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src/java.base/share/classes/java/lang/ref/SoftReference.java

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 13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 14  * version 2 for more details (a copy is included in the LICENSE file that
 15  * accompanied this code).
 16  *
 17  * You should have received a copy of the GNU General Public License version
 18  * 2 along with this work; if not, write to the Free Software Foundation,
 19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 20  *
 21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 22  * or visit www.oracle.com if you need additional information or have any
 23  * questions.
 24  */
 25 
 26 package java.lang.ref;
 27 
 28 
 29 /**
 30  * Soft reference objects, which are cleared at the discretion of the garbage
 31  * collector in response to memory demand.  Soft references are most often used
 32  * to implement memory-sensitive caches.




 33  *
 34  * <p> Suppose that the garbage collector determines at a certain point in time
 35  * that an object is <a href="package-summary.html#reachability">softly
 36  * reachable</a>.  At that time it may choose to clear atomically all soft
 37  * references to that object and all soft references to any other
 38  * softly-reachable objects from which that object is reachable through a chain
 39  * of strong references.  At the same time or at some later time it will
 40  * enqueue those newly-cleared soft references that are registered with
 41  * reference queues.
 42  *
 43  * <p> All soft references to softly-reachable objects are guaranteed to have
 44  * been cleared before the virtual machine throws an
 45  * {@code OutOfMemoryError}.  Otherwise no constraints are placed upon the
 46  * time at which a soft reference will be cleared or the order in which a set
 47  * of such references to different objects will be cleared.  Virtual machine
 48  * implementations are, however, encouraged to bias against clearing
 49  * recently-created or recently-used soft references.
 50  *
 51  * <p> Direct instances of this class may be used to implement simple caches;
 52  * this class or derived subclasses may also be used in larger data structures

 64 
 65 public non-sealed class SoftReference<T> extends Reference<T> {
 66 
 67     /**
 68      * Timestamp clock, updated by the garbage collector
 69      */
 70     private static long clock;
 71 
 72     /**
 73      * Timestamp updated by each invocation of the get method.  The VM may use
 74      * this field when selecting soft references to be cleared, but it is not
 75      * required to do so.
 76      */
 77     private long timestamp;
 78 
 79     /**
 80      * Creates a new soft reference that refers to the given object.  The new
 81      * reference is not registered with any queue.
 82      *
 83      * @param referent object the new soft reference will refer to


 84      */
 85     public SoftReference(T referent) {
 86         super(referent);
 87         this.timestamp = clock;
 88     }
 89 
 90     /**
 91      * Creates a new soft reference that refers to the given object and is
 92      * registered with the given queue.
 93      *
 94      * @param referent object the new soft reference will refer to
 95      * @param q the queue with which the reference is to be registered,
 96      *          or {@code null} if registration is not required
 97      *

 98      */
 99     public SoftReference(T referent, ReferenceQueue<? super T> q) {
100         super(referent, q);
101         this.timestamp = clock;
102     }
103 
104     /**
105      * Returns this reference object's referent.  If this reference object has
106      * been cleared, either by the program or by the garbage collector, then
107      * this method returns {@code null}.
108      *
109      * @return   The object to which this reference refers, or
110      *           {@code null} if this reference object has been cleared
111      */
112     public T get() {
113         T o = super.get();
114         if (o != null && this.timestamp != clock)
115             this.timestamp = clock;
116         return o;
117     }

 13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 14  * version 2 for more details (a copy is included in the LICENSE file that
 15  * accompanied this code).
 16  *
 17  * You should have received a copy of the GNU General Public License version
 18  * 2 along with this work; if not, write to the Free Software Foundation,
 19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 20  *
 21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 22  * or visit www.oracle.com if you need additional information or have any
 23  * questions.
 24  */
 25 
 26 package java.lang.ref;
 27 
 28 
 29 /**
 30  * Soft reference objects, which are cleared at the discretion of the garbage
 31  * collector in response to memory demand.  Soft references are most often used
 32  * to implement memory-sensitive caches.
 33  * <p>
 34  * The referent must not be an instance of a {@linkplain Class#isValue()
 35  * value class}; such a value can never have another reference to it
 36  * and cannot be held in a reference type.
 37  *
 38  * <p> Suppose that the garbage collector determines at a certain point in time
 39  * that an object is <a href="package-summary.html#reachability">softly
 40  * reachable</a>.  At that time it may choose to clear atomically all soft
 41  * references to that object and all soft references to any other
 42  * softly-reachable objects from which that object is reachable through a chain
 43  * of strong references.  At the same time or at some later time it will
 44  * enqueue those newly-cleared soft references that are registered with
 45  * reference queues.
 46  *
 47  * <p> All soft references to softly-reachable objects are guaranteed to have
 48  * been cleared before the virtual machine throws an
 49  * {@code OutOfMemoryError}.  Otherwise no constraints are placed upon the
 50  * time at which a soft reference will be cleared or the order in which a set
 51  * of such references to different objects will be cleared.  Virtual machine
 52  * implementations are, however, encouraged to bias against clearing
 53  * recently-created or recently-used soft references.
 54  *
 55  * <p> Direct instances of this class may be used to implement simple caches;
 56  * this class or derived subclasses may also be used in larger data structures

 68 
 69 public non-sealed class SoftReference<T> extends Reference<T> {
 70 
 71     /**
 72      * Timestamp clock, updated by the garbage collector
 73      */
 74     private static long clock;
 75 
 76     /**
 77      * Timestamp updated by each invocation of the get method.  The VM may use
 78      * this field when selecting soft references to be cleared, but it is not
 79      * required to do so.
 80      */
 81     private long timestamp;
 82 
 83     /**
 84      * Creates a new soft reference that refers to the given object.  The new
 85      * reference is not registered with any queue.
 86      *
 87      * @param referent object the new soft reference will refer to
 88      * @throws IllegalArgumentException if the referent is an instance of a
 89      *         {@link Class#isValue() value class}
 90      */
 91     public SoftReference(T referent) {
 92         super(referent);
 93         this.timestamp = clock;
 94     }
 95 
 96     /**
 97      * Creates a new soft reference that refers to the given object and is
 98      * registered with the given queue.
 99      *
100      * @param referent object the new soft reference will refer to
101      * @param q the queue with which the reference is to be registered,
102      *          or {@code null} if registration is not required
103      * @throws IllegalArgumentException if the referent is an instance of a
104      *         {@link Class#isValue() value class}
105      */
106     public SoftReference(T referent, ReferenceQueue<? super T> q) {
107         super(referent, q);
108         this.timestamp = clock;
109     }
110 
111     /**
112      * Returns this reference object's referent.  If this reference object has
113      * been cleared, either by the program or by the garbage collector, then
114      * this method returns {@code null}.
115      *
116      * @return   The object to which this reference refers, or
117      *           {@code null} if this reference object has been cleared
118      */
119     public T get() {
120         T o = super.get();
121         if (o != null && this.timestamp != clock)
122             this.timestamp = clock;
123         return o;
124     }
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