Mocking vs. Spying in mocking frameworks

Mock object replace mocked class entirely, returning recorded or default values. You can create mock out of "thin air". This is what is mostly used during unit testing.

When spying, you take an existing object and "replace" only some methods. This is useful when you have a huge class and only want to mock certain methods (partial mocking). Let me quote Mockito documentation:

You can create spies of real objects. When you use the spy then the real methods are called (unless a method was stubbed). Real spies should be used carefully and occasionally, for example when dealing with legacy code.

When in doubt, use mocks.

I'll try to explain using an example here:

// Difference between mocking, stubbing and spying
@Test
public void differenceBetweenMockingSpyingAndStubbing() {
    List list = new ArrayList();
    list.add("abc");
    assertEquals(1, list.size());

    List mockedList = spy(list);
    when(mockedList.size()).thenReturn(10);
    assertEquals(10, mockedList.size());
}

Here, we had initial real object list, in which we added one element and expected size to be one.

We spy real object meaning that we can instruct which method to be stubbed. So we declared that we stubbed method - size() on spy object which will return 10, no matter what is actual size.

In a nutshell, you will spy real object and stub some of the methods.

Mockito warns that partial mocking isn't a good practice and you should revise your Object Oriented architecture. Spy (or partial mocking) is recommended to test legacy code.

Based on Mocks Aren't Stubs by Martin Fowler:

Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists. Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example). Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Spies are stubs that also record some information based on how they were called. One form of this might be an email service that records how many messages it was sent. Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.

In Mockito if you assign any object to instance variable of Mock Object then does not affect on Mock Object.

But in case of Spy, if you assign any object to instance variable of Spy Object then does affect on Spy Object because of Spy act like real-time object modification.

For a reference example are

@RunWith(MockitoJUnitRunner.class)
public class MockSpyExampleTest {

    @Mock
    private List<String> mockList;

    @Spy
    private List<String> spyList = new ArrayList();

    @Test
    public void testMockList() {
        //by default, calling the methods of mock object will do nothing
        mockList.add("test");
        assertNull(mockList.get(0));
    }

    @Test
    public void testSpyList() {
        //spy object will call the real method when not stub
        spyList.add("test");
        assertEquals("test", spyList.get(0));
    }
}

Reference: http://javapointers.com/tutorial/difference-between-spy-and-mock-in-mockito/

When using mock objects, the default behavior of the method when not stub is do nothing. Simple means, if its a void method, then it will do nothing when you call the method or if its a method with a return then it may return null, empty or the default value.

While in spy objects, of course, since it is a real method, when you are not stubbing the method, then it will call the real method behavior. If you want to change and mock the method, then you need to stub it.

Spies have two definitions. One, is where the real method is called, another where, no functionality is called and only null or null equivalent values are returned, but methods were called, and they're state was recorded, commonly like, method x was called y times.

If we want to avoid calling external services and just want to test the logic inside the method then use mock.

If we want to call the external services and use the real dependency i.e to run the program as it is and just stub specific methods then use spy