    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 */
    017package org.apache.commons.collections4.map;
    018
    019import java.io.IOException;
    020import java.io.ObjectInputStream;
    021import java.io.ObjectOutputStream;
    022import java.io.Serializable;
    023import java.lang.ref.Reference;
    024
    025/**
    026 * A {@code Map} implementation that allows mappings to be
    027 * removed by the garbage collector and matches keys and values based
    028 * on {@code ==} not {@code equals()}.
    029 * <p>
    030 * When you construct a {@code ReferenceIdentityMap}, 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 * <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 * <p>
    043 * This map is similar to
    044 * {@link org.apache.commons.collections4.map.ReferenceMap ReferenceMap}.
    045 * It differs in that keys and values in this class are compared using {@code ==}.
    046 * </p>
    047 * <p>
    048 * This map will violate the detail of various Map and map view contracts.
    049 * As a general rule, don't compare this map to other maps.
    050 * </p>
    051 * <p>
    052 * This {@link java.util.Map Map} implementation does <em>not</em> allow null elements.
    053 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
    054 * </p>
    055 * <p>
    056 * This implementation is not synchronized.
    057 * You can use {@link java.util.Collections#synchronizedMap} to
    058 * provide synchronized access to a {@code ReferenceIdentityMap}.
    059 * Remember that synchronization will not stop the garbage collector removing entries.
    060 * </p>
    061 * <p>
    062 * All the available iterators can be reset back to the start by casting to
    063 * {@code ResettableIterator} and calling {@code reset()}.
    064 * </p>
    065 * <p>
    066 * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
    067 * If you wish to use this map from multiple threads concurrently, you must use
    068 * appropriate synchronization. The simplest approach is to wrap this map
    069 * using {@link java.util.Collections#synchronizedMap}. This class may throw
    070 * exceptions when accessed by concurrent threads without synchronization.
    071 * </p>
    072 *
    073 * @param <K> the type of the keys in this map
    074 * @param <V> the type of the values in this map
    075 * @see java.lang.ref.Reference
    076 * @since 3.0 (previously in main package v2.1)
    077 */
    078public class ReferenceIdentityMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
    079
    080    /** Serialization version */
    081    private static final long serialVersionUID = -1266190134568365852L;
    082
    083    /**
    084     * Constructs a new {@code ReferenceIdentityMap} that will
    085     * use hard references to keys and soft references to values.
    086     */
    087    public ReferenceIdentityMap() {
    088        super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
    089                DEFAULT_LOAD_FACTOR, false);
    090    }
    091
    092    /**
    093     * Constructs a new {@code ReferenceIdentityMap} that will
    094     * use the specified types of references.
    095     *
    096     * @param keyType  the type of reference to use for keys;
    097     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    098     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    099     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    100     * @param valueType  the type of reference to use for values;
    101     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    102     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    103     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    104     */
    105    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
    106        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    107    }
    108
    109    /**
    110     * Constructs a new {@code ReferenceIdentityMap} that will
    111     * use the specified types of references.
    112     *
    113     * @param keyType  the type of reference to use for keys;
    114     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    115     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    116     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    117     * @param valueType  the type of reference to use for values;
    118     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    119     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    120     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    121     * @param purgeValues should the value be automatically purged when the
    122     *   key is garbage collected
    123     */
    124    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
    125            final boolean purgeValues) {
    126        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
    127    }
    128
    129    /**
    130     * Constructs a new {@code ReferenceIdentityMap} 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 AbstractReferenceMap.ReferenceStrength#HARD HARD},
    135     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    136     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    137     * @param valueType  the type of reference to use for values;
    138     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    139     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    140     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    141     * @param capacity  the initial capacity for the map
    142     * @param loadFactor  the load factor for the map
    143     */
    144    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
    145            final int capacity, final float loadFactor) {
    146        super(keyType, valueType, capacity, loadFactor, false);
    147    }
    148
    149    /**
    150     * Constructs a new {@code ReferenceIdentityMap} with the
    151     * specified reference types, load factor and initial capacity.
    152     *
    153     * @param keyType  the type of reference to use for keys;
    154     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    155     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    156     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    157     * @param valueType  the type of reference to use for values;
    158     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    159     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    160     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    161     * @param capacity  the initial capacity for the map
    162     * @param loadFactor  the load factor for the map
    163     * @param purgeValues  should the value be automatically purged when the
    164     *   key is garbage collected
    165     */
    166    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
    167            final int capacity, final float loadFactor, final boolean purgeValues) {
    168        super(keyType, valueType, capacity, loadFactor, purgeValues);
    169    }
    170
    171    /**
    172     * Gets the hash code for the key specified.
    173     * <p>
    174     * This implementation uses the identity hash code.
    175     * </p>
    176     *
    177     * @param key  the key to get a hash code for
    178     * @return the hash code
    179     */
    180    @Override
    181    protected int hash(final Object key) {
    182        return System.identityHashCode(key);
    183    }
    184
    185    /**
    186     * Gets the hash code for a MapEntry.
    187     * <p>
    188     * This implementation uses the identity hash code.
    189     * </p>
    190     *
    191     * @param key  the key to get a hash code for, may be null
    192     * @param value  the value to get a hash code for, may be null
    193     * @return the hash code, as per the MapEntry specification
    194     */
    195    @Override
    196    protected int hashEntry(final Object key, final Object value) {
    197        return System.identityHashCode(key) ^
    198               System.identityHashCode(value);
    199    }
    200
    201    /**
    202     * Compares two keys for equals.
    203     * <p>
    204     * This implementation converts the key from the entry to a real reference
    205     * before comparison and uses {@code ==}.
    206     * </p>
    207     *
    208     * @param key1  the first key to compare passed in from outside
    209     * @param key2  the second key extracted from the entry via {@code entry.key}
    210     * @return true if equal by identity
    211     */
    212    @Override
    213    protected boolean isEqualKey(final Object key1, Object key2) {
    214        key2 = isKeyType(ReferenceStrength.HARD) ? key2 : ((Reference<?>) key2).get();
    215        return key1 == key2;
    216    }
    217
    218    /**
    219     * Compares two values for equals.
    220     * <p>
    221     * This implementation uses {@code ==}.
    222     * </p>
    223     *
    224     * @param value1  the first value to compare passed in from outside
    225     * @param value2  the second value extracted from the entry via {@code getValue()}
    226     * @return true if equal by identity
    227     */
    228    @Override
    229    protected boolean isEqualValue(final Object value1, final Object value2) {
    230        return value1 == value2;
    231    }
    232
    233    /**
    234     * Deserializes the map in using a custom routine.
    235     *
    236     * @param in the input stream
    237     * @throws IOException if an error occurs while reading from the stream
    238     * @throws ClassNotFoundException if an object read from the stream cannot be loaded
    239     */
    240    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    241        in.defaultReadObject();
    242        doReadObject(in);
    243    }
    244
    245    /**
    246     * Serializes this object to an ObjectOutputStream.
    247     *
    248     * @param out the target ObjectOutputStream.
    249     * @throws IOException thrown when an I/O errors occur writing to the target stream.
    250     */
    251    private void writeObject(final ObjectOutputStream out) throws IOException {
    252        out.defaultWriteObject();
    253        doWriteObject(out);
    254    }
    255
    256}