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/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
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* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
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* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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package java.lang.ref;
// Android-changed: Javadoc change to recommend against using SoftReferences for caches.
/**
* Soft reference objects, which are cleared at the discretion of the garbage
* collector in response to memory demand.
*
* 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.
*
*
Avoid Soft References for Caching
* In practice, soft references are inefficient for caching. The runtime doesn't
* have enough information on which references to clear and which to keep. Most
* fatally, it doesn't know what to do when given the choice between clearing a
* soft reference and growing the heap.
*
* The lack of information on the value to your application of each reference
* limits the usefulness of soft references. References that are cleared too
* early cause unnecessary work; those that are cleared too late waste memory.
*
*
Most applications should use an {@code android.util.LruCache} instead of
* soft references. LruCache has an effective eviction policy and lets the user
* tune how much memory is allotted.
*
* @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;
}
}