/*
* Copyright 2004 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections.map;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.ref.Reference;
/**
* A <code>Map</code> implementation that allows mappings to be
* removed by the garbage collector and matches keys and values based
* on <code>==</code> not <code>equals()</code>.
* <p>
* <p>
* When you construct a <code>ReferenceIdentityMap</code>, you can specify what kind
* of references are used to store the map's keys and values.
* If non-hard references are used, then the garbage collector can remove
* mappings if a key or value becomes unreachable, or if the JVM's memory is
* running low. For information on how the different reference types behave,
* see {@link Reference}.
* <p>
* Different types of references can be specified for keys and values.
* The default constructor uses hard keys and soft values, providing a
* memory-sensitive cache.
* <p>
* This map is similar to
* {@link org.apache.commons.collections.map.ReferenceMap ReferenceMap}.
* It differs in that keys and values in this class are compared using <code>==</code>.
* <p>
* This map will violate the detail of various Map and map view contracts.
* As a general rule, don't compare this map to other maps.
* <p>
* This {@link java.util.Map Map} implementation does <i>not</i> allow null elements.
* Attempting to add a null key or value to the map will raise a <code>NullPointerException</code>.
* <p>
* This implementation is not synchronized.
* You can use {@link java.util.Collections#synchronizedMap} to
* provide synchronized access to a <code>ReferenceIdentityMap</code>.
* Remember that synchronization will not stop the garbage collecter removing entries.
* <p>
* All the available iterators can be reset back to the start by casting to
* <code>ResettableIterator</code> and calling <code>reset()</code>.
* <p>
* <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
* If you wish to use this map from multiple threads concurrently, you must use
* appropriate synchronization. The simplest approach is to wrap this map
* using {@link java.util.Collections#synchronizedMap}. This class may throw
* exceptions when accessed by concurrent threads without synchronization.
*
* @see java.lang.ref.Reference
*
* @since Commons Collections 3.0 (previously in main package v2.1)
* @version $Revision: 348007 $ $Date: 2005-11-21 22:52:57 +0000 (Mon, 21 Nov 2005) $
*
* @author Stephen Colebourne
*/
public class ReferenceIdentityMap extends AbstractReferenceMap implements Serializable {
/** Serialization version */
private static final long serialVersionUID = -1266190134568365852L;
/**
* Constructs a new <code>ReferenceIdentityMap</code> that will
* use hard references to keys and soft references to values.
*/
public ReferenceIdentityMap() {
super(HARD, SOFT, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
}
/**
* Constructs a new <code>ReferenceIdentityMap</code> that will
* use the specified types of references.
*
* @param keyType the type of reference to use for keys;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param valueType the type of reference to use for values;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
*/
public ReferenceIdentityMap(int keyType, int valueType) {
super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
}
/**
* Constructs a new <code>ReferenceIdentityMap</code> that will
* use the specified types of references.
*
* @param keyType the type of reference to use for keys;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param valueType the type of reference to use for values;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param purgeValues should the value be automatically purged when the
* key is garbage collected
*/
public ReferenceIdentityMap(int keyType, int valueType, boolean purgeValues) {
super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
}
/**
* Constructs a new <code>ReferenceIdentityMap</code> with the
* specified reference types, load factor and initial capacity.
*
* @param keyType the type of reference to use for keys;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param valueType the type of reference to use for values;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param capacity the initial capacity for the map
* @param loadFactor the load factor for the map
*/
public ReferenceIdentityMap(int keyType, int valueType, int capacity, float loadFactor) {
super(keyType, valueType, capacity, loadFactor, false);
}
/**
* Constructs a new <code>ReferenceIdentityMap</code> with the
* specified reference types, load factor and initial capacity.
*
* @param keyType the type of reference to use for keys;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param valueType the type of reference to use for values;
* must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
* @param capacity the initial capacity for the map
* @param loadFactor the load factor for the map
* @param purgeValues should the value be automatically purged when the
* key is garbage collected
*/
public ReferenceIdentityMap(int keyType, int valueType, int capacity,
float loadFactor, boolean purgeValues) {
super(keyType, valueType, capacity, loadFactor, purgeValues);
}
//-----------------------------------------------------------------------
/**
* Gets the hash code for the key specified.
* <p>
* This implementation uses the identity hash code.
*
* @param key the key to get a hash code for
* @return the hash code
*/
protected int hash(Object key) {
return System.identityHashCode(key);
}
/**
* Gets the hash code for a MapEntry.
* <p>
* This implementation uses the identity hash code.
*
* @param key the key to get a hash code for, may be null
* @param value the value to get a hash code for, may be null
* @return the hash code, as per the MapEntry specification
*/
protected int hashEntry(Object key, Object value) {
return System.identityHashCode(key) ^
System.identityHashCode(value);
}
/**
* Compares two keys for equals.
* <p>
* This implementation converts the key from the entry to a real reference
* before comparison and uses <code>==</code>.
*
* @param key1 the first key to compare passed in from outside
* @param key2 the second key extracted from the entry via <code>entry.key</code>
* @return true if equal by identity
*/
protected boolean isEqualKey(Object key1, Object key2) {
key2 = (keyType > HARD ? ((Reference) key2).get() : key2);
return (key1 == key2);
}
/**
* Compares two values for equals.
* <p>
* This implementation uses <code>==</code>.
*
* @param value1 the first value to compare passed in from outside
* @param value2 the second value extracted from the entry via <code>getValue()</code>
* @return true if equal by identity
*/
protected boolean isEqualValue(Object value1, Object value2) {
return (value1 == value2);
}
//-----------------------------------------------------------------------
/**
* Write the map out using a custom routine.
*/
private void writeObject(ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
doWriteObject(out);
}
/**
* Read the map in using a custom routine.
*/
private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
doReadObject(in);
}
}