/*

 * 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;

/**

 * <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>();

}