@Spy

@Spy is used to create a spy object in a unit test.

It creates a real object and calls real method. 

When we do not stub the method it calls the real method on the created spy object.

While @Mock creates a completely mock or fake object. It doesnt call the actual real method.

Note: spy Object created for interface using @Spy will be useless because you could not test the actual  data on it.

Example:
@Spy ArrayList arrayList;
@Test
public void test_ArrayList_with_Spy_Annotation(){
arrayList.add("good");
arrayList.add("night");

Mockito.verify(arrayList).add("good");
Mockito.verify(arrayList).add("night");
}