    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}