In JDK 6, by default javac retrieves the bootclasspath information ct.sym instead of rt.jar. The ct.sym file represents a subset of rt.jar; classes just used to implement the JDK itself are hidden from ct.sym. Therefore, such classes are not available at compile time (a possible source incompatabilty), but they remain available at runtime (keeping binary compatability).

It has been suggested (http://blogs.sun.com/darcy/entry/deprecation_in_the_jdk#comments) that deprecated elements be hidden during compilation but kept at runtime; the same difference currently supported by ct.sym.

If that policy is provided as an option, ct.sym would be a fine mechanism to support it.