ref.java

来自「This is a resource based on j2me embedde」· Java 代码 · 共 124 行

/*
 * @(#)Ref.java	1.35 06/10/10
 *
 * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
 * 
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License version
 * 2 only, as published by the Free Software Foundation. 
 * 
 * This program 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 at /legal/license.txt). 
 * 
 * You should have received a copy of the GNU General Public License
 * version 2 along with this work; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA 
 * 
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
 * Clara, CA 95054 or visit www.sun.com if you need additional
 * information or have any questions. 
 */

package sun.misc;
import java.lang.ref.SoftReference;


/**
 * A "Ref" is an indirect reference to an object that the garbage collector
 * knows about.  An application should override the reconstitute() method with one
 * that will construct the object based on information in the Ref, often by
 * reading from a file.  The get() method retains a cache of the result of the last call to
 * reconstitute() in the Ref.  When space gets tight, the garbage collector
 * will clear old Ref cache entries when there are no other pointers to the
 * object.  In normal usage, Ref will always be subclassed.  The subclass will add the 
 * instance variables necessary for the reconstitute() method to work.  It will also add a 
 * constructor to set them up, and write a version of reconstitute().
 *
 * @deprecated This class has been replaced by
 * <code>java.util.SoftReference</code>.
 *
 * @see java.util.SoftReference
 *
 * @version     1.35, 10/10/06
 */

public abstract class Ref {

    private SoftReference soft = null;

    /**
     * Returns a pointer to the object referenced by this Ref.  If the object
     * has been thrown away by the garbage collector, it will be
     * reconstituted. This method does everything necessary to ensure that the garbage
     * collector throws things away in Least Recently Used(LRU) order.  Applications should 
     * never override this method. The get() method effectively caches calls to
     * reconstitute().
     */
    public synchronized Object get() {
	Object t = check();
	if (t == null) {
	    t = reconstitute();
	    setThing(t);
	}
	return t;
    }

    /**
     * Returns a pointer to the object referenced by this Ref by 
     * reconstituting it from some external source (such as a file).  This method should not 
     * bother with caching since the method get() will deal with that.
     * <p>
     * In normal usage, Ref will always be subclassed.  The subclass will add
     * the instance variables necessary for reconstitute() to work.  It will
     * also add a constructor to set them up, and write a version of
     * reconstitute().
     */
    public abstract Object reconstitute();

    /**
     * Flushes the cached object.  Forces the next invocation of get() to
     * invoke reconstitute().
     */
    public synchronized void flush() {
	SoftReference s = soft;
	if (s != null) s.clear();
	soft = null;
    }

    /**
     * Sets the thing to the specified object.
     * @param thing the specified object
     */
    public synchronized void setThing(Object thing) {
	flush();
	soft = new SoftReference(thing);
    }

    /**
     * Checks to see what object is being pointed at by this Ref and returns it.
     */
    public synchronized Object check() {
	SoftReference s = soft;
	if (s == null) return null;
	return s.get();
    }

    /**
     * Constructs a new Ref.
     */
    public Ref() { }

    /**
     * Constructs a new Ref that initially points to thing.
     */
    public Ref(Object thing) {
	setThing(thing);
    }

}