How We Refactor and How We Mine it ? A Large Scale Study on Refactoring Activities in Open Source Systems

Refactoring, as coined by WIlliam Obdyke in 1992, is the art of optimizing the syntactic design of a software system without altering its external behavior. Refactoring was also cataloged by Martin Fowler as a response to the existence of design defects that negatively impact the software’s design. Since then, the research in refactoring has been driven by improving systems structures. However, recent studies have been showing that developers may incorporate refactoring strategies in other development related activities that go beyond improving the design. In this context, we aim in better understanding the developer’s perception of refactoring by mining and automatically classifying refactoring activities in 1,706 open source Java projects. We perform a differentiated replication of the pioneering work by Tsantalis et al. We revisit five research questions presented in this previous empirical study and compare our results to their original work. The original study investigates various types of refactorings applied to different source types (i.e., production vs. test), the degree to which experienced developers contribute to refactoring efforts, the chronological collocation of refactoring with release and testing periods, and the developer’s intention behind specific types of refactorings. We reexamine the same questions but on a larger number of systems. To do this, our approach relies on mining refactoring instances executed throughout several releases of each project we studied. We also mined several properties related to these projects; namely their commits, contributors, issues, test files, etc. Our findings confirm some of the results of the previous study and we highlight some differences for discussion. We found that 1) feature addition and bug fixes are strong motivators for developers to refactor their code base, rather than the traditional design improvement motivation; 2) a variety of refactoring types are applied when refactoring both production and test code. 3) refactorings tend to be applied by experienced developers who have contributed a wide range of commits to the code. 4) there is a correlation between the type of refactoring activities taking place and whether the source code is undergoing a release or a test period.