java.lang.ref

Class Reference<T>

java.lang.Object

java.lang.ref.Reference<T>

Direct Known Subclasses:

FinalizerReference, PhantomReference, SoftReference, WeakReference

public abstract class Reference<T>
extends Object

Provides an abstract class which describes behavior common to all reference objects. It is not possible to create immediate subclasses of Reference in addition to the ones provided by this package. It is also not desirable to do so, since references require very close cooperation with the system's garbage collector. The existing, specialized reference classes should be used instead.

Three different type of references exist, each being weaker than the preceding one: SoftReference, WeakReference, and PhantomReference. "Weakness" here means that less restrictions are being imposed on the garbage collector as to when it is allowed to actually garbage-collect the referenced object.

In order to use reference objects properly it is important to understand the different types of reachability that trigger their clearing and enqueueing. The following table lists these, from strongest to weakest. For each row, an object is said to have the reachability on the left side if (and only if) it fulfills all of the requirements on the right side. In all rows, consider the root set to be a set of references that are "resistant" to garbage collection (that is, running threads, method parameters, local variables, static fields and the like).

Strongly reachable	There exists at least one path from the root set to the object that does not traverse any instance of a java.lang.ref.Reference subclass.
Softly reachable	The object is not strongly reachable. There exists at least one path from the root set to the object that does traverse a java.lang.ref.SoftReference instance, but no java.lang.ref.WeakReference or java.lang.ref.PhantomReference instances.
Weakly reachable	The object is neither strongly nor softly reachable. There exists at least one path from the root set to the object that does traverse a java.lang.ref.WeakReference instance, but no java.lang.ref.PhantomReference instances.
Phantom-reachable	The object is neither strongly, softly, nor weakly reachable. The object is referenced by a java.lang.ref.PhantomReference instance. The object has already been finalized.

Field Summary

Fields

Modifier and Type	Field and Description
Reference<?>	pendingNext Used internally by the VM.

Method Summary

Modifier and Type	Method and Description
void	clear() Makes the referent null.
boolean	enqueue() Forces the reference object to be enqueued if it has been associated with a queue.
boolean	enqueueInternal() Adds an object to its reference queue.
T	get() Returns the referent of the reference object.
boolean	isEnqueued() Checks whether the reference object has been enqueued.

Methods inherited from class java.lang.Object

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

Field Detail

pendingNext

public volatile Reference<?> pendingNext

Used internally by the VM. This field forms a circular and singly linked list of reference objects discovered by the garbage collector and awaiting processing by the reference queue thread.

Method Detail

clear

public void clear()

Makes the referent null. This does not force the reference object to be enqueued.

enqueueInternal

public final boolean enqueueInternal()

Adds an object to its reference queue.

Returns:

true if this call has caused the Reference to become enqueued, or false otherwise

enqueue

public boolean enqueue()

Forces the reference object to be enqueued if it has been associated with a queue.

Returns:

true if this call has caused the Reference to become enqueued, or false otherwise

get

public T get()

Returns the referent of the reference object.

Returns:

the referent to which reference refers, or null if the object has been cleared.

isEnqueued

public boolean isEnqueued()

Checks whether the reference object has been enqueued.

Returns:

true if the Reference has been enqueued, false otherwise