1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.collections4.map;
18
19 import java.io.IOException;
20 import java.io.ObjectInputStream;
21 import java.io.ObjectOutputStream;
22 import java.io.Serializable;
23 import java.lang.ref.Reference;
24
25 /**
26 * A {@code Map} implementation that allows mappings to be
27 * removed by the garbage collector and matches keys and values based
28 * on {@code ==} not {@code equals()}.
29 * <p>
30 * When you construct a {@code ReferenceIdentityMap}, you can specify what kind
31 * of references are used to store the map's keys and values.
32 * If non-hard references are used, then the garbage collector can remove
33 * mappings if a key or value becomes unreachable, or if the JVM's memory is
34 * running low. For information on how the different reference types behave,
35 * see {@link Reference}.
36 * </p>
37 * <p>
38 * Different types of references can be specified for keys and values.
39 * The default constructor uses hard keys and soft values, providing a
40 * memory-sensitive cache.
41 * </p>
42 * <p>
43 * This map is similar to
44 * {@link org.apache.commons.collections4.map.ReferenceMap ReferenceMap}.
45 * It differs in that keys and values in this class are compared using {@code ==}.
46 * </p>
47 * <p>
48 * This map will violate the detail of various Map and map view contracts.
49 * As a general rule, don't compare this map to other maps.
50 * </p>
51 * <p>
52 * This {@link java.util.Map Map} implementation does <em>not</em> allow null elements.
53 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
54 * </p>
55 * <p>
56 * This implementation is not synchronized.
57 * You can use {@link java.util.Collections#synchronizedMap} to
58 * provide synchronized access to a {@code ReferenceIdentityMap}.
59 * Remember that synchronization will not stop the garbage collector removing entries.
60 * </p>
61 * <p>
62 * All the available iterators can be reset back to the start by casting to
63 * {@code ResettableIterator} and calling {@code reset()}.
64 * </p>
65 * <p>
66 * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
67 * If you wish to use this map from multiple threads concurrently, you must use
68 * appropriate synchronization. The simplest approach is to wrap this map
69 * using {@link java.util.Collections#synchronizedMap}. This class may throw
70 * exceptions when accessed by concurrent threads without synchronization.
71 * </p>
72 *
73 * @param <K> the type of the keys in this map
74 * @param <V> the type of the values in this map
75 * @see java.lang.ref.Reference
76 * @since 3.0 (previously in main package v2.1)
77 */
78 public class ReferenceIdentityMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
79
80 /** Serialization version */
81 private static final long serialVersionUID = -1266190134568365852L;
82
83 /**
84 * Constructs a new {@code ReferenceIdentityMap} that will
85 * use hard references to keys and soft references to values.
86 */
87 public ReferenceIdentityMap() {
88 super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
89 DEFAULT_LOAD_FACTOR, false);
90 }
91
92 /**
93 * Constructs a new {@code ReferenceIdentityMap} that will
94 * use the specified types of references.
95 *
96 * @param keyType the type of reference to use for keys;
97 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
98 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
99 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
100 * @param valueType the type of reference to use for values;
101 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
102 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
103 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
104 */
105 public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
106 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
107 }
108
109 /**
110 * Constructs a new {@code ReferenceIdentityMap} that will
111 * use the specified types of references.
112 *
113 * @param keyType the type of reference to use for keys;
114 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
115 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
116 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
117 * @param valueType the type of reference to use for values;
118 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
119 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
120 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
121 * @param purgeValues should the value be automatically purged when the
122 * key is garbage collected
123 */
124 public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
125 final boolean purgeValues) {
126 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
127 }
128
129 /**
130 * Constructs a new {@code ReferenceIdentityMap} with the
131 * specified reference types, load factor and initial capacity.
132 *
133 * @param keyType the type of reference to use for keys;
134 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
135 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
136 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
137 * @param valueType the type of reference to use for values;
138 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
139 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
140 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
141 * @param capacity the initial capacity for the map
142 * @param loadFactor the load factor for the map
143 */
144 public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
145 final int capacity, final float loadFactor) {
146 super(keyType, valueType, capacity, loadFactor, false);
147 }
148
149 /**
150 * Constructs a new {@code ReferenceIdentityMap} with the
151 * specified reference types, load factor and initial capacity.
152 *
153 * @param keyType the type of reference to use for keys;
154 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
155 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
156 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
157 * @param valueType the type of reference to use for values;
158 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
159 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
160 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
161 * @param capacity the initial capacity for the map
162 * @param loadFactor the load factor for the map
163 * @param purgeValues should the value be automatically purged when the
164 * key is garbage collected
165 */
166 public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
167 final int capacity, final float loadFactor, final boolean purgeValues) {
168 super(keyType, valueType, capacity, loadFactor, purgeValues);
169 }
170
171 /**
172 * Gets the hash code for the key specified.
173 * <p>
174 * This implementation uses the identity hash code.
175 * </p>
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 * </p>
190 *
191 * @param key the key to get a hash code for, may be null
192 * @param value the value to get a hash code for, may be null
193 * @return the hash code, as per the MapEntry specification
194 */
195 @Override
196 protected int hashEntry(final Object key, final Object value) {
197 return System.identityHashCode(key) ^
198 System.identityHashCode(value);
199 }
200
201 /**
202 * Compares two keys for equals.
203 * <p>
204 * This implementation converts the key from the entry to a real reference
205 * before comparison and uses {@code ==}.
206 * </p>
207 *
208 * @param key1 the first key to compare passed in from outside
209 * @param key2 the second key extracted from the entry via {@code entry.key}
210 * @return true if equal by identity
211 */
212 @Override
213 protected boolean isEqualKey(final Object key1, Object key2) {
214 key2 = isKeyType(ReferenceStrength.HARD) ? key2 : ((Reference<?>) key2).get();
215 return key1 == key2;
216 }
217
218 /**
219 * Compares two values for equals.
220 * <p>
221 * This implementation uses {@code ==}.
222 * </p>
223 *
224 * @param value1 the first value to compare passed in from outside
225 * @param value2 the second value extracted from the entry via {@code getValue()}
226 * @return true if equal by identity
227 */
228 @Override
229 protected boolean isEqualValue(final Object value1, final Object value2) {
230 return value1 == value2;
231 }
232
233 /**
234 * Deserializes the map in using a custom routine.
235 *
236 * @param in the input stream
237 * @throws IOException if an error occurs while reading from the stream
238 * @throws ClassNotFoundException if an object read from the stream cannot be loaded
239 */
240 private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
241 in.defaultReadObject();
242 doReadObject(in);
243 }
244
245 /**
246 * Serializes this object to an ObjectOutputStream.
247 *
248 * @param out the target ObjectOutputStream.
249 * @throws IOException thrown when an I/O errors occur writing to the target stream.
250 */
251 private void writeObject(final ObjectOutputStream out) throws IOException {
252 out.defaultWriteObject();
253 doWriteObject(out);
254 }
255
256 }