public class PhantomReference<T> extends Reference<T>Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used to schedule post-mortem cleanup actions.
Suppose the garbage collector determines at a certain point in time that an object is phantom reachable. At that time it will atomically clear all phantom references to that object and all phantom references to any other phantom-reachable objects from which that object is reachable. At the same time or at some later time it will enqueue those newly-cleared phantom references that are registered with reference queues.
In order to ensure that a reclaimable object remains so, the referent of a phantom reference may not be retrieved: The
getmethod of a phantom reference always returns
public PhantomReference(T referent, ReferenceQueue<? super T> q)Creates a new phantom reference that refers to the given object and is registered with the given queue.
It is possible to create a phantom reference with a
nullqueue, but such a reference is completely useless: Its
getmethod will always return
nulland, since it does not have a queue, it will never be enqueued.
referent- the object the new phantom reference will refer to
q- the queue with which the reference is to be registered, or
nullif registration is not required