/**
 * General-purpose phantom-reference-based cleaners.
 *
 * <p> Cleaners are a lightweight and more robust alternative to finalization.
 * They are lightweight because they are not created by the VM and thus do not
 * require a JNI upcall to be created, and because their cleanup code is
 * invoked directly by the reference-handler thread rather than by the
 * finalizer thread.  They are more robust because they use phantom references,
 * the weakest type of reference object, thereby avoiding the nasty ordering
 * problems inherent to finalization.
 *
 * <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
 * cleanup code.  Some time after the GC detects that a cleaner's referent has
 * become phantom-reachable, the reference-handler thread will run the cleaner.
 * Cleaners may also be invoked directly; they are thread safe and ensure that
 * they run their thunks at most once.
 *
 * <p> Cleaners are not a replacement for finalization.  They should be used
 * only when the cleanup code is extremely simple and straightforward.
 * Nontrivial cleaners are inadvisable since they risk blocking the
 * reference-handler thread and delaying further cleanup and finalization.
 *
 *
 * @author Mark Reinhold
 */
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