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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 | } | ||