SoftReference – JAVA編程語言程序開發技術文章

SoftReference的語義就是當內存不夠用的時候,GC會回收SoftReference所引用的對象。所以,在memory sensitive的程序中將某些大型數據設置成SoftReference再合適不過瞭。
創建一個SoftReference:
[java] 
Object obj = new Object(); 
SoftReference softRef = new SoftReference(obj);      
obj = null;  
最後那句“obj = null”很重要。如果不將obj設置成null,那麼new出來的Object就會有一個"strong reference",如果這樣,softRef就不能發揮作用瞭。
使用SoftReferene:
[java] 
Object obj2;      
obj2 = sr.get();      
if (obj2 == null) // GC freed this      
      sr = new SoftReference(obj2 = new Object());   
 因為是SoftReference,所以有可能已經被GC回收瞭,所以需要判斷sr.get()的返回值是否為null。如果是的話,就再重新new一個Object。這裡的最後一行代碼也很有講究,比如,也許會這麼寫:
[java] 
Object obj2;      
obj2 = sr.get();      
if (obj2 == null) {      
    sr = new SoftReference(new Object());      
    obj2 = sr.get();      
}    
SoftReference源碼:
[java] 
/**
 * @(#)SoftReference.java   1.36 09/02/26
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */ 
 
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.
 *
 * <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</code>.  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.
 *
 * @version  1.36, 02/26/09
 * @author   Mark Reinhold
 * @since    1.2
 */ 
 
public class SoftReference<T> extends Reference<T> { 
 
    /** 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 <tt>null</tt> 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 <code>null</code>.
     *
     * @return   The object to which this reference refers, or
     *           <code>null</code> 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; 
    } 
 

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