Difference between Mock & Spy

Difference between Mock & Spy

In the context of software testing and test-driven development, "mock" and "spy" refer to different approaches for creating test doubles or objects that simulate the behavior of real components. Both are used to isolate the code being tested and ensure that tests are focused on specific units of functionality. Here's a brief explanation of the differences between mocks and spies:

1. Mock Objects:

   • Purpose: Mock objects are designed to replace real objects in a test scenario. They are used to verify interactions and behavior between the code being tested and the dependencies.
   • Behavior: Mocks are set up with specific expectations regarding the methods or functions that will be called during the test. They record these expectations and, after the test is run, verify that the expected interactions occurred.
   • Verification: The main focus of mocks is on interaction verification. You check whether the expected methods were called with the correct arguments and in the expected order.

2. Spy Objects:

   • Purpose: Spy objects, on the other hand, are real objects that have been instrumented to record information about their interactions. They allow you to monitor the behavior of a real object without completely replacing it.
   • Behavior: Spies observe the methods or functions of the actual object and record information such as the number of times a method was called, the arguments passed, or even the return values. Unlike mocks, spies don't typically have pre-set expectations; they passively observe the behavior.
   • Verification: The verification with spies is more about checking the state or the history of interactions after the fact. You inspect the recorded information to ensure that the code being tested behaves as expected.

In summary, mocks are used to set expectations about method calls and verify those expectations after the test, focusing on interaction verification. Spies, on the other hand, allow you to observe the behavior of real objects and inspect the recorded information after the test to ensure correct behavior. The choice between mocks and spies often depends on the specific requirements of the test and the desired level of isolation.