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package java.lang.ref;
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.
Author: Mark Reinhold Since: 1.2
/**
* 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.
*
* <p> Suppose that the garbage collector determines at a certain point in time
* that an object is <a href="package-summary.html#reachability">softly
* reachable</a>. 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.
*
* <p> All soft references to softly-reachable objects are guaranteed to have
* been cleared before the virtual machine throws an
* {@code 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.
*
* <p> 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.
*
* @author Mark Reinhold
* @since 1.2
*/
public class SoftReference<T> extends Reference<T> {
Timestamp clock, updated by the garbage collector
/**
* Timestamp clock, updated by the garbage collector
*/
private static long clock;
Timestamp updated by each invocation of the get method. The VM may use
this field when selecting soft references to be cleared, but it is not
required to do so.
/**
* Timestamp updated by each invocation of the get method. The VM may use
* this field when selecting soft references to be cleared, but it is not
* required to do so.
*/
private long timestamp;
Creates a new soft reference that refers to the given object. The new
reference is not registered with any queue.
Params: - referent – object the new soft reference will refer to
/**
* Creates a new soft reference that refers to the given object. The new
* reference is not registered with any queue.
*
* @param referent object the new soft reference will refer to
*/
public SoftReference(T referent) {
super(referent);
this.timestamp = clock;
}
Creates a new soft reference that refers to the given object and is
registered with the given queue.
Params: - referent – object the new soft reference will refer to
- q – the queue with which the reference is to be registered, or
null if registration is not required
/**
* Creates a new soft reference that refers to the given object and is
* registered with the given queue.
*
* @param referent object the new soft reference will refer to
* @param q the queue with which the reference is to be registered,
* or {@code null} if registration is not required
*
*/
public SoftReference(T referent, ReferenceQueue<? super T> q) {
super(referent, q);
this.timestamp = clock;
}
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. Returns: The object to which this reference refers, or null if this reference object has been cleared
/**
* 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 {@code null}.
*
* @return The object to which this reference refers, or
* {@code null} if this reference object has been cleared
*/
public T get() {
T o = super.get();
if (o != null && this.timestamp != clock)
this.timestamp = clock;
return o;
}
}