WeakValueCache.java revision 1508:a661018d34b8 
1/*
2 * Copyright (c) 2015, 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
26package jdk.nashorn.internal;
27
28import java.lang.ref.ReferenceQueue;
29import java.lang.ref.WeakReference;
30import java.util.HashMap;
31import java.util.function.Function;
32
33/**
34 * This class provides a map based cache with weakly referenced values. Cleared references are
35 * purged from the underlying map when values are retrieved or created.
36 * It uses a {@link java.util.HashMap} to store values and needs to be externally synchronized.
37 *
38 * @param <K> the key type
39 * @param <V> the value type
40 */
41public final class WeakValueCache<K, V> {
42
43    private final HashMap<K, KeyValueReference<K, V>> map = new HashMap<>();
44    private final ReferenceQueue<V> refQueue = new ReferenceQueue<>();
45
46    /**
47     * Returns the value associated with {@code key}, or {@code null} if no such value exists.
48     *
49     * @param key the key
50     * @return the value or null if none exists
51     */
52    public V get(final K key) {
53        removeClearedEntries();
54        return findValue(key);
55    }
56
57    /**
58     * Returns the value associated with {@code key}, or creates and returns a new value if
59     * no value exists using the {@code creator} function.
60     *
61     * @param key the key
62     * @param creator function to create a new value
63     * @return the existing value, or a new one if none existed
64     */
65    public V getOrCreate(final K key, final Function<? super K, ? extends V> creator) {
66        removeClearedEntries();
67
68        V value = findValue(key);
69
70        if (value == null) {
71            // Define a new value if it does not exist
72            value = creator.apply(key);
73            map.put(key, new KeyValueReference<>(key, value));
74        }
75
76        return value;
77    }
78
79    private V findValue(final K key) {
80        final KeyValueReference<K, V> ref = map.get(key);
81        if (ref != null) {
82            return ref.get();
83        }
84        return null;
85    }
86
87    private void removeClearedEntries() {
88        // Remove cleared entries
89        for (;;) {
90            final KeyValueReference ref = (KeyValueReference) refQueue.poll();
91            if (ref == null) {
92                break;
93            }
94            map.remove(ref.key, ref);
95        }
96    }
97
98    private static class KeyValueReference<K, V> extends WeakReference<V> {
99        final K key;
100
101        KeyValueReference(final K key, final V value) {
102            super(value);
103            this.key = key;
104        }
105    }
106
107}
108