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 import java.lang.ref.Reference; 024 025 /** 026 * A <code>Map</code> implementation that allows mappings to be 027 * removed by the garbage collector and matches keys and values based 028 * on <code>==</code> not <code>equals()</code>. 029 * <p> 030 * <p> 031 * When you construct a <code>ReferenceIdentityMap</code>, you can specify what kind 032 * of references are used to store the map's keys and values. 033 * If non-hard references are used, then the garbage collector can remove 034 * mappings if a key or value becomes unreachable, or if the JVM's memory is 035 * running low. For information on how the different reference types behave, 036 * see {@link Reference}. 037 * <p> 038 * Different types of references can be specified for keys and values. 039 * The default constructor uses hard keys and soft values, providing a 040 * memory-sensitive cache. 041 * <p> 042 * This map is similar to 043 * {@link org.apache.commons.collections.map.ReferenceMap ReferenceMap}. 044 * It differs in that keys and values in this class are compared using <code>==</code>. 045 * <p> 046 * This map will violate the detail of various Map and map view contracts. 047 * As a general rule, don't compare this map to other maps. 048 * <p> 049 * This {@link java.util.Map Map} implementation does <i>not</i> allow null elements. 050 * Attempting to add a null key or value to the map will raise a <code>NullPointerException</code>. 051 * <p> 052 * This implementation is not synchronized. 053 * You can use {@link java.util.Collections#synchronizedMap} to 054 * provide synchronized access to a <code>ReferenceIdentityMap</code>. 055 * Remember that synchronization will not stop the garbage collecter removing entries. 056 * <p> 057 * All the available iterators can be reset back to the start by casting to 058 * <code>ResettableIterator</code> and calling <code>reset()</code>. 059 * <p> 060 * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong> 061 * If you wish to use this map from multiple threads concurrently, you must use 062 * appropriate synchronization. The simplest approach is to wrap this map 063 * using {@link java.util.Collections#synchronizedMap}. This class may throw 064 * exceptions when accessed by concurrent threads without synchronization. 065 * 066 * @see java.lang.ref.Reference 067 * 068 * @since Commons Collections 3.0 (previously in main package v2.1) 069 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $ 070 * 071 * @author Stephen Colebourne 072 */ 073 public class ReferenceIdentityMap extends AbstractReferenceMap implements Serializable { 074 075 /** Serialization version */ 076 private static final long serialVersionUID = -1266190134568365852L; 077 078 /** 079 * Constructs a new <code>ReferenceIdentityMap</code> that will 080 * use hard references to keys and soft references to values. 081 */ 082 public ReferenceIdentityMap() { 083 super(HARD, SOFT, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false); 084 } 085 086 /** 087 * Constructs a new <code>ReferenceIdentityMap</code> that will 088 * use the specified types of references. 089 * 090 * @param keyType the type of reference to use for keys; 091 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 092 * @param valueType the type of reference to use for values; 093 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 094 */ 095 public ReferenceIdentityMap(int keyType, int valueType) { 096 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false); 097 } 098 099 /** 100 * Constructs a new <code>ReferenceIdentityMap</code> that will 101 * use the specified types of references. 102 * 103 * @param keyType the type of reference to use for keys; 104 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 105 * @param valueType the type of reference to use for values; 106 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 107 * @param purgeValues should the value be automatically purged when the 108 * key is garbage collected 109 */ 110 public ReferenceIdentityMap(int keyType, int valueType, boolean purgeValues) { 111 super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues); 112 } 113 114 /** 115 * Constructs a new <code>ReferenceIdentityMap</code> with the 116 * specified reference types, load factor and initial capacity. 117 * 118 * @param keyType the type of reference to use for keys; 119 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 120 * @param valueType the type of reference to use for values; 121 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 122 * @param capacity the initial capacity for the map 123 * @param loadFactor the load factor for the map 124 */ 125 public ReferenceIdentityMap(int keyType, int valueType, int capacity, float loadFactor) { 126 super(keyType, valueType, capacity, loadFactor, false); 127 } 128 129 /** 130 * Constructs a new <code>ReferenceIdentityMap</code> with the 131 * specified reference types, load factor and initial capacity. 132 * 133 * @param keyType the type of reference to use for keys; 134 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 135 * @param valueType the type of reference to use for values; 136 * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK} 137 * @param capacity the initial capacity for the map 138 * @param loadFactor the load factor for the map 139 * @param purgeValues should the value be automatically purged when the 140 * key is garbage collected 141 */ 142 public ReferenceIdentityMap(int keyType, int valueType, int capacity, 143 float loadFactor, boolean purgeValues) { 144 super(keyType, valueType, capacity, loadFactor, purgeValues); 145 } 146 147 //----------------------------------------------------------------------- 148 /** 149 * Gets the hash code for the key specified. 150 * <p> 151 * This implementation uses the identity hash code. 152 * 153 * @param key the key to get a hash code for 154 * @return the hash code 155 */ 156 protected int hash(Object key) { 157 return System.identityHashCode(key); 158 } 159 160 /** 161 * Gets the hash code for a MapEntry. 162 * <p> 163 * This implementation uses the identity hash code. 164 * 165 * @param key the key to get a hash code for, may be null 166 * @param value the value to get a hash code for, may be null 167 * @return the hash code, as per the MapEntry specification 168 */ 169 protected int hashEntry(Object key, Object value) { 170 return System.identityHashCode(key) ^ 171 System.identityHashCode(value); 172 } 173 174 /** 175 * Compares two keys for equals. 176 * <p> 177 * This implementation converts the key from the entry to a real reference 178 * before comparison and uses <code>==</code>. 179 * 180 * @param key1 the first key to compare passed in from outside 181 * @param key2 the second key extracted from the entry via <code>entry.key</code> 182 * @return true if equal by identity 183 */ 184 protected boolean isEqualKey(Object key1, Object key2) { 185 key2 = (keyType > HARD ? ((Reference) key2).get() : key2); 186 return (key1 == key2); 187 } 188 189 /** 190 * Compares two values for equals. 191 * <p> 192 * This implementation uses <code>==</code>. 193 * 194 * @param value1 the first value to compare passed in from outside 195 * @param value2 the second value extracted from the entry via <code>getValue()</code> 196 * @return true if equal by identity 197 */ 198 protected boolean isEqualValue(Object value1, Object value2) { 199 return (value1 == value2); 200 } 201 202 //----------------------------------------------------------------------- 203 /** 204 * Write the map out using a custom routine. 205 */ 206 private void writeObject(ObjectOutputStream out) throws IOException { 207 out.defaultWriteObject(); 208 doWriteObject(out); 209 } 210 211 /** 212 * Read the map in using a custom routine. 213 */ 214 private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { 215 in.defaultReadObject(); 216 doReadObject(in); 217 } 218 219 }