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 * https://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 import java.util.Map;
25
26 /**
27 * A {@code Map} implementation that allows mappings to be
28 * removed by the garbage collector.
29 * <p>
30 * When you construct a {@code ReferenceMap}, 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 Reference}.
36 * </p>
37 * <p>
38 * Different types of references can be specified for keys and values.
39 * The keys can be configured to be weak but the values hard,
40 * in which case this class will behave like a
41 * <a href="https://docs.oracle.com/javase/8/docs/api/java/util/WeakHashMap.html">
42 * {@code WeakHashMap}</a>. However, you can also specify hard keys and
43 * weak values, or any other combination. The default constructor uses
44 * hard keys and soft values, providing a memory-sensitive cache.
45 * </p>
46 * <p>
47 * This map is similar to
48 * {@link ReferenceIdentityMap ReferenceIdentityMap}.
49 * It differs in that keys and values in this class are compared using {@code equals()}.
50 * </p>
51 * <p>
52 * This {@link 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 ReferenceMap}.
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 ReferenceMap 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 * <p>
73 * NOTE: As from Commons Collections 3.1 this map extends {@code AbstractReferenceMap}
74 * (previously it extended AbstractMap). As a result, the implementation is now
75 * extensible and provides a {@code MapIterator}.
76 * </p>
77 *
78 * @param <K> The type of the keys in the map
79 * @param <V> The type of the values in the map
80 * @see java.lang.ref.Reference
81 * @since 3.0 (previously in main package v2.1)
82 */
83 public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
84
85 /** Serialization version */
86 private static final long serialVersionUID = 1555089888138299607L;
87
88 /**
89 * Constructs a new {@code ReferenceMap} that will
90 * use hard references to keys and soft references to values.
91 */
92 public ReferenceMap() {
93 super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
94 DEFAULT_LOAD_FACTOR, false);
95 }
96
97 /**
98 * Constructs a new {@code ReferenceMap} that will
99 * use the specified types of references.
100 *
101 * @param keyType The type of reference to use for keys;
102 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
103 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
104 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
105 * @param valueType The type of reference to use for values;
106 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
107 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
108 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
109 */
110 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
111 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
112 }
113
114 /**
115 * Constructs a new {@code ReferenceMap} that will
116 * use the specified types of references.
117 *
118 * @param keyType The type of reference to use for keys;
119 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
120 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
121 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
122 * @param valueType The type of reference to use for values;
123 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
124 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
125 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
126 * @param purgeValues should the value be automatically purged when the
127 * key is garbage collected
128 */
129 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final boolean purgeValues) {
130 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
131 }
132
133 /**
134 * Constructs a new {@code ReferenceMap} with the
135 * specified reference types, load factor and initial
136 * capacity.
137 *
138 * @param keyType The type of reference to use for keys;
139 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
140 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
141 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
142 * @param valueType The type of reference to use for values;
143 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
144 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
145 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
146 * @param capacity The initial capacity for the map
147 * @param loadFactor The load factor for the map
148 */
149 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
150 final float loadFactor) {
151 super(keyType, valueType, capacity, loadFactor, false);
152 }
153
154 /**
155 * Constructs a new {@code ReferenceMap} with the
156 * specified reference types, load factor and initial
157 * capacity.
158 *
159 * @param keyType The type of reference to use for keys;
160 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
161 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
162 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
163 * @param valueType The type of reference to use for values;
164 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
165 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
166 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
167 * @param capacity The initial capacity for the map
168 * @param loadFactor The load factor for the map
169 * @param purgeValues should the value be automatically purged when the
170 * key is garbage collected
171 */
172 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
173 final float loadFactor, final boolean purgeValues) {
174 super(keyType, valueType, capacity, loadFactor, purgeValues);
175 }
176
177 /**
178 * Deserializes the map in using a custom routine.
179 *
180 * @param in The input stream
181 * @throws IOException Thrown if an error occurs while reading from the stream
182 * @throws ClassNotFoundException if an object read from the stream cannot be loaded
183 */
184 private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
185 in.defaultReadObject();
186 doReadObject(in);
187 }
188
189 /**
190 * Serializes this object to an ObjectOutputStream.
191 *
192 * @param out The target ObjectOutputStream.
193 * @throws IOException thrown when an I/O errors occur writing to the target stream.
194 */
195 private void writeObject(final ObjectOutputStream out) throws IOException {
196 out.defaultWriteObject();
197 doWriteObject(out);
198 }
199
200 }