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        // Remove cleared entries
54        for (;;) {
55            final KeyValueReference<?, ?> ref = (KeyValueReference) refQueue.poll();
56            if (ref == null) {
57                break;
58            }
59            map.remove(ref.key, ref);
60        }
61
62        final KeyValueReference<K, V> ref = map.get(key);
63        if (ref != null) {
64            return ref.get();
65        }
66        return null;
67    }
68
69    /**
70     * Returns the value associated with {@code key}, or creates and returns a new value if
71     * no value exists using the {@code creator} function.
72     *
73     * @param key the key
74     * @param creator function to create a new value
75     * @return the existing value, or a new one if none existed
76     */
77    public V getOrCreate(final K key, final Function<? super K, ? extends V> creator) {
78        V value = get(key);
79
80        if (value == null) {
81            // Define a new value if it does not exist
82            value = creator.apply(key);
83            map.put(key, new KeyValueReference<>(key, value));
84        }
85
86        return value;
87    }
88
89    private static class KeyValueReference<K, V> extends WeakReference<V> {
90        final K key;
91
92        KeyValueReference(final K key, final V value) {
93            super(value);
94            this.key = key;
95        }
96    }
97
98}
99