Cleaner

General-purpose phantom-reference-based cleaners.

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.

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.