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 */ 017 package org.apache.commons.collections.map; 018 019 import java.io.IOException; 020 import java.io.ObjectInputStream; 021 import java.io.ObjectOutputStream; 022 import 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.collections.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 collecter 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 * @see java.lang.ref.Reference 069 * 070 * @since Commons Collections 3.0 (previously in main package v2.1) 071 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $ 072 * 073 * @author Paul Jack 074 * @author Stephen Colebourne 075 */ 076 public class ReferenceMap extends AbstractReferenceMap implements Serializable { 077 078 /** Serialization version */ 079 private static final long serialVersionUID = 1555089888138299607L; 080 081 /** 082 * Constructs a new <code>ReferenceMap</code> that will 083 * use hard references to keys and soft references to values. 084 */ 085 public ReferenceMap() { 086 super(HARD, SOFT, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false); 087 } 088 089 /** 090 * Constructs a new <code>ReferenceMap</code> that will 091 * use the specified types of references. 092 * 093 * @param keyType the type of reference to use for keys; 094 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 095 * @param valueType the type of reference to use for values; 096 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 097 */ 098 public ReferenceMap(int keyType, int valueType) { 099 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false); 100 } 101 102 /** 103 * Constructs a new <code>ReferenceMap</code> that will 104 * use the specified types of references. 105 * 106 * @param keyType the type of reference to use for keys; 107 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 108 * @param valueType the type of reference to use for values; 109 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 110 * @param purgeValues should the value be automatically purged when the 111 * key is garbage collected 112 */ 113 public ReferenceMap(int keyType, int valueType, boolean purgeValues) { 114 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues); 115 } 116 117 /** 118 * Constructs a new <code>ReferenceMap</code> with the 119 * specified reference types, load factor and initial 120 * capacity. 121 * 122 * @param keyType the type of reference to use for keys; 123 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 124 * @param valueType the type of reference to use for values; 125 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 126 * @param capacity the initial capacity for the map 127 * @param loadFactor the load factor for the map 128 */ 129 public ReferenceMap(int keyType, int valueType, int capacity, float loadFactor) { 130 super(keyType, valueType, capacity, loadFactor, false); 131 } 132 133 /** 134 * Constructs a new <code>ReferenceMap</code> 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 #HARD}, {@link #SOFT}, {@link #WEAK} 140 * @param valueType the type of reference to use for values; 141 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 142 * @param capacity the initial capacity for the map 143 * @param loadFactor the load factor for the map 144 * @param purgeValues should the value be automatically purged when the 145 * key is garbage collected 146 */ 147 public ReferenceMap(int keyType, int valueType, int capacity, 148 float loadFactor, boolean purgeValues) { 149 super(keyType, valueType, capacity, loadFactor, purgeValues); 150 } 151 152 //----------------------------------------------------------------------- 153 /** 154 * Write the map out using a custom routine. 155 */ 156 private void writeObject(ObjectOutputStream out) throws IOException { 157 out.defaultWriteObject(); 158 doWriteObject(out); 159 } 160 161 /** 162 * Read the map in using a custom routine. 163 */ 164 private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { 165 in.defaultReadObject(); 166 doReadObject(in); 167 } 168 169 }