowerMock: https://code.google.com/p/powermock/

PowerMock extends Mockito (so you don't have to learn a new mock framework), providing additional functionality. This includes the ability to have a constructor return a mock. Powerful, but a little complicated - so use it judiciously.

You use a different Mock runner. And you need to prepare the class that is going to invoke the constructor. (Note that this is a common gotcha - prepare the class that calls the constructor, not the constructed class)

@RunWith(PowerMockRunner.class)
@PrepareForTest({First.class})

Then in your test set-up, you can use the whenNew method to have the constructor return a mock

whenNew(Second.class).withAnyArguments().thenReturn(mock(Second.class));

Answer 2

This is not possible if you can't change your code. But I like dependency injection and Mockito supports it:

public class First {    
    @Resource
    Second second;

    public First() {
        second = new Second();
    }

    public String doSecond() {
        return second.doSecond();
    }
}

Your test:

@RunWith(MockitoJUnitRunner.class)
public class YourTest {
   @Mock
   Second second;

   @InjectMocks
   First first = new First();

   public void testFirst(){
      when(second.doSecond()).thenReturn("Stubbed Second");
      assertEquals("Stubbed Second", first.doSecond());
   }
}

This is very nice and easy.

Answer 3

If you can't change the member variable, then the other way around this is to use powerMockit and call

Second second = mock(Second.class)
when(second.doSecond()).thenReturn("Stubbed Second");
whenNew(Second.class).withAnyArguments.thenReturn(second);

Now the problem is that ANY call to new Second will return the same mocked instance. But in your simple case this will work.