    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 *      http://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;
    023
    024/**
    025 * A {@code Map} implementation that allows mappings to be
    026 * removed by the garbage collector.
    027 * <p>
    028 * When you construct a {@code ReferenceMap}, you can specify what kind
    029 * of references are used to store the map's keys and values.
    030 * If non-hard references are used, then the garbage collector can remove
    031 * mappings if a key or value becomes unreachable, or if the JVM's memory is
    032 * running low. For information on how the different reference types behave,
    033 * see {@link java.lang.ref.Reference Reference}.
    034 * </p>
    035 * <p>
    036 * Different types of references can be specified for keys and values.
    037 * The keys can be configured to be weak but the values hard,
    038 * in which case this class will behave like a
    039 * <a href="https://docs.oracle.com/javase/8/docs/api/java/util/WeakHashMap.html">
    040 * {@code WeakHashMap}</a>. However, you can also specify hard keys and
    041 * weak values, or any other combination. The default constructor uses
    042 * hard keys and soft values, providing a memory-sensitive cache.
    043 * </p>
    044 * <p>
    045 * This map is similar to
    046 * {@link org.apache.commons.collections4.map.ReferenceIdentityMap ReferenceIdentityMap}.
    047 * It differs in that keys and values in this class are compared using {@code equals()}.
    048 * </p>
    049 * <p>
    050 * This {@link java.util.Map Map} implementation does <i>not</i> allow null elements.
    051 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
    052 * </p>
    053 * <p>
    054 * This implementation is not synchronized.
    055 * You can use {@link java.util.Collections#synchronizedMap} to
    056 * provide synchronized access to a {@code ReferenceMap}.
    057 * Remember that synchronization will not stop the garbage collector removing entries.
    058 * </p>
    059 * <p>
    060 * All the available iterators can be reset back to the start by casting to
    061 * {@code ResettableIterator} and calling {@code reset()}.
    062 * </p>
    063 * <p>
    064 * <strong>Note that ReferenceMap is not synchronized and is not thread-safe.</strong>
    065 * If you wish to use this map from multiple threads concurrently, you must use
    066 * appropriate synchronization. The simplest approach is to wrap this map
    067 * using {@link java.util.Collections#synchronizedMap}. This class may throw
    068 * exceptions when accessed by concurrent threads without synchronization.
    069 * </p>
    070 * <p>
    071 * NOTE: As from Commons Collections 3.1 this map extends {@code AbstractReferenceMap}
    072 * (previously it extended AbstractMap). As a result, the implementation is now
    073 * extensible and provides a {@code MapIterator}.
    074 * </p>
    075 *
    076 * @param <K> the type of the keys in the map
    077 * @param <V> the type of the values in the map
    078 *
    079 * @see java.lang.ref.Reference
    080 * @since 3.0 (previously in main package v2.1)
    081 */
    082public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
    083
    084    /** Serialization version */
    085    private static final long serialVersionUID = 1555089888138299607L;
    086
    087    /**
    088     * Constructs a new {@code ReferenceMap} that will
    089     * use hard references to keys and soft references to values.
    090     */
    091    public ReferenceMap() {
    092        super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
    093                DEFAULT_LOAD_FACTOR, false);
    094    }
    095
    096    /**
    097     * Constructs a new {@code ReferenceMap} that will
    098     * use the specified types of references.
    099     *
    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}