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| 1 | +/* | |
| 2 | + * Copyright (c) 2003, 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 | +package sun.rmi.server; | |
| 26 | + | |
| 27 | +import java.lang.ref.Reference; | |
| 28 | +import java.lang.ref.SoftReference; | |
| 29 | +import java.util.Map; | |
| 30 | +import java.util.WeakHashMap; | |
| 31 | + | |
| 32 | +/** | |
| 33 | + * Abstract class that maps Class objects to lazily-computed values of | |
| 34 | + * type V. A concrete subclass must implement the computeValue method | |
| 35 | + * to determine how the values are computed. | |
| 36 | + * | |
| 37 | + * The keys are only weakly reachable through this map, so this map | |
| 38 | + * does not prevent a class (along with its class loader, etc.) from | |
| 39 | + * being garbage collected if it is not otherwise strongly reachable. | |
| 40 | + * The values are only softly reachable through this map, so that the | |
| 41 | + * computed values generally persist while not otherwise strongly | |
| 42 | + * reachable, but their storage may be reclaimed if necessary. Also, | |
| 43 | + * note that if a key is strongly reachable from a value, then the key | |
| 44 | + * is effectively softly reachable through this map, which may delay | |
| 45 | + * garbage collection of classes (see 4429536). | |
| 46 | + **/ | |
| 47 | +public abstract class WeakClassHashMap<V> { | |
| 48 | + | |
| 49 | + private Map<Class<?>,ValueCell<V>> internalMap = | |
| 50 | + new WeakHashMap<Class<?>,ValueCell<V>>(); | |
| 51 | + | |
| 52 | + protected WeakClassHashMap() { } | |
| 53 | + | |
| 54 | + public V get(Class<?> remoteClass) { | |
| 55 | + /* | |
| 56 | + * Use a mutable cell (a one-element list) to hold the soft | |
| 57 | + * reference to a value, to allow the lazy value computation | |
| 58 | + * to be synchronized with entry-level granularity instead of | |
| 59 | + * by locking the whole table. | |
| 60 | + */ | |
| 61 | + ValueCell<V> valueCell; | |
| 62 | + synchronized (internalMap) { | |
| 63 | + valueCell = internalMap.get(remoteClass); | |
| 64 | + if (valueCell == null) { | |
| 65 | + valueCell = new ValueCell<V>(); | |
| 66 | + internalMap.put(remoteClass, valueCell); | |
| 67 | + } | |
| 68 | + } | |
| 69 | + synchronized (valueCell) { | |
| 70 | + V value = null; | |
| 71 | + if (valueCell.ref != null) { | |
| 72 | + value = valueCell.ref.get(); | |
| 73 | + } | |
| 74 | + if (value == null) { | |
| 75 | + value = computeValue(remoteClass); | |
| 76 | + valueCell.ref = new SoftReference<V>(value); | |
| 77 | + } | |
| 78 | + return value; | |
| 79 | + } | |
| 80 | + } | |
| 81 | + | |
| 82 | + protected abstract V computeValue(Class<?> remoteClass); | |
| 83 | + | |
| 84 | + private static class ValueCell<T> { | |
| 85 | + Reference<T> ref = null; | |
| 86 | + ValueCell() { } | |
| 87 | + } | |
| 88 | +} | |