When the garbage collector finds only phantom references to an object, it enqueues the phantom reference. The program polls the reference queue and upon notification that the phantom reference is enqueued, performs post-finalization cleanup processing that requires the object to be unreachable (such as the deallocation of resources outside the Java heap). At the end of the post-finalization cleanup code, the program should call the clear method on the phantom reference object to set the reference field to null to make the referent eligible for garbage collection.