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
24 /**
25 * A {@code Map} implementation that allows mappings to be
26 * removed by the garbage collector.
27 * <p>
28 * When you construct a {@code ReferenceMap}, you can specify what kind
29 * of references are used to store the map's keys and values.
30 * If non-hard references are used, then the garbage collector can remove
31 * mappings if a key or value becomes unreachable, or if the JVM's memory is
32 * running low. For information on how the different reference types behave,
33 * see {@link java.lang.ref.Reference Reference}.
34 * </p>
35 * <p>
36 * Different types of references can be specified for keys and values.
37 * The keys can be configured to be weak but the values hard,
38 * in which case this class will behave like a
39 * <a href="https://docs.oracle.com/javase/8/docs/api/java/util/WeakHashMap.html">
40 * {@code WeakHashMap}</a>. However, you can also specify hard keys and
41 * weak values, or any other combination. The default constructor uses
42 * hard keys and soft values, providing a memory-sensitive cache.
43 * </p>
44 * <p>
45 * This map is similar to
46 * {@link org.apache.commons.collections4.map.ReferenceIdentityMap ReferenceIdentityMap}.
47 * It differs in that keys and values in this class are compared using {@code equals()}.
48 * </p>
49 * <p>
50 * This {@link java.util.Map Map} implementation does <em>not</em> allow null elements.
51 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
52 * </p>
53 * <p>
54 * This implementation is not synchronized.
55 * You can use {@link java.util.Collections#synchronizedMap} to
56 * provide synchronized access to a {@code ReferenceMap}.
57 * Remember that synchronization will not stop the garbage collector removing entries.
58 * </p>
59 * <p>
60 * All the available iterators can be reset back to the start by casting to
61 * {@code ResettableIterator} and calling {@code reset()}.
62 * </p>
63 * <p>
64 * <strong>Note that ReferenceMap is not synchronized and is not thread-safe.</strong>
65 * If you wish to use this map from multiple threads concurrently, you must use
66 * appropriate synchronization. The simplest approach is to wrap this map
67 * using {@link java.util.Collections#synchronizedMap}. This class may throw
68 * exceptions when accessed by concurrent threads without synchronization.
69 * </p>
70 * <p>
71 * NOTE: As from Commons Collections 3.1 this map extends {@code AbstractReferenceMap}
72 * (previously it extended AbstractMap). As a result, the implementation is now
73 * extensible and provides a {@code MapIterator}.
74 * </p>
75 *
76 * @param <K> the type of the keys in the map
77 * @param <V> the type of the values in the map
78 * @see java.lang.ref.Reference
79 * @since 3.0 (previously in main package v2.1)
80 */
81 public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
82
83 /** Serialization version */
84 private static final long serialVersionUID = 1555089888138299607L;
85
86 /**
87 * Constructs a new {@code ReferenceMap} that will
88 * use hard references to keys and soft references to values.
89 */
90 public ReferenceMap() {
91 super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
92 DEFAULT_LOAD_FACTOR, false);
93 }
94
95 /**
96 * Constructs a new {@code ReferenceMap} that will
97 * use the specified types of references.
98 *
99 * @param keyType the type of reference to use for keys;
100 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
101 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
102 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
103 * @param valueType the type of reference to use for values;
104 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
105 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
106 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
107 */
108 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
109 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
110 }
111
112 /**
113 * Constructs a new {@code ReferenceMap} that will
114 * use the specified types of references.
115 *
116 * @param keyType the type of reference to use for keys;
117 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
118 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
119 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
120 * @param valueType the type of reference to use for values;
121 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
122 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
123 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
124 * @param purgeValues should the value be automatically purged when the
125 * key is garbage collected
126 */
127 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final boolean purgeValues) {
128 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
129 }
130
131 /**
132 * Constructs a new {@code ReferenceMap} with the
133 * specified reference types, load factor and initial
134 * capacity.
135 *
136 * @param keyType the type of reference to use for keys;
137 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
138 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
139 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
140 * @param valueType the type of reference to use for values;
141 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
142 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
143 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
144 * @param capacity the initial capacity for the map
145 * @param loadFactor the load factor for the map
146 */
147 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
148 final float loadFactor) {
149 super(keyType, valueType, capacity, loadFactor, false);
150 }
151
152 /**
153 * Constructs a new {@code ReferenceMap} with the
154 * specified reference types, load factor and initial
155 * capacity.
156 *
157 * @param keyType the type of reference to use for keys;
158 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
159 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
160 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
161 * @param valueType the type of reference to use for values;
162 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
163 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
164 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
165 * @param capacity the initial capacity for the map
166 * @param loadFactor the load factor for the map
167 * @param purgeValues should the value be automatically purged when the
168 * key is garbage collected
169 */
170 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
171 final float loadFactor, final boolean purgeValues) {
172 super(keyType, valueType, capacity, loadFactor, purgeValues);
173 }
174
175 /**
176 * Deserializes the map in using a custom routine.
177 *
178 * @param in the input stream
179 * @throws IOException if an error occurs while reading from the stream
180 * @throws ClassNotFoundException if an object read from the stream cannot be loaded
181 */
182 private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
183 in.defaultReadObject();
184 doReadObject(in);
185 }
186
187 /**
188 * Serializes this object to an ObjectOutputStream.
189 *
190 * @param out the target ObjectOutputStream.
191 * @throws IOException thrown when an I/O errors occur writing to the target stream.
192 */
193 private void writeObject(final ObjectOutputStream out) throws IOException {
194 out.defaultWriteObject();
195 doWriteObject(out);
196 }
197
198 }