Does refactoring improve software structural quality? A longitudinal study of 25 projects

Abstract

Code smells in a program represent indications of structural quality problems, which can be addressed by software refactoring. Refactoring is widely practiced by developers, and considerable development effort has been invested in refactoring tooling support. There is an explicit assumption that software refactoring improves the structural quality of a program by reducing its density of code smells. However, little has been reported about whether and to what extent developers successfully remove code smells through refactoring. This paper reports a first longitudinal study intended to address this gap. We analyze how often the commonly-used refactoring types affect the density of 5 types of code smells along the version histories of 25 projects. Our findings are based on the analysis of 2,635 refactorings distributed in 11 different types. Surprisingly, 2,506 refactorings (95.1%) did not reduce or introduce code smells. Thus, these findings suggest that refactorings lead to smell reduction less often than what has been reported. According to our data, only 2.24% of refactoring changes removed code smells and 2.66% introduced new ones. Moreover, several smells induced by refactoring tended to live long, i.e., 146 days on average. These smells were only eventually removed when smelly elements started to exhibit poor structural quality and, as a consequence, started to be more costly to get rid of.