TimedSoftReference.java

/*
 *                 Sun Public License Notice
 * 
 * The contents of this file are subject to the Sun Public License
 * Version 1.0 (the "License"). You may not use this file except in
 * compliance with the License. A copy of the License is available at
 * http://www.sun.com/
 * 
 * The Original Code is NetBeans. The Initial Developer of the Original
 * Code is Sun Microsystems, Inc. Portions Copyright 1997-2003 Sun
 * Microsystems, Inc. All Rights Reserved.
 */

package org.openide.util;

import java.lang.ref.SoftReference;
import java.util.Map;

/**
 * A soft reference which is held strongly for a while after last access.
 * Lifecycle:
 * <ol>
 * <li>Created. Referent held strongly. A task is scheduled into the request
 *     processor for some time in the future (currently 30 seconds).</li>
 * <li>Expired. After the timeout, the reference switches to a normal soft
 *     reference.</li>
 * <li>Touched. If the value is accessed before it is garbage collected,
 *     whether the reference is expired or not, the reference is "touched".
 *     This means that the referent is again held strongly and the timeout
 *     is started from scratch.</li>
 * <li>Dead. If after expiry there is no access before the next full GC cycle,
 *     the GC algorithm may reclaim the reference. In this case the reference
 *     of course dies. As a bonus, it will try to remove itself as the value
 *     from a map of your choice, to make it convenient to use these references
 *     as values in a caching map without leaking memory for the key.</li>
 * </ol>
 * @author Jesse Glick
 */
final class TimedSoftReference extends SoftReference implements Runnable {
    
    private static final int TIMEOUT = 30000;
    
    private static final RequestProcessor RP = new RequestProcessor("TimedSoftReference"); // NOI18N
    
    private RequestProcessor.Task task;
    
    private Object o;
    
    private final Map m;
    
    private final Object k;
    
    /** Time when the object was last time touched */
    private long touched;
    
    /**
     * Create a soft reference with timeout.
     * The supplied map serves double duty as a synchronization lock
     * for the reference's state changes.
     * @param o the referent
     * @param m a map in which this reference may serve as a value
     * @param k the key whose value in <code>m</code> may be this reference
     */
    public TimedSoftReference(Object o, Map m, Object k) {
        super(o, Utilities.activeReferenceQueue());
        this.o = o;
        this.m = m;
        this.k = k;
        task = RP.create(this);
        task.schedule(TIMEOUT);
    }
    
    public void run() {
        synchronized (m) {
            if (o != null) {
                //System.err.println("Expire " + k);
                // how long we've really been idle
                long unused  = System.currentTimeMillis() - touched;
                if (unused > TIMEOUT/2) {
                    o = null;
                    touched = 0;
                } else {
                    task.schedule(TIMEOUT - (int)unused);
                }
            } else {
                // clean up map ref, we are dead
                //System.err.println("Die " + k);
                m.remove(k);
            }
        }
    }
    
    public Object get() {
        synchronized (m) {
            if (o == null) {
                o = super.get();
            }
            if (o != null) {
                // touch me
                //System.err.println("Touch " + k);
                if (touched == 0) {
                    task.schedule(TIMEOUT);
                } 
                touched = System.currentTimeMillis();
                return o;
            } else {
                return null;
            }
        }
    }
    
}