Characterizing and Identifying Composite Refactorings: Concepts, Heuristics and Patterns

Refactoring consists of a transformation applied to improve the program internal structure, for instance, by contributing to remove code smells. Developers often apply multiple interrelated refactorings called composite refactoring. Even though composite refactoring is a common practice, an investigation from different points of view on how composite refactoring manifests in practice is missing. Previous empirical studies also neglect how different kinds of composite refactorings affect the removal, prevalence or introduction of smells. To address these matters, we provide a conceptual framework and two heuristics to respectively characterize and identify composite refactorings within and across commits. Then, we mined the commit history of 48 GitHub software projects. We identified and analyzed 24,911 composite refactorings involving 104,505 single refactorings. Amongst several findings, we observed that most composite refactorings occur in the same commit and have the same refactoring type. We found that several refactorings are semantically related to each other, which occur in different parts of the system but are still related to the same task. Our study is the first to reveal that many smells are introduced in a program due to “incomplete” composite refactorings. Our study is also the first to reveal 111 patterns of composite refactorings that frequently introduce or remove certain smell types. These patterns can be used as guidelines for developers to improve their refactoring practices as well as for designers of recommender systems.