java.lang.ref.SoftReference Maven / Gradle / Ivy
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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
*/
public class SoftReference extends Reference {
/**
* Timestamp clock, updated by the garbage collector
*/
static private 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.
*/
private long timestamp;
/**
* 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.
*
* @param referent object the new soft reference will refer to
* @param q the queue with which the reference is to be registered,
* or null if registration is not required
*
*/
public SoftReference(T referent, ReferenceQueue 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.
*
* @return The object to which this reference refers, or
* 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;
}
}