Test double


In computer programming and computer science, programmers and developers employ a technique called automated unit testing to enhance the quality of the software. Frequently, the final release software consists of a complex set of objects or procedures interacting together to create the final result. In automated unit testing, it may be necessary to use objects or procedures that look and behave like their release-intended counterparts, but are actually simplified versions that reduce the complexity and facilitate testing. A test double is a generic term used for these objects or procedures.

Types of test doubles

Gerard Meszaros identified several different terms for what he calls, "Test Doubles." Using his vocabulary, there are at least five types of Test Doubles:
For both manual and automated black box testing of service oriented architecture systems or microservices software developers and testers use test doubles that communicate with the system under test over a network protocol. These test doubles are called different names depending on the tool vendor. A commonly used term is service virtualization. Other names used include API simulation, API mock, HTTP stub, HTTP mock, over the wire test double
Another form of test double is the Verified Fake, a Fake object whose behavior has been verified to match that of the real object using a set of tests that run against both the Verified Fake and the real implementation.
While there is no open standard for test double and the various types, there is momentum for continued use of these terms in this manner. Martin Fowler used these terms in his article, Mocks Aren't Stubs referring to Meszaros' book. Microsoft also used the same terms and definitions in an article titled, Exploring The Continuum Of Test Doubles.