javac uses an architectural model which until the addition of annotation processing assumed the existence of a single "Context" object for the compilation.

With the introduction of annotation processing in JDK 5, the model was (imperfectly) changed to one Context per processing round.

One major issue is that any custom objects registered in the initial context are not propogated to any subsequent contexts.

This is a problem for the issue of reducing the incompatibilities between NetBeans and javac, because the general direction there is to have NetBeans register custom objects in the compilation context.