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 <i>not</i> 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 *
79 * @see java.lang.ref.Reference
80 * @since 3.0 (previously in main package v2.1)
81 */
82 public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
83
84 /** Serialization version */
85 private static final long serialVersionUID = 1555089888138299607L;
86
87 /**
88 * Constructs a new {@code ReferenceMap} that will
89 * use hard references to keys and soft references to values.
90 */
91 public ReferenceMap() {
92 super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
93 DEFAULT_LOAD_FACTOR, false);
94 }
95
96 /**
97 * Constructs a new {@code ReferenceMap} that will
98 * use the specified types of references.
99 *
100 * @param keyType the type of reference to use for keys;
101 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
102 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
103 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
104 * @param valueType the type of reference to use for values;
105 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
106 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
107 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
108 */
109 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
110 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
111 }
112
113 /**
114 * Constructs a new {@code ReferenceMap} that will
115 * use the specified types of references.
116 *
117 * @param keyType the type of reference to use for keys;
118 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
119 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
120 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
121 * @param valueType the type of reference to use for values;
122 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
123 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
124 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
125 * @param purgeValues should the value be automatically purged when the
126 * key is garbage collected
127 */
128 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final boolean purgeValues) {
129 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
130 }
131
132 /**
133 * Constructs a new {@code ReferenceMap} with the
134 * specified reference types, load factor and initial
135 * capacity.
136 *
137 * @param keyType the type of reference to use for keys;
138 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
139 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
140 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
141 * @param valueType the type of reference to use for values;
142 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
143 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
144 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
145 * @param capacity the initial capacity for the map
146 * @param loadFactor the load factor for the map
147 */
148 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
149 final float loadFactor) {
150 super(keyType, valueType, capacity, loadFactor, false);
151 }
152
153 /**
154 * Constructs a new {@code ReferenceMap} with the
155 * specified reference types, load factor and initial
156 * capacity.
157 *
158 * @param keyType the type of reference to use for keys;
159 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
160 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
161 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
162 * @param valueType the type of reference to use for values;
163 * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
164 * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
165 * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
166 * @param capacity the initial capacity for the map
167 * @param loadFactor the load factor for the map
168 * @param purgeValues should the value be automatically purged when the
169 * key is garbage collected
170 */
171 public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
172 final float loadFactor, final boolean purgeValues) {
173 super(keyType, valueType, capacity, loadFactor, purgeValues);
174 }
175
176 /**
177 * Read the map in using a custom routine.
178 *
179 * @param in the input stream
180 * @throws IOException if an error occurs while reading from the stream
181 * @throws ClassNotFoundException if an object read from the stream can not be loaded
182 */
183 private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
184 in.defaultReadObject();
185 doReadObject(in);
186 }
187
188 /**
189 * Write the map out using a custom routine.
190 *
191 * @param out the output stream
192 * @throws IOException if an error occurs while writing to the stream
193 */
194 private void writeObject(final ObjectOutputStream out) throws IOException {
195 out.defaultWriteObject();
196 doWriteObject(out);
197 }
198
199 }