A test double is a generic term used to describe the various ways of mimicking a system and can be classified as follows
- Dummy objects - objects that passed around but never actually used. Usually they are just used to fill parameter lists.
- Fake objects - objects that actually have working implementations, but usually take some shortcut which makes them not suitable for production (an InMemoryTestDatabase 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.
- Spies - These are stubs that also record some information based on how they were called. One form of this might be an email service that records how many messages it was sent.
- Mocks - These are pre-programmed with expectations which form a specification of the calls they are expected to receive. They can throw an exception if they receive a call they don't expect and are checked during verification to ensure they got all the calls they were expecting.
Test doubles can be used across the entire test life cycle, facilitating the delivery of code by solving tricky integration or environment problems. However, at some point it may be necessary to remove those doubles and carry out more realistic system integration test. You should always assess the risk of using test doubles and have at the back of your mind that these are not real systems or objects, and are just helping development move along.
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