Showing the changes to java/pkg/com/sun/jmx/mbeanserver/WeakIdentityHashMap.java (1 of 18352 files). Show all
Unified · Split
| @@ -0,0 +1,137 @@ | ||
| 1 | +/* | |
| 2 | + * Copyright (c) 2005, 2008, Oracle and/or its affiliates. All rights reserved. | |
| 3 | + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | |
| 4 | + * | |
| 5 | + * This code is free software; you can redistribute it and/or modify it | |
| 6 | + * under the terms of the GNU General Public License version 2 only, as | |
| 7 | + * published by the Free Software Foundation. Oracle designates this | |
| 8 | + * particular file as subject to the "Classpath" exception as provided | |
| 9 | + * by Oracle in the LICENSE file that accompanied this code. | |
| 10 | + * | |
| 11 | + * This code is distributed in the hope that it will be useful, but WITHOUT | |
| 12 | + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
| 13 | + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
| 14 | + * version 2 for more details (a copy is included in the LICENSE file that | |
| 15 | + * accompanied this code). | |
| 16 | + * | |
| 17 | + * You should have received a copy of the GNU General Public License version | |
| 18 | + * 2 along with this work; if not, write to the Free Software Foundation, | |
| 19 | + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | |
| 20 | + * | |
| 21 | + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | |
| 22 | + * or visit www.oracle.com if you need additional information or have any | |
| 23 | + * questions. | |
| 24 | + */ | |
| 25 | + | |
| 26 | +package com.sun.jmx.mbeanserver; | |
| 27 | + | |
| 28 | +import static com.sun.jmx.mbeanserver.Util.*; | |
| 29 | + | |
| 30 | +import java.lang.ref.Reference; | |
| 31 | +import java.lang.ref.ReferenceQueue; | |
| 32 | +import java.lang.ref.WeakReference; | |
| 33 | + | |
| 34 | +import java.util.Map; | |
| 35 | + | |
| 36 | + | |
| 37 | +/** | |
| 38 | + * <p>A map where keys are compared using identity comparison (like | |
| 39 | + * IdentityHashMap) but where the presence of an object as a key in | |
| 40 | + * the map does not prevent it being garbage collected (like | |
| 41 | + * WeakHashMap). This class does not implement the Map interface | |
| 42 | + * because it is difficult to ensure correct semantics for iterators | |
| 43 | + * over the entrySet().</p> | |
| 44 | + * | |
| 45 | + * <p>Because we do not implement Map, we do not copy the questionable | |
| 46 | + * interface where you can call get(k) or remove(k) for any type of k, | |
| 47 | + * which of course can only have an effect if k is of type K.</p> | |
| 48 | + * | |
| 49 | + * <p>This map does not support null keys.</p> | |
| 50 | + */ | |
| 51 | +/* | |
| 52 | + * The approach | |
| 53 | + * is to wrap each key in a WeakReference and use the wrapped value as | |
| 54 | + * a key in an ordinary HashMap. The WeakReference has to be a | |
| 55 | + * subclass IdentityWeakReference (IWR) where two IWRs are equal if | |
| 56 | + * they refer to the same object. This enables us to find the entry | |
| 57 | + * again. | |
| 58 | + */ | |
| 59 | +class WeakIdentityHashMap<K, V> { | |
| 60 | + private WeakIdentityHashMap() {} | |
| 61 | + | |
| 62 | + static <K, V> WeakIdentityHashMap<K, V> make() { | |
| 63 | + return new WeakIdentityHashMap<K, V>(); | |
| 64 | + } | |
| 65 | + | |
| 66 | + V get(K key) { | |
| 67 | + expunge(); | |
| 68 | + WeakReference<K> keyref = makeReference(key); | |
| 69 | + return map.get(keyref); | |
| 70 | + } | |
| 71 | + | |
| 72 | + public V put(K key, V value) { | |
| 73 | + expunge(); | |
| 74 | + if (key == null) | |
| 75 | + throw new IllegalArgumentException("Null key"); | |
| 76 | + WeakReference<K> keyref = makeReference(key, refQueue); | |
| 77 | + return map.put(keyref, value); | |
| 78 | + } | |
| 79 | + | |
| 80 | + public V remove(K key) { | |
| 81 | + expunge(); | |
| 82 | + WeakReference<K> keyref = makeReference(key); | |
| 83 | + return map.remove(keyref); | |
| 84 | + } | |
| 85 | + | |
| 86 | + private void expunge() { | |
| 87 | + Reference<? extends K> ref; | |
| 88 | + while ((ref = refQueue.poll()) != null) | |
| 89 | + map.remove(ref); | |
| 90 | + } | |
| 91 | + | |
| 92 | + private WeakReference<K> makeReference(K referent) { | |
| 93 | + return new IdentityWeakReference<K>(referent); | |
| 94 | + } | |
| 95 | + | |
| 96 | + private WeakReference<K> makeReference(K referent, ReferenceQueue<K> q) { | |
| 97 | + return new IdentityWeakReference<K>(referent, q); | |
| 98 | + } | |
| 99 | + | |
| 100 | + /** | |
| 101 | + * WeakReference where equals and hashCode are based on the | |
| 102 | + * referent. More precisely, two objects are equal if they are | |
| 103 | + * identical or if they both have the same non-null referent. The | |
| 104 | + * hashCode is the value the original referent had. Even if the | |
| 105 | + * referent is cleared, the hashCode remains. Thus, objects of | |
| 106 | + * this class can be used as keys in hash-based maps and sets. | |
| 107 | + */ | |
| 108 | + private static class IdentityWeakReference<T> extends WeakReference<T> { | |
| 109 | + IdentityWeakReference(T o) { | |
| 110 | + this(o, null); | |
| 111 | + } | |
| 112 | + | |
| 113 | + IdentityWeakReference(T o, ReferenceQueue<T> q) { | |
| 114 | + super(o, q); | |
| 115 | + this.hashCode = (o == null) ? 0 : System.identityHashCode(o); | |
| 116 | + } | |
| 117 | + | |
| 118 | + public boolean equals(Object o) { | |
| 119 | + if (this == o) | |
| 120 | + return true; | |
| 121 | + if (!(o instanceof IdentityWeakReference<?>)) | |
| 122 | + return false; | |
| 123 | + IdentityWeakReference<?> wr = (IdentityWeakReference<?>) o; | |
| 124 | + Object got = get(); | |
| 125 | + return (got != null && got == wr.get()); | |
| 126 | + } | |
| 127 | + | |
| 128 | + public int hashCode() { | |
| 129 | + return hashCode; | |
| 130 | + } | |
| 131 | + | |
| 132 | + private final int hashCode; | |
| 133 | + } | |
| 134 | + | |
| 135 | + private Map<WeakReference<K>, V> map = newMap(); | |
| 136 | + private ReferenceQueue<K> refQueue = new ReferenceQueue<K>(); | |
| 137 | +} | |