1/*
2 * Copyright (c) 2003, 2012, 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 */
25package sun.rmi.server;
26
27import java.lang.ref.Reference;
28import java.lang.ref.SoftReference;
29import java.util.Map;
30import 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 **/
47public 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}
88