To dispose of Java2D-related resources we
  (Java2D) create a Java2D-Disposer thread
  the first time any Java2D-related activity
  is encountered (it is started from static block
  of Disposer class). This thread then lives
  forever.

  There is a couple of problems: first, if the
  customer's thread has an InheritableThreadLocal,
  our thread inherits it and thus it never gets
  disposed of even if customer's thread went away.

  This is not specific to our Disposer
  thread - if, say, an EventDispatchThread is started
  from the customer's thread it could also run
  into this issue.

  Another issue is that the Disposer thread
  inherits the parent thread's contextClassLoader,
  and since our thread lives forever, any classes
  referenced by that loader are not being collected
  (and since those could be customer's classes, they
  could contain some large data, leading to huge
  memory leaks).

Here's a thread which started the discussion:
   http://forums.java.net/jive/thread.jspa?messageID=169608&#169608