PhantomReference

AI-generated Key Takeaways

• Phantom references are used to schedule pre-mortem cleanup actions, offering more flexibility than Java finalization.

• The get method of a phantom reference always returns null because its referent is inaccessible.

• They are enqueued when the garbage collector identifies their referents as phantom reachable, but they are not automatically cleared.

• Objects remain reachable via phantom references until all such references are cleared or become unreachable themselves.

• Phantom references can be created with a ReferenceQueue for tracking, although a null queue renders them useless.

public class PhantomReference 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 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.

Public Constructor Summary

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.

Public Method Summary

T	get() Returns this reference object's referent.

Inherited Method Summary

From class java.lang.ref.Reference

void	clear() Clears this reference object.
boolean	enqueue() Adds this reference object to the queue with which it is registered, if any.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
T	get() Returns this reference object's referent.
boolean	isEnqueued() Tells whether or not this reference object has been enqueued, either by the program or by the garbage collector.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

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 null queue, but such a reference is completely useless: Its get method will always return null and, since it does not have a queue, it will never be enqueued.

Parameters

referent	the object the new phantom reference will refer to
q	the queue with which the reference is to be registered, or null if registration is not required

Public Methods

public T get ()

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

Returns

• null