Mockito Deep Dive — @Mock, @Spy, verify(), ArgumentCaptor

@Mock creates stubs that return defaults. @Spy wraps real objects — real methods execute unless overridden with doReturn().when(). The key mistake is using when().thenReturn() with spies, which calls the real method before stubbing.

verify() confirms interactions — did the service call save()? Did it send the notification? verify(mock, never()) is equally important for testing that error paths don't trigger side effects.

ArgumentCaptor captures arguments passed to mocks for detailed assertions. When verify() tells you the method was called, the captor tells you what was passed. This is essential for testing services that transform data between layers — asserting that the request DTO was correctly mapped to the domain object.

My pattern: stub the inputs, call the method, verify the interactions, capture arguments for important assertions.