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 = new WeakHashMap<>(); 50 51 protected WeakClassHashMap() { } 52 53 public V get(Class<?> remoteClass) { 54 /* 55 * Use a mutable cell (a one-element list) to hold the soft 56 * reference to a value, to allow the lazy value computation 57 * to be synchronized with entry-level granularity instead of 58 * by locking the whole table. 59 */ 60 ValueCell<V> valueCell; 61 synchronized (internalMap) { 62 valueCell = internalMap.get(remoteClass); 63 if (valueCell == null) { 64 valueCell = new ValueCell<V>(); 65 internalMap.put(remoteClass, valueCell); 66 } 67 } 68 synchronized (valueCell) { 69 V value = null; 70 if (valueCell.ref != null) { 71 value = valueCell.ref.get(); 72 } 73 if (value == null) { 74 value = computeValue(remoteClass); 75 valueCell.ref = new SoftReference<V>(value); 76 } 77 return value; 78 } 79 } 80 81 protected abstract V computeValue(Class<?> remoteClass); 82 83 private static class ValueCell<T> { 84 Reference<T> ref = null; 85 ValueCell() { } 86 } 87 }