org.apache.openjpa.lib.util.collections.ReferenceMap Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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.openjpa.lib.util.collections;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
/**
* A Map implementation that allows mappings to be
* removed by the garbage collector.
*
* When you construct a ReferenceMap, 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 java.lang.ref.Reference Reference}.
*
*
* Different types of references can be specified for keys and values.
* The keys can be configured to be weak but the values hard,
* in which case this class will behave like a
*
* WeakHashMap. However, you can also specify hard keys and
* weak values, or any other combination. The default constructor uses
* hard keys and soft values, providing a memory-sensitive cache.
*
*
* This map is similar to
* {@link ReferenceIdentityMap ReferenceIdentityMap}.
* It differs in that keys and values in this class are compared using equals().
*
*
* This {@link java.util.Map Map} implementation does not allow null elements.
* Attempting to add a null key or value to the map will raise a NullPointerException.
*
*
* This implementation is not synchronized.
* You can use {@link java.util.Collections#synchronizedMap} to
* provide synchronized access to a ReferenceMap.
* Remember that synchronization will not stop the garbage collector removing entries.
*
*
* All the available iterators can be reset back to the start by casting to
* ResettableIterator and calling reset().
*
*
* Note that ReferenceMap is not synchronized and is not thread-safe.
* 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.
*
*
* NOTE: As from Commons Collections 3.1 this map extends AbstractReferenceMap
* (previously it extended AbstractMap). As a result, the implementation is now
* extensible and provides a MapIterator.
*
*
* @param the type of the keys in the map
* @param the type of the values in the map
*
* @see java.lang.ref.Reference
* @since 3.0 (previously in main package v2.1)
*/
public class ReferenceMap extends AbstractReferenceMap implements Serializable {
/** Serialization version */
private static final long serialVersionUID = 1555089888138299607L;
/**
* Constructs a new ReferenceMap that will
* use hard references to keys and soft references to values.
*/
public ReferenceMap() {
super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
DEFAULT_LOAD_FACTOR, false);
}
/**
* Constructs a new ReferenceMap that will
* use the specified types of references.
*
* @param keyType the type of reference to use for keys;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
* @param valueType the type of reference to use for values;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
*/
public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
}
/**
* Constructs a new ReferenceMap that will
* use the specified types of references.
*
* @param keyType the type of reference to use for keys;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
* @param valueType the type of reference to use for values;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
* @param purgeValues should the value be automatically purged when the
* key is garbage collected
*/
public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final boolean purgeValues) {
super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
}
/**
* Constructs a new ReferenceMap with the
* specified reference types, load factor and initial
* capacity.
*
* @param keyType the type of reference to use for keys;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
* @param valueType the type of reference to use for values;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
* @param capacity the initial capacity for the map
* @param loadFactor the load factor for the map
*/
public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
final float loadFactor) {
super(keyType, valueType, capacity, loadFactor, false);
}
/**
* Constructs a new ReferenceMap with the
* specified reference types, load factor and initial
* capacity.
*
* @param keyType the type of reference to use for keys;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK WEAK}
* @param valueType the type of reference to use for values;
* must be {@link ReferenceStrength#HARD HARD},
* {@link ReferenceStrength#SOFT SOFT},
* {@link ReferenceStrength#WEAK 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 ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity,
final float loadFactor, final boolean purgeValues) {
super(keyType, valueType, capacity, loadFactor, purgeValues);
}
//-----------------------------------------------------------------------
/**
* Write the map out using a custom routine.
*
* @param out the output stream
* @throws IOException if an error occurs while writing to the stream
*/
private void writeObject(final ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
doWriteObject(out);
}
/**
* Read the map in using a custom routine.
*
* @param in the input stream
* @throws IOException if an error occurs while reading from the stream
* @throws ClassNotFoundException if an object read from the stream can not be loaded
*/
private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
doReadObject(in);
}
}