001: /*
002: * Copyright 2004 The Apache Software Foundation
003: *
004: * Licensed under the Apache License, Version 2.0 (the "License");
005: * you may not use this file except in compliance with the License.
006: * You may obtain a copy of the License at
007: *
008: * http://www.apache.org/licenses/LICENSE-2.0
009: *
010: * Unless required by applicable law or agreed to in writing, software
011: * distributed under the License is distributed on an "AS IS" BASIS,
012: * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013: * See the License for the specific language governing permissions and
014: * limitations under the License.
015: */
016: package org.apache.commons.collections.map;
017:
018: import java.io.IOException;
019: import java.io.ObjectInputStream;
020: import java.io.ObjectOutputStream;
021: import java.io.Serializable;
022: import java.lang.ref.Reference;
023:
024: /**
025: * A <code>Map</code> implementation that allows mappings to be
026: * removed by the garbage collector and matches keys and values based
027: * on <code>==</code> not <code>equals()</code>.
028: * <p>
029: * <p>
030: * When you construct a <code>ReferenceIdentityMap</code>, 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}.
036: * <p>
037: * Different types of references can be specified for keys and values.
038: * The default constructor uses hard keys and soft values, providing a
039: * memory-sensitive cache.
040: * <p>
041: * This map is similar to
042: * {@link org.apache.commons.collections.map.ReferenceMap ReferenceMap}.
043: * It differs in that keys and values in this class are compared using <code>==</code>.
044: * <p>
045: * This map will violate the detail of various Map and map view contracts.
046: * As a general rule, don't compare this map to other maps.
047: * <p>
048: * This {@link java.util.Map Map} implementation does <i>not</i> allow null elements.
049: * Attempting to add a null key or value to the map will raise a <code>NullPointerException</code>.
050: * <p>
051: * This implementation is not synchronized.
052: * You can use {@link java.util.Collections#synchronizedMap} to
053: * provide synchronized access to a <code>ReferenceIdentityMap</code>.
054: * Remember that synchronization will not stop the garbage collecter removing entries.
055: * <p>
056: * All the available iterators can be reset back to the start by casting to
057: * <code>ResettableIterator</code> and calling <code>reset()</code>.
058: * <p>
059: * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
060: * If you wish to use this map from multiple threads concurrently, you must use
061: * appropriate synchronization. The simplest approach is to wrap this map
062: * using {@link java.util.Collections#synchronizedMap}. This class may throw
063: * exceptions when accessed by concurrent threads without synchronization.
064: *
065: * @see java.lang.ref.Reference
066: *
067: * @since Commons Collections 3.0 (previously in main package v2.1)
068: * @version $Revision: 348007 $ $Date: 2005-11-21 22:52:57 +0000 (Mon, 21 Nov 2005) $
069: *
070: * @author Stephen Colebourne
071: */
072: public class ReferenceIdentityMap extends AbstractReferenceMap
073: 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,
097: DEFAULT_LOAD_FACTOR, false);
098: }
099:
100: /**
101: * Constructs a new <code>ReferenceIdentityMap</code> that will
102: * use the specified types of references.
103: *
104: * @param keyType the type of reference to use for keys;
105: * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
106: * @param valueType the type of reference to use for values;
107: * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
108: * @param purgeValues should the value be automatically purged when the
109: * key is garbage collected
110: */
111: public ReferenceIdentityMap(int keyType, int valueType,
112: boolean purgeValues) {
113: super (keyType, valueType, DEFAULT_CAPACITY,
114: DEFAULT_LOAD_FACTOR, purgeValues);
115: }
116:
117: /**
118: * Constructs a new <code>ReferenceIdentityMap</code> with the
119: * specified reference types, load factor and initial capacity.
120: *
121: * @param keyType the type of reference to use for keys;
122: * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
123: * @param valueType the type of reference to use for values;
124: * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
125: * @param capacity the initial capacity for the map
126: * @param loadFactor the load factor for the map
127: */
128: public ReferenceIdentityMap(int keyType, int valueType,
129: int capacity, float loadFactor) {
130: super (keyType, valueType, capacity, loadFactor, false);
131: }
132:
133: /**
134: * Constructs a new <code>ReferenceIdentityMap</code> with the
135: * specified reference types, load factor and initial capacity.
136: *
137: * @param keyType the type of reference to use for keys;
138: * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
139: * @param valueType the type of reference to use for values;
140: * must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
141: * @param capacity the initial capacity for the map
142: * @param loadFactor the load factor for the map
143: * @param purgeValues should the value be automatically purged when the
144: * key is garbage collected
145: */
146: public ReferenceIdentityMap(int keyType, int valueType,
147: int capacity, float loadFactor, boolean purgeValues) {
148: super (keyType, valueType, capacity, loadFactor, purgeValues);
149: }
150:
151: //-----------------------------------------------------------------------
152: /**
153: * Gets the hash code for the key specified.
154: * <p>
155: * This implementation uses the identity hash code.
156: *
157: * @param key the key to get a hash code for
158: * @return the hash code
159: */
160: protected int hash(Object key) {
161: return System.identityHashCode(key);
162: }
163:
164: /**
165: * Gets the hash code for a MapEntry.
166: * <p>
167: * This implementation uses the identity hash code.
168: *
169: * @param key the key to get a hash code for, may be null
170: * @param value the value to get a hash code for, may be null
171: * @return the hash code, as per the MapEntry specification
172: */
173: protected int hashEntry(Object key, Object value) {
174: return System.identityHashCode(key)
175: ^ System.identityHashCode(value);
176: }
177:
178: /**
179: * Compares two keys for equals.
180: * <p>
181: * This implementation converts the key from the entry to a real reference
182: * before comparison and uses <code>==</code>.
183: *
184: * @param key1 the first key to compare passed in from outside
185: * @param key2 the second key extracted from the entry via <code>entry.key</code>
186: * @return true if equal by identity
187: */
188: protected boolean isEqualKey(Object key1, Object key2) {
189: key2 = (keyType > HARD ? ((Reference) key2).get() : key2);
190: return (key1 == key2);
191: }
192:
193: /**
194: * Compares two values for equals.
195: * <p>
196: * This implementation uses <code>==</code>.
197: *
198: * @param value1 the first value to compare passed in from outside
199: * @param value2 the second value extracted from the entry via <code>getValue()</code>
200: * @return true if equal by identity
201: */
202: protected boolean isEqualValue(Object value1, Object value2) {
203: return (value1 == value2);
204: }
205:
206: //-----------------------------------------------------------------------
207: /**
208: * Write the map out using a custom routine.
209: */
210: private void writeObject(ObjectOutputStream out) throws IOException {
211: out.defaultWriteObject();
212: doWriteObject(out);
213: }
214:
215: /**
216: * Read the map in using a custom routine.
217: */
218: private void readObject(ObjectInputStream in) throws IOException,
219: ClassNotFoundException {
220: in.defaultReadObject();
221: doReadObject(in);
222: }
223:
224: }
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