Object graphs with ownership domains: an empirical study

Researchers have proposed many ownership type systems but reported limited experience with most of them on real object-oriented code. Only a few systems have been implemented, and there have been few substantial case studies done with those systems. In order to better empirically evaluate ownership type systems, we have therefore conducted a number of case studies applying the Ownership Domains type system to programs at a larger scale. To facilitate the study of legacy code, we reimplemented Ownership Domains using available language support for annotations. After annotating and typechecking a range of object-oriented systems, we extracted global, hierarchical, Ownership Object Graphs (OOGs) using static analysis. OOGs provide an abstracted view that is consistent with programmer design intent, compared to flat object graphs that can be extracted without the benefit of the ownership annotations. An OOG also visualizes the system's ownership structure and helps developers refine the annotations they add to better express the system's design. This paper shares our observations from studying the annotations and the extracted OOGs across several subject systems. We compute metrics on the annotations and on the extracted OOGs, to gain insights into the ownership relationships latent within object-oriented code and to evaluate the effectiveness of the abstraction mechanisms in OOGs.