Software Architect's Handbook

Isolated and independent

Each unit test should have the ability to be run independently of other tests and in any order. If tests are isolated and independent, it should be possible to execute any unit test at any time.

Unit tests should not depend on anything except the class we are testing. It should not rely on other classes, nor should it depend on things such as connecting to a database, using a hardware device, accessing files on a file system, or communicating across a network.

With a testing framework and a DI framework, we can mock dependencies for our unit tests. A mock object is a simulated object that is instantiated, perhaps with the help of a testing/mocking framework, that mimics the behaviors of the real object. By mocking ...