Does Unit-Tested Code Crash? A Case Study of Eclipse

Context: Software development projects increasingly adopt unit testing as a way to identify and correct program faults early in the construction process. Code that is unit tested should therefore have fewer failures associated with it. Objective: Compare the number of field failures arising in unit tested code against those arising in code that has not been unit tested. Method: We retrieved 2,083,979 crash incident reports associated with the Eclipse integrated development environment project, and processed them to obtain a set of 126,026 unique program failure stack traces associated with a specific popular release. We then run the JaCoCo code test coverage analysis on the same release, obtaining results on the line, instruction, and branch-level coverage of 216,392 methods. We also extracted from the source code the classes that are linked to a corresponding test class so as to limit test code coverage results to 1,267 classes with actual tests. Finally, we correlated unit tests with failures at the level of 9,523 failing tested methods. Results: Unit-tested code does not appear to be associated with fewer failures. Conclusion: Unit testing on its own may not be a sufficient method for preventing program failures.