/*
 * Copyright (c) 2005, 2008, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code 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.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code 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 in the LICENSE file that
 * accompanied this code).
 *
 * 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

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;


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().
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.
This map does not support null keys.
/**
 * <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.
/**
     * 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>();
}