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 | } | ||