Inferring Specifications of Object Oriented APIs from API Source Code

API libraries are becoming increasingly popular in modern software industries because these libraries provide various methods and classes for reuse. However, as pointed out by researchers, libraries are typically difficult to use. It is desirable to infer some specifications for libraries so that programmers can learn the correct usages of these libraries. In this paper, we propose an approach to infer specifications from source code of API libraries. Our approach is based on the observation that rules in object-oriented programs can be traced from basic constraints such as memory usage, file usage, and network protocol. In addition, rules of one class spread to its dependent classes through the features of object-oriented programs such as derivation, invocation relationship, and field access among methods. Based on our approach, we implemented a prototype named Java Rule Finder (JRF) to infer specifications from source code of API libraries in Java. We conducted four case studies using JRF. The result shows that JRF infers some rules correctly. We further conducted an experiment on three open source API libraries. The results show that JRF scales well with real API libraries.