Mockito: Mocking "Blackbox" Dependencies

Question

So I have been asked to read up on mocking and BDD for our development team and play around with mocks so as to improve a handful of our existing unit tests (as an experiment).

I have ultimately chosen to go with Mockito for a number of reasons (some outside the scope of my control), but namely because it supports both stubbing and mocking for instances when mocking would not be appropriate.

I have spent all day learning about Mockito, mocking (in general) and BDD. And now I am ready to dig in and start augmenting our unit tests.

So we have a class called WebAdaptor that has a run() method:

public class WebAdaptor {

    private Subscriber subscriber;

    public void run() {

        subscriber = new Subscriber();
        subscriber.init();
    }
}

Please note: I do not have a way to modify this code (for reasons outside the scope of this question!). Thus I do not have the ability to add a setter method for Subscriber, and thus it can be thought of as an unreachable "blackbox" inside of my WebAdaptor.

I want to write a unit test which incorporates a Mockito mock, and uses that mock to verify that executing WebAdaptor::run() causes Subscriber::init() to be called.

So here's what I've got so far (inside WebAdaptorUnitTest):

@Test
public void runShouldInvokeSubscriberInit() {

    // Given
    Subscriber mockSubscriber = mock(Subscriber.class);
    WebAdaptor adaptor = new WebAdaptor();

    // When
    adaptor.run();

    // Then
    verify(mockSubscriber).init();
}

When I run this test, the actual Subscriber::init() method gets executed (I can tell from the console output and seeing files being generated on my local system), not the mockSubscriber, which shouldn't do (or return) anything.

I have checked and re-checked: init is public, is neither static or final, and it returns void. According to the docs, Mockito should have no problem mocking this object.

So it got me thinking: do I need to explictly associate the mockSubscriber with the adaptor? If this is a case, then ordinarily, the following would normally fix it:

adaptor.setSubscriber(mockSubscriber);

But since I cannot add any such setter (please read my note above), I'm at a loss as to how I could force such an association. So, several very-closely-related questions:

• Can anyone confirm that I've set the test up correctly (using the Mockito API)?
• Is my suspicion about the missing setter correct? (Do I need to associate these objects via a setter?)
• If my above suspicion is true, and I can't modify WebAdaptor, are there any circumventions at my dispose?

Thanks in advance!

Answer 1

You need to inject the mock into the class which you are testing. You do not need access to Subscriber. The way mockito and other mocking frameworks help is that you do not need access to objects which you are interacting with. You do however need a way to get mock objects into the class you are testing.

public class WebAdaptor {

    public WebAdaptor(Subscriber subscriber) { /* Added a new constructor */
       this.subscriber = subscriber;
    }

    private Subscriber subscriber;

    public void run() {
        subscriber.init();
    }
}

Now you can verify your interactions on the mock, rather than on the real object.

@Test
public void runShouldInvokeSubscriberInit() {

    // Given
    Subscriber mockSubscriber = mock(Subscriber.class);
    WebAdaptor adaptor = new WebAdaptor(mockSubscriber);  // Use the new constructor

    // When
    adaptor.run();

    // Then
    verify(mockSubscriber).init();
}

If adding the Subscriber to the constructor is not the correct approach, you could also consider using a factory to allow WebAdaptor to instantiate new Subscriber objects from a factory which you control. You could then mock the factory to provider mock Subscribers.

Answer 2

If you don't want to change the production code and still be able to mock the functionality of the Subscriber class you should have a look at PowerMock. It works fine together with Mockito and allows you to mock the creation of new objects.

Subscriber mockSubscriber = mock(Subscriber.class);
whenNew(Subscriber.class).withNoArguments().thenReturn(mockSubscriber);

Further details are explained in the documentation for the PowerMock framework.