Currently with the langtools, corba, jaxp, jaxws pre-components split out and built separately, when someone builds the j2se workspace it needs to find replacements for the classes these components deliver. Currently it uses jar to extract out the packages from the JDK_IMPORT_PATH jdk (the import jdk for the build, e.g. the most recent promoted build for this jdk release). The problem is that these "imported" classes can accidently be too many, e.g. when importing in the corba javax/rmi/ packages from rt.jar, we also get the javax/rmi/ssl packages, which are built and delivered by the j2se workspace not corba.
So we have over installed into the classes directory.

Maybe an "impclasses" directory should be used for this, added to the bootclasspath during the build, and then in Release.gmk when the rt.jar is created, it can be smarter about these impclasses files getting put into the rt.jar? Checking for overlap and problems?

Once copied into the classes directory (which is the javac destination directory), these classes can mess up the javac compilations (class file timestamps being newer than the sources, when the classes are actually OLD).
langtools also delivers javax.annotation.processing so watch out for javax.annotation, it's a package that is split between workspaces.