Assessing and Improving the Mutation Testing Practice of PIT

Mutation testing is extensively used in software testing studies. However, popular mutation testing tools use a restrictive set of mutants which does not conform to the community standards and mutation testing literature. This can be problematic since the effectiveness of mutation strongly depends on the used mutants. To investigate this issue we form an extended set of mutants and implement it on a popular mutation testing tool named PIT. We then show that in real-world projects the original mutants of PIT are easier to kill and lead to tests that score statistically lower than those of the extended set of mutants for a range of 35% to 70% of the studied classes. These results raise serious concerns regarding the validity of mutation-based experiments that use PIT. To further show the strengths of the extended mutants we also performed an analysis using a benchmark with mutation-adequate test cases and identified equivalent mutants. Our results confirmed that the extended mutants are more effective than a) the original version of PIT and b) two other popular mutation testing tools (major and muJava). In particular, our results demonstrate that the extended mutants are more effective by 23%, 12% and 7% than the mutants of the original PIT, major and muJava. They also show that the extended mutants are at least as strong as the mutants of all the other three tools together. To support future research, we make the new version of PIT, which is equipped with the extended mutants, publicly available.