weakidentityhashmap.java

java1.6众多例子参考

/*
 * @(#)WeakIdentityHashMap.java	1.2 05/11/17
 * 
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package com.sun.jmx.mbeanserver;

import static com.sun.jmx.mbeanserver.Util.*;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;

import java.util.Map;


/**
 * <p>A map where keys are compared using identity comparison (like
 * IdentityHashMap) but where the presence of an object as a key in
 * the map does not prevent it being garbage collected (like
 * WeakHashMap).  This class does not implement the Map interface
 * because it is difficult to ensure correct semantics for iterators
 * over the entrySet().</p>
 *
 * <p>Because we do not implement Map, we do not copy the questionable
 * interface where you can call get(k) or remove(k) for any type of k,
 * which of course can only have an effect if k is of type K.</p>
 *
 * <p>This map does not support null keys.</p>
 */
/*
 * The approach
 * is to wrap each key in a WeakReference and use the wrapped value as
 * a key in an ordinary HashMap.  The WeakReference has to be a
 * subclass IdentityWeakReference (IWR) where two IWRs are equal if
 * they refer to the same object.  This enables us to find the entry
 * again.
 */
class WeakIdentityHashMap<K, V> {
    private WeakIdentityHashMap() {}
    
    static <K, V> WeakIdentityHashMap<K, V> make() {
        return new WeakIdentityHashMap<K, V>();
    }
    
    V get(K key) {
	expunge();
	WeakReference<K> keyref = makeReference(key);
	return map.get(keyref);
    }

    public V put(K key, V value) {
	expunge();
	if (key == null)
	    throw new IllegalArgumentException("Null key");
	WeakReference<K> keyref = makeReference(key, refQueue);
	return map.put(keyref, value);
    }

    public V remove(K key) {
	expunge();
	WeakReference<K> keyref = makeReference(key);
	return map.remove(keyref);
    }

    private void expunge() {
	Reference<? extends K> ref;
	while ((ref = refQueue.poll()) != null)
	    map.remove(ref);
    }

    private WeakReference<K> makeReference(K referent) {
	return new IdentityWeakReference<K>(referent);
    }

    private WeakReference<K> makeReference(K referent, ReferenceQueue<K> q) {
	return new IdentityWeakReference<K>(referent, q);
    }

    /**
     * WeakReference where equals and hashCode are based on the
     * referent.  More precisely, two objects are equal if they are
     * identical or if they both have the same non-null referent.  The
     * hashCode is the value the original referent had.  Even if the
     * referent is cleared, the hashCode remains.  Thus, objects of
     * this class can be used as keys in hash-based maps and sets.
     */
    private static class IdentityWeakReference<T> extends WeakReference<T> {
	IdentityWeakReference(T o) {
	    this(o, null);
	}

	IdentityWeakReference(T o, ReferenceQueue<T> q) {
	    super(o, q);
	    this.hashCode = (o == null) ? 0 : System.identityHashCode(o);
	}

	public boolean equals(Object o) {
	    if (this == o)
		return true;
	    if (!(o instanceof IdentityWeakReference))
		return false;
	    IdentityWeakReference wr = (IdentityWeakReference) o;
	    Object got = get();
	    return (got != null && got == wr.get());
	}

	public int hashCode() {
	    return hashCode;
	}

	private final int hashCode;
    }

    private Map<WeakReference<K>, V> map = newMap();
    private ReferenceQueue<K> refQueue = new ReferenceQueue<K>();
}