What’s the difference between a mock & stub?


There are several definitions of objects, that are not real. The general term is test double. This term encompasses: dummy, fake, stub, mock.


According to Martin Fowler’s article:

  • Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
  • Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example).
  • Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what’s programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it ‘sent’, or maybe only how many messages it ‘sent’.
  • Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.


Mocks vs Stubs = Behavioral testing vs State testing


According to the principle of Test only one thing per test, there may be several stubs in one test, but generally there is only one mock.


Test lifecycle with stubs:

  1. Setup – Prepare object that is being tested and its stubs collaborators.
  2. Exercise – Test the functionality.
  3. Verify state – Use asserts to check object’s state.
  4. Teardown – Clean up resources.

Test lifecycle with mocks:

  1. Setup data – Prepare object that is being tested.
  2. Setup expectations – Prepare expectations in mock that is being used by primary object.
  3. Exercise – Test the functionality.
  4. Verify expectations – Verify that correct methods has been invoked in mock.
  5. Verify state – Use asserts to check object’s state.
  6. Teardown – Clean up resources.


Both mocks and stubs testing give an answer for the question: What is the result?

Testing with mocks are also interested in: How the result has been achieved?

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