Martin Fowler - Test Double

Bliki entry documenting Gerard Meszaros’s taxonomy of test doubles from the book xUnit Test Patterns.

Key Takeaways

“Test Double” is the umbrella term for any production object replacement used in testing (coined by Meszaros, analogous to a film stunt double).
Dummy: passed but never used; satisfies a method signature.
Fake: working implementation with a shortcut unsuitable for production (e.g., in-memory database).
Stub: returns canned answers to calls; does not respond to calls outside what’s programmed.
Spy: a stub that also records how it was called; used for post-hoc verification.
Mock: pre-programmed with expectations; verifies calls were made; throws on unexpected calls.
Key distinction: stubs for state verification; mocks for behaviour verification.
Over-mocking leads to tests that are tightly coupled to implementation and break on refactoring.

High — read directly from source.