Revapi has been written in extensibility in mind. Everything from API scanning, filtering to report generation is extensible. Revapi comes with a couple of extensions that make it useful without a need to write any code yourself (even though writing an extension is super simple and fun ;) ).
This is a collection of the most basic and rudimentary extensions that make the work with an evolving API useful. Included are extensible filtering, reclassification of found problems, etc.
Revapi itself is written in Java so it would be strange for it not to have a Java API checker. The java extension is just that - a full featured Java API checker checking both source and binary compatibility and even some hints of behavioral compatibility (veeery simple so far - don’t get too excited ;) ).