001/**
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.camel.util;
018
019import java.lang.ref.WeakReference;
020import java.util.ArrayList;
021import java.util.Collection;
022import java.util.LinkedHashSet;
023import java.util.Map;
024import java.util.Set;
025
026/**
027 * A Least Recently Used Cache which uses {@link java.lang.ref.WeakReference}.
028 * <p/>
029 * This implementation uses {@link java.lang.ref.WeakReference} for stored values in the cache, to support the JVM
030 * when it wants to reclaim objects for example during garbage collection. Therefore this implementation does
031 * not support <b>all</b> the {@link java.util.Map} methods.
032 * <p/>
033 * The following methods is <b>only</b> be be used:
034 * <ul>
035 *   <li>containsKey - To determine if the key is in the cache and refers to a value</li>
036 *   <li>entrySet - To return a set of all the entries (as key/value paris)</li>
037 *   <li>get - To get a value from the cache</li>
038 *   <li>isEmpty - To determine if the cache contains any values</li>
039 *   <li>keySet - To return a set of the current keys which refers to a value</li>
040 *   <li>put - To add a value to the cache</li>
041 *   <li>putAll - To add values to the cache</li>
042 *   <li>remove - To remove a value from the cache by its key</li>
043 *   <li>size - To get the current size</li>
044 *   <li>values - To return a copy of all the value in a list</li>
045 * </ul>
046 * <p/>
047 * The {@link #containsValue(Object)} method should <b>not</b> be used as it's not adjusted to check
048 * for the existence of a value without catering for the soft references.
049 * <p/>
050 * Notice that if the JVM reclaim memory the content of this cache may be garbage collected, without any
051 * eviction notifications.
052 *
053 * @see LRUCache
054 * @see LRUSoftCache
055 */
056public class LRUWeakCache<K, V> extends LRUCache<K, V> {
057    private static final long serialVersionUID = 1L;
058
059    public LRUWeakCache(int maximumCacheSize) {
060        super(maximumCacheSize);
061    }
062
063    public LRUWeakCache(int initialCapacity, int maximumCacheSize) {
064        super(initialCapacity, maximumCacheSize);
065    }
066
067    @Override
068    @SuppressWarnings("unchecked")
069    public V put(K key, V value) {
070        WeakReference<V> put = new WeakReference<V>(value);
071        WeakReference<V> prev = (WeakReference<V>) super.put(key, (V) put);
072        return prev != null ? prev.get() : null;
073    }
074
075    @Override
076    @SuppressWarnings("unchecked")
077    public V get(Object o) {
078        WeakReference<V> ref = (WeakReference<V>) super.get(o);
079        return ref != null ? ref.get() : null;
080    }
081
082    @Override
083    public void putAll(Map<? extends K, ? extends V> map) {
084        for (Entry<? extends K, ? extends V> entry : map.entrySet()) {
085            put(entry.getKey(), entry.getValue());
086        }
087    }
088
089    @Override
090    @SuppressWarnings("unchecked")
091    public V remove(Object o) {
092        WeakReference<V> ref = (WeakReference<V>) super.remove(o);
093        return ref != null ? ref.get() : null;
094    }
095
096    @Override
097    @SuppressWarnings("unchecked")
098    public Collection<V> values() {
099        // return a copy of all the active values
100        Collection<WeakReference<V>> col = (Collection<WeakReference<V>>) super.values();
101        Collection<V> answer = new ArrayList<V>();
102        for (WeakReference<V> ref : col) {
103            V value = ref.get();
104            if (value != null) {
105                answer.add(value);
106            }
107        }
108        return answer;
109    }
110
111    @Override
112    public int size() {
113        // only count as a size if there is a value
114        int size = 0;
115        for (V value : super.values()) {
116            WeakReference<?> ref = (WeakReference<?>) value;
117            if (ref != null && ref.get() != null) {
118                size++;
119            }
120        }
121        return size;
122    }
123
124    @Override
125    public boolean isEmpty() {
126        return size() == 0;
127    }
128
129    @Override
130    public boolean containsKey(Object o) {
131        // must lookup if the key has a value, as we only regard a key to be contained
132        // if the value is still there (the JVM can remove the soft reference if it need memory)
133        return get(o) != null;
134    }
135
136    @Override
137    public Set<Entry<K, V>> entrySet() {
138        Set<Entry<K, V>> original = super.entrySet();
139
140        // must use a copy to avoid concurrent modifications and be able to get/set value using
141        // the soft reference so the returned set is without the soft reference, and thus is
142        // use able for the caller to use
143        Set<Entry<K, V>> answer = new LinkedHashSet<Entry<K, V>>(original.size());
144        for (final Entry<K, V> entry : original) {
145            Entry<K, V> view = new Entry<K, V>() {
146                @Override
147                public K getKey() {
148                    return entry.getKey();
149                }
150
151                @Override
152                @SuppressWarnings("unchecked")
153                public V getValue() {
154                    WeakReference<V> ref = (WeakReference<V>) entry.getValue();
155                    return ref != null ? ref.get() : null;
156                }
157
158                @Override
159                @SuppressWarnings("unchecked")
160                public V setValue(V v) {
161                    V put = (V) new WeakReference<V>(v);
162                    WeakReference<V> prev = (WeakReference<V>) entry.setValue(put);
163                    return prev != null ? prev.get() : null;
164                }
165            };
166            answer.add(view);
167        }
168
169        return answer;
170    }
171
172    @Override
173    public String toString() {
174        return "LRUWeakCache@" + ObjectHelper.getIdentityHashCode(this);
175    }
176}