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 *      https://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;
024import java.util.Map;
025
026/**
027 * A {@code Map} implementation that allows mappings to be
028 * removed by the garbage collector.
029 * <p>
030 * When you construct a {@code ReferenceMap}, 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 Reference}.
036 * </p>
037 * <p>
038 * Different types of references can be specified for keys and values.
039 * The keys can be configured to be weak but the values hard,
040 * in which case this class will behave like a
041 * <a href="https://docs.oracle.com/javase/8/docs/api/java/util/WeakHashMap.html">
042 * {@code WeakHashMap}</a>. However, you can also specify hard keys and
043 * weak values, or any other combination. The default constructor uses
044 * hard keys and soft values, providing a memory-sensitive cache.
045 * </p>
046 * <p>
047 * This map is similar to
048 * {@link ReferenceIdentityMap ReferenceIdentityMap}.
049 * It differs in that keys and values in this class are compared using {@code equals()}.
050 * </p>
051 * <p>
052 * This {@link Map Map} implementation does <em>not</em> 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 ReferenceMap}.
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 ReferenceMap 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 * <p>
073 * NOTE: As from Commons Collections 3.1 this map extends {@code AbstractReferenceMap}
074 * (previously it extended AbstractMap). As a result, the implementation is now
075 * extensible and provides a {@code MapIterator}.
076 * </p>
077 *
078 * @param <K> The type of the keys in the map
079 * @param <V> The type of the values in the map
080 * @see java.lang.ref.Reference
081 * @since 3.0 (previously in main package v2.1)
082 */
083public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
084
085    /** Serialization version */
086    private static final long serialVersionUID = 1555089888138299607L;
087
088    /**
089     * Constructs a new {@code ReferenceMap} that will
090     * use hard references to keys and soft references to values.
091     */
092    public ReferenceMap() {
093        super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
094                DEFAULT_LOAD_FACTOR, false);
095    }
096
097    /**
098     * Constructs a new {@code ReferenceMap} that will
099     * 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}