What is mutation testing?

Mutation testing is a software testing technique that evaluates the effectiveness of a test suite by intentionally introducing small changes, or mutations, into the source code and observing if the test suite can detect these alterations. This method identifies gaps in test coverage that traditional code coverage metrics might miss, by ensuring that the tests not only execute every line of code but also catch defects when the code behaves unexpectedly. Despite being computationally expensive, recent advancements in hardware, parallel execution, and smarter tools have made mutation testing feasible for integration into CI/CD pipelines. Tools like Stryker and PIT facilitate this process by generating mutants, running tests against them, and reporting surviving mutants that indicate weaknesses in the test suite. Mutation testing complements code coverage by highlighting areas where additional assertions are needed to improve test effectiveness, particularly in critical code areas such as business logic. The approach requires careful implementation to balance its high cost; typically, it's run on a schedule or scoped to specific changes, ensuring it enhances software quality without excessive resource consumption.