Mocking vs. Spying in mocking frameworks

In mocking frameworks, you can mock an object or spy on it. What's the difference between the two and when would/should I use one over the other?

Looking at Mockito, for example, I see similar things being done using spies and mocks, but I am unsure as to the distinction between the two.

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.

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));
    }
}