Mock object creation for test factoring

Test factoring creates fast, focused unit tests from slow system-wide tests; each new unit test exercises only a subset of the functionality exercised by the system tests. Augmenting a test suite with factored unit tests, and prioritizing the tests, should catch errors earlier in a test run.One way to factor a test is to introduce mock objects. If a test exercises a component A, which is designed to issue queries against or mutate another component B, the implementation of B can be replaced by a mock. The mock has two purposes: it checks that A's calls to B are as expected, and it simulates B's behavior in response. Given a system test for A and B, and a record of A's and B's behavior when the system test is run, we would like to automatically generate unit tests for A in which B is mocked. The factored tests can isolate bugs in A from bugs in B and, if B is slow or expensive, improve test performance or cost.This paper motivates test factoring with an illustrative example, proposes a simple procedure for automatically generating mock objects for factored tests, and gives examples of how the procedure can be extended to produce more robust factored tests.