Types of Test Doubles

Mock is an overloaded term in software development. Sadly this leads to developers answering with "mock it" when a mock object may not be the right solution. Test Doubles are a more general term. I should try to use this naming more than I do at present - a goal I aim to work towards. The result of choosing the wrong test double may seem innocent but the effect will be a very different style of test method, with increased coupling to implementation details. The following definitions are ordered in terms of complexity and increased coupling.


Provide canned responses. By their nature stubs would respond to queries. Stubs allow you to test paths of the code that would be otherwise difficult as they always provide the same answer.


Similar to a stub but with the addition that a spy records its actions. When responding to a query or a command the spy keeps track of what happened, how often and anything else relevant. The test can then inspect the spy for the answer, deciding whether to pass or fail. Unlike Mocks, spies play well with the Arrange-Act-Assert pattern. Spies let you answer the question has something happened whereas Mocks tend to lead you towards how has something happened.


Fake objects tend to be used in higher level tests. These are fake implementations of the object they are standing in for. A fake repository would be implemented in a simple manner, instead opting for a simple in memory hash table for its implementation. This allows tests to be run with some confidence that the system will behave as expected. Combined with Contract Tests, fakes can turbo charge the speed of your test execution while still providing confidence.


Similar to spies mocks are primarily in charge with recording what happens. However while spies are silent in their nature relying on the test to interrogate them, mocks differ by throwing exceptions if their expectations are not met. Mocks natural partner is commands. Unlike spies Mocks can struggle to fit into the Arrange-Act-Assert pattern. Of all the test doubles Mocks are the most coupled to implementation details so their use should be limited.