Topic: Unit Testing Applications

Extreme Programming

The cornerstone of Extreme Programming (XP) is the unit test. XP relies on an automated unit testing framework. This automated unit testing framework can be either third party, e.g. xUnit, or created within the development group.

Extreme Programming uses the creation of unit tests for Test Driven Development. The developer writes a unit test that exposes either a software requirement or a defect. This test will fail because either the requirement isn't implemented yet, or because it intentionally exposes a defect in the existing code. Then, the developer writes the simplest code to make the test, along with other tests, pass.

All classes in the system are unit tested. Developers release unit testing code to the code repository in conjunction with the code it tests. XP's thorough unit testing allows the benefits mentioned above, such as simpler and more confident code development and refactoring, simplified code integration, accurate documentation, and more modular designs. These unit tests are also constantly run as a form of regression test.

Techniques

Unit testing is commonly automated, but may still be performed manually. As with many reputable standards bodies, the IEEE^[1] prescribes neither an automated nor a manual approach. A manual approach to unit testing may employ a step-by-step instructional document. Nevertheless, the objective in unit testing is to isolate a unit and validate its correctness. Automation is efficient for achieving this, and enables the many benefits listed in this article. Conversely, if not planned carefully, a careless manual unit test case may execute as an integration test case that involves many software components, and thus preclude the achievement of most if not all of the goals established for unit testing.

Under the automated approach, to fully realize the effect of isolation, the unit or code body subjected to the unit test is executed within a framework outside of its natural environment, that is, outside of the product or calling context for which it was originally created. Testing in an isolated manner has the benefit of revealing unnecessary dependencies between the code being tested and other units or data spaces in the product. These dependencies can then be eliminated through refactoring, or if necessary, re-design.

Using a unit testing framework, the developer codifies criteria into the unit test to verify the correctness of the unit under test. During execution of the unit test(s), the framework logs test cases that fail any criterion. Many frameworks will also automatically flag and report in a summary these failed test cases. Depending upon the severity of a failure, the framework may halt subsequent testing.

As a consequence, unit testing is traditionally a motivator for programmers to create decoupled and cohesive code bodies. This practice promotes healthy habits in software development. Design patterns, unit testing, and refactoring often work together so that the most ideal solution may emerge.