The DbC4J library allowed enforcing contract clauses using regular Java methods via naming conventions. Internally, it was implemented using AspectJ, the original Aspect-Oriented Programming (AOP) framework, and a lot reflection. The library has been available on-line since 2007, but the since the research group I was part of closed their website, it has been unavailable. Currently the source code available at GitHub is the last release from 2007, unchanged, so I'm not sure how the code compiles under the latest versions of Java/AspectJ (this was originally implemented under JDK/AspectJ/AJDT 5 with Eclipse). Also available on GitHub is my Master Thesis dissertation, in case anyone in interested in the research underlying the prototype.