/*
 * %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

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 * @deprecated This class has been replaced by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     %I%, %G%
 */
@Deprecated

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);
    }

}