    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</code> implementation that allows mappings to be
    026 * removed by the garbage collector.
    027 * <p>
    028 * When you construct a <code>ReferenceMap</code>, 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 * Different types of references can be specified for keys and values.
    036 * The keys can be configured to be weak but the values hard,
    037 * in which case this class will behave like a
    038 * <a href="http://java.sun.com/j2se/1.4/docs/api/java/util/WeakHashMap.html">
    039 * <code>WeakHashMap</code></a>. However, you can also specify hard keys and
    040 * weak values, or any other combination. The default constructor uses
    041 * hard keys and soft values, providing a memory-sensitive cache.
    042 * <p>
    043 * This map is similar to
    044 * {@link org.apache.commons.collections4.map.ReferenceIdentityMap ReferenceIdentityMap}.
    045 * It differs in that keys and values in this class are compared using <code>equals()</code>.
    046 * <p>
    047 * This {@link java.util.Map Map} implementation does <i>not</i> allow null elements.
    048 * Attempting to add a null key or value to the map will raise a <code>NullPointerException</code>.
    049 * <p>
    050 * This implementation is not synchronized.
    051 * You can use {@link java.util.Collections#synchronizedMap} to
    052 * provide synchronized access to a <code>ReferenceMap</code>.
    053 * Remember that synchronization will not stop the garbage collector removing entries.
    054 * <p>
    055 * All the available iterators can be reset back to the start by casting to
    056 * <code>ResettableIterator</code> and calling <code>reset()</code>.
    057 * <p>
    058 * <strong>Note that ReferenceMap is not synchronized and is not thread-safe.</strong>
    059 * If you wish to use this map from multiple threads concurrently, you must use
    060 * appropriate synchronization. The simplest approach is to wrap this map
    061 * using {@link java.util.Collections#synchronizedMap}. This class may throw
    062 * exceptions when accessed by concurrent threads without synchronization.
    063 * <p>
    064 * NOTE: As from Commons Collections 3.1 this map extends <code>AbstractReferenceMap</code>
    065 * (previously it extended AbstractMap). As a result, the implementation is now
    066 * extensible and provides a <code>MapIterator</code>.
    067 *
    068 * @param <K> the type of the keys in the map
    069 * @param <V> the type of the values in the map
    070 *
    071 * @see java.lang.ref.Reference
    072 * @since 3.0 (previously in main package v2.1)
    073 */
    074public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
    075
    076    /** Serialization version */
    077    private static final long serialVersionUID = 1555089888138299607L;
    078
    079    /**
    080     * Constructs a new <code>ReferenceMap</code> that will
    081     * use hard references to keys and soft references to values.
    082     */
    083    public ReferenceMap() {
    084        super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
    085                DEFAULT_LOAD_FACTOR, false);
    086    }
    087
    088    /**
    089     * Constructs a new <code>ReferenceMap</code> that will
    090     * use the specified types of references.
    091     *
    092     * @param keyType  the type of reference to use for keys;
    093     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    094     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    095     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    096     * @param valueType  the type of reference to use for values;
    097     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    098     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    099     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    100     */
    101    public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
    102        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    103    }
    104
    105    /**
    106     * Constructs a new <code>ReferenceMap</code> that will
    107     * use the specified types of references.
    108     *
    109     * @param keyType  the type of reference to use for keys;
    110     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    111     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    112     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    113     * @param valueType  the type of reference to use for values;
    114     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    115     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    116     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    117     * @param purgeValues should the value be automatically purged when the
    118     *   key is garbage collected
    119     */
    120    public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final boolean purgeValues) {
    121        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
    122    }
    123
    124    /**
    125     * Constructs a new <code>ReferenceMap</code> with the
    126     * specified reference types, load factor and initial
    127     * capacity.
    128     *
    129     * @param keyType  the type of reference to use for keys;
    130     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    131     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    132     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    133     * @param valueType  the type of reference to use for values;
    134     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    135     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    136     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    137     * @param capacity  the initial capacity for the map
    138     * @param loadFactor  the load factor for the map
    139     */
    140    public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
    141            final float loadFactor) {
    142        super(keyType, valueType, capacity, loadFactor, false);
    143    }
    144
    145    /**
    146     * Constructs a new <code>ReferenceMap</code> with the
    147     * specified reference types, load factor and initial
    148     * capacity.
    149     *
    150     * @param keyType  the type of reference to use for keys;
    151     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    152     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    153     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    154     * @param valueType  the type of reference to use for values;
    155     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    156     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    157     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    158     * @param capacity  the initial capacity for the map
    159     * @param loadFactor  the load factor for the map
    160     * @param purgeValues  should the value be automatically purged when the
    161     *   key is garbage collected
    162     */
    163    public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
    164            final float loadFactor, final boolean purgeValues) {
    165        super(keyType, valueType, capacity, loadFactor, purgeValues);
    166    }
    167
    168    //-----------------------------------------------------------------------
    169    /**
    170     * Write the map out using a custom routine.
    171     *
    172     * @param out  the output stream
    173     * @throws IOException if an error occurs while writing to the stream
    174     */
    175    private void writeObject(final ObjectOutputStream out) throws IOException {
    176        out.defaultWriteObject();
    177        doWriteObject(out);
    178    }
    179
    180    /**
    181     * Read the map in using a custom routine.
    182     *
    183     * @param in the input stream
    184     * @throws IOException if an error occurs while reading from the stream
    185     * @throws ClassNotFoundException if an object read from the stream can not be loaded
    186     */
    187    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    188        in.defaultReadObject();
    189        doReadObject(in);
    190    }
    191
    192}