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.commons.collections4.map;
018
019import java.io.IOException;
020import java.io.ObjectInputStream;
021import java.io.ObjectOutputStream;
022import java.io.Serializable;
023import java.lang.ref.Reference;
024
025/**
026 * A {@code Map} implementation that allows mappings to be
027 * removed by the garbage collector and matches keys and values based
028 * on {@code ==} not {@code equals()}.
029 * <p>
030 * When you construct a {@code ReferenceIdentityMap}, you can specify what kind
031 * of references are used to store the map's keys and values.
032 * If non-hard references are used, then the garbage collector can remove
033 * mappings if a key or value becomes unreachable, or if the JVM's memory is
034 * running low. For information on how the different reference types behave,
035 * see {@link Reference}.
036 * </p>
037 * <p>
038 * Different types of references can be specified for keys and values.
039 * The default constructor uses hard keys and soft values, providing a
040 * memory-sensitive cache.
041 * </p>
042 * <p>
043 * This map is similar to
044 * {@link org.apache.commons.collections4.map.ReferenceMap ReferenceMap}.
045 * It differs in that keys and values in this class are compared using {@code ==}.
046 * </p>
047 * <p>
048 * This map will violate the detail of various Map and map view contracts.
049 * As a general rule, don't compare this map to other maps.
050 * </p>
051 * <p>
052 * This {@link java.util.Map Map} implementation does <i>not</i> allow null elements.
053 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
054 * </p>
055 * <p>
056 * This implementation is not synchronized.
057 * You can use {@link java.util.Collections#synchronizedMap} to
058 * provide synchronized access to a {@code ReferenceIdentityMap}.
059 * Remember that synchronization will not stop the garbage collector removing entries.
060 * </p>
061 * <p>
062 * All the available iterators can be reset back to the start by casting to
063 * {@code ResettableIterator} and calling {@code reset()}.
064 * </p>
065 * <p>
066 * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
067 * If you wish to use this map from multiple threads concurrently, you must use
068 * appropriate synchronization. The simplest approach is to wrap this map
069 * using {@link java.util.Collections#synchronizedMap}. This class may throw
070 * exceptions when accessed by concurrent threads without synchronization.
071 * </p>
072 *
073 * @param <K> the type of the keys in this map
074 * @param <V> the type of the values in this map
075 *
076 * @see java.lang.ref.Reference
077 * @since 3.0 (previously in main package v2.1)
078 */
079public class ReferenceIdentityMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
080
081    /** Serialization version */
082    private static final long serialVersionUID = -1266190134568365852L;
083
084    /**
085     * Constructs a new {@code ReferenceIdentityMap} that will
086     * use hard references to keys and soft references to values.
087     */
088    public ReferenceIdentityMap() {
089        super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
090                DEFAULT_LOAD_FACTOR, false);
091    }
092
093    /**
094     * Constructs a new {@code ReferenceIdentityMap} that will
095     * use the specified types of references.
096     *
097     * @param keyType  the type of reference to use for keys;
098     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
099     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
100     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
101     * @param valueType  the type of reference to use for values;
102     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
103     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
104     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
105     */
106    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
107        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
108    }
109
110    /**
111     * Constructs a new {@code ReferenceIdentityMap} that will
112     * use the specified types of references.
113     *
114     * @param keyType  the type of reference to use for keys;
115     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
116     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
117     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
118     * @param valueType  the type of reference to use for values;
119     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
120     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
121     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
122     * @param purgeValues should the value be automatically purged when the
123     *   key is garbage collected
124     */
125    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
126            final boolean purgeValues) {
127        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
128    }
129
130    /**
131     * Constructs a new {@code ReferenceIdentityMap} with the
132     * specified reference types, load factor and initial capacity.
133     *
134     * @param keyType  the type of reference to use for keys;
135     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
136     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
137     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
138     * @param valueType  the type of reference to use for values;
139     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
140     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
141     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
142     * @param capacity  the initial capacity for the map
143     * @param loadFactor  the load factor for the map
144     */
145    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
146            final int capacity, final float loadFactor) {
147        super(keyType, valueType, capacity, loadFactor, false);
148    }
149
150    /**
151     * Constructs a new {@code ReferenceIdentityMap} with the
152     * specified reference types, load factor and initial capacity.
153     *
154     * @param keyType  the type of reference to use for keys;
155     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
156     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
157     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
158     * @param valueType  the type of reference to use for values;
159     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
160     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
161     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
162     * @param capacity  the initial capacity for the map
163     * @param loadFactor  the load factor for the map
164     * @param purgeValues  should the value be automatically purged when the
165     *   key is garbage collected
166     */
167    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
168            final int capacity, final float loadFactor, final boolean purgeValues) {
169        super(keyType, valueType, capacity, loadFactor, purgeValues);
170    }
171
172    /**
173     * Gets the hash code for the key specified.
174     * <p>
175     * This implementation uses the identity hash code.
176     *
177     * @param key  the key to get a hash code for
178     * @return the hash code
179     */
180    @Override
181    protected int hash(final Object key) {
182        return System.identityHashCode(key);
183    }
184
185    /**
186     * Gets the hash code for a MapEntry.
187     * <p>
188     * This implementation uses the identity hash code.
189     *
190     * @param key  the key to get a hash code for, may be null
191     * @param value  the value to get a hash code for, may be null
192     * @return the hash code, as per the MapEntry specification
193     */
194    @Override
195    protected int hashEntry(final Object key, final Object value) {
196        return System.identityHashCode(key) ^
197               System.identityHashCode(value);
198    }
199
200    /**
201     * Compares two keys for equals.
202     * <p>
203     * This implementation converts the key from the entry to a real reference
204     * before comparison and uses {@code ==}.
205     *
206     * @param key1  the first key to compare passed in from outside
207     * @param key2  the second key extracted from the entry via {@code entry.key}
208     * @return true if equal by identity
209     */
210    @Override
211    protected boolean isEqualKey(final Object key1, Object key2) {
212        key2 = isKeyType(ReferenceStrength.HARD) ? key2 : ((Reference<?>) key2).get();
213        return key1 == key2;
214    }
215
216    /**
217     * Compares two values for equals.
218     * <p>
219     * This implementation uses {@code ==}.
220     *
221     * @param value1  the first value to compare passed in from outside
222     * @param value2  the second value extracted from the entry via {@code getValue()}
223     * @return true if equal by identity
224     */
225    @Override
226    protected boolean isEqualValue(final Object value1, final Object value2) {
227        return value1 == value2;
228    }
229
230    /**
231     * Read the map in using a custom routine.
232     *
233     * @param in the input stream
234     * @throws IOException if an error occurs while reading from the stream
235     * @throws ClassNotFoundException if an object read from the stream can not be loaded
236     */
237    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
238        in.defaultReadObject();
239        doReadObject(in);
240    }
241
242    /**
243     * Write the map out using a custom routine.
244     *
245     * @param out the output stream
246     * @throws IOException if an error occurs while writing to the stream
247     */
248    private void writeObject(final ObjectOutputStream out) throws IOException {
249        out.defaultWriteObject();
250        doWriteObject(out);
251    }
252
253}