Abstract
Software licenses determine, from a legal point of view, under which conditions software can be integrated, used, and above all, redistributed. Licenses evolve over time to meet the needs of development communities and to cope with emerging legal issues and new development paradigms. Such evolution of licenses is likely to be accompanied by changes in the way how software uses such licenses, resulting in some licenses being adopted while others are abandoned. This paper reports a large empirical study aimed at quantitatively and qualitatively investigating when and why developer change software licenses. Specifically, we first identify licenses' changes in 1,731,828 commits, representing the entire history of 16,221 Java projects hosted on GitHub. Then, to understand the rationale of license changes, we perform a qualitative analysis---following a grounded theory approach---of commit notes and issue tracker discussions concerning licensing topics and, whenever possible, try to build traceability links between discussions and changes. Our results point out a lack of traceability of when and why licensing changes are made. This can be a major concern, because a change in the license of a system can negatively impact those that reuse it.