Spy on existing classes

Rather than mocking an object, you can create spies of real objects. Spies will run the real methods in a class, unlike mocks which don’t run anything. Stubbing methods on a spy will run the stub instead, so you can have a mixture of real methods and stubbed methods.

Spies get their name because you can spy on the code you’re testing and see what methods it called. Spies let you verify that a method is called, just like a mock.