The Classloader API doesn't have an "enumerate" method, because class loading is an "on-demand" activity -- you usually have thousands of classes in your classpath, only a fraction of which will ever be needed (the rt.jar alone is 48MB nowadays!).
So, even if you could enumerate all classes, this would be very time- and memory-consuming.
The simple approach is to list the concerned classes in a setup file (xml or whatever suits your fancy); if you want to do this automatically, restrict yourself to one JAR or one class directory.