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 1 263 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.