An Empirical Study of Long-Lived Code Clones

Previous research has shown that refactoring code clones as soon as they are formed or discovered is not always feasible or worthwhile to perform, since some clones never change during evolution and some disappear in a short amount of time, while some undergo repetitive similar edits over their long lifetime. Toward a long-term goal of developing a recommendation system that selectively identifies clones to refactor, as a first step, we conducted an empirical investigation into the characteristics of long-lived clones. Our study of 13558 clone genealogies from 7 large open source projects, over the history of 33.25 years in total, found surprising results. The size of a clone, the number of clones in the same group, and the method-level distribution of clones are not strongly correlated with the survival time of clones. However, the number of developers who modified clones and the time since the last addition or removal of a clone to its group are highly correlated with the survival time of clones. This result indicates that the evolutionary characteristics of clones may be a better indicator for refactoring needs than static or spatial characteristics such as LOC, the number of clones in the same group, or the dispersion of clones in a system.