java.lang.ref
Class PhantomReference

java.lang.Object
  java.lang.ref.Reference
      java.lang.ref.PhantomReference

public class PhantomReference

extends Reference

Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

If the garbage collector determines at a certain point in time that the referent of a phantom reference is phantom reachable, then at that time or at some later time it will enqueue the reference.

In order to ensure that a reclaimable object remains so, the referent of a phantom reference may not be retrieved: The get method of a phantom reference always returns null.

Unlike soft and weak references, phantom references are not automatically cleared by the garbage collector as they are enqueued. An object that is reachable via phantom references will remain so until all such references are cleared or themselves become unreachable.

Since:

1.2

Author:

Mark Reinhold

Nested Class Summary

Nested classes inherited from class java.lang.ref.Reference

Field Summary

Fields inherited from class java.lang.ref.Reference

next, queue

Constructor Summary

PhantomReference(Object referent, ReferenceQueue q)
Creates a new phantom reference that refers to the given object and is registered with the given queue.

Method Summary

Object

get() Returns this reference object's referent.

Methods inherited from class java.lang.ref.Reference

clear, enqueue, isEnqueued

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

PhantomReference

public PhantomReference(Object referent,
                        ReferenceQueue q)

Creates a new phantom reference that refers to the given object and is registered with the given queue.

Parameters:

referent - the object the new phantom reference will refer to

q - queue the phantom reference is registered with

Throws:

NullPointerException - If the queue argument is null

Method Detail

get

public Object get()

Returns this reference object's referent. Because the referent of a phantom reference is always inaccessible, this method always returns null.

Overrides:

get in class Reference

Returns:

null