java.lang.ref
Class SoftReference

java.lang.Object
  java.lang.ref.Reference
      java.lang.ref.SoftReference

public class SoftReference

extends Reference

Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand. Soft references are most often used to implement memory-sensitive caches.

Suppose that the garbage collector determines at a certain point in time that an object is softly reachable. At that time it may choose to clear atomically all soft references to that object and all soft references to any other softly-reachable objects from which that object is reachable through a chain of strong references. At the same time or at some later time it will enqueue those newly-cleared soft references that are registered with reference queues.

All soft references to softly-reachable objects are guaranteed to have been cleared before the virtual machine throws an OutOfMemoryError. Otherwise no constraints are placed upon the time at which a soft reference will be cleared or the order in which a set of such references to different objects will be cleared. Virtual machine implementations are, however, encouraged to bias against clearing recently-created or recently-used soft references.

Direct instances of this class may be used to implement simple caches; this class or derived subclasses may also be used in larger data structures to implement more sophisticated caches. As long as the referent of a soft reference is strongly reachable, that is, is actually in use, the soft reference will not be cleared. Thus a sophisticated cache can, for example, prevent its most recently used entries from being discarded by keeping strong referents to those entries, leaving the remaining entries to be discarded at the discretion of the garbage collector.

Since:

1.2

Author:

Mark Reinhold

Nested Class Summary

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

Field Summary

private static long	clock
private long	timestamp

Fields inherited from class java.lang.ref.Reference

next, queue

Constructor Summary

SoftReference(Object referent)
Creates a new soft reference that refers to the given object.

SoftReference(Object referent, ReferenceQueue q)
Creates a new soft 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

Field Detail

clock

private static long clock

timestamp

private long timestamp

Constructor Detail

SoftReference

public SoftReference(Object referent)

Creates a new soft reference that refers to the given object. The new reference is not registered with any queue.

Parameters:

referent - object the new soft reference will refer to

SoftReference

public SoftReference(Object referent,
                     ReferenceQueue q)

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

Parameters:

referent - object the new soft reference will refer to

q - queue the soft reference is registered with

Throws:

NullPointerException - If the queue argument is null

Method Detail

get

public Object get()

Returns this reference object's referent. If this reference object has been cleared, either by the program or by the garbage collector, then this method returns null.

Overrides:

get in class Reference

Returns:

The object to which this reference refers, or null if this reference object has been cleared