OpenJDK 6 and 7 by default have UseConcMarkSweepGC disabled. Thus, setting just CMSClassUnloadingEnabled will not work as intended. This can be verified by examining the OpenJDK 6 and 7 source: /hotspot/src/share/vm/runtime/globals.hpp
Merge pull request #40 from rockymadden/master

OpenJDK 6 and 7 issues with CMSClassUnloadingEnabled being set without UseConcMarkSweepGC