    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 *      https://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;
    024import java.util.Map;
    025
    026/**
    027 * A {@code Map} implementation that allows mappings to be
    028 * removed by the garbage collector and matches keys and values based
    029 * on {@code ==} not {@code equals()}.
    030 * <p>
    031 * When you construct a {@code ReferenceIdentityMap}, 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 * <p>
    039 * Different types of references can be specified for keys and values.
    040 * The default constructor uses hard keys and soft values, providing a
    041 * memory-sensitive cache.
    042 * </p>
    043 * <p>
    044 * This map is similar to
    045 * {@link ReferenceMap ReferenceMap}.
    046 * It differs in that keys and values in this class are compared using {@code ==}.
    047 * </p>
    048 * <p>
    049 * This map will violate the detail of various Map and map view contracts.
    050 * As a general rule, don't compare this map to other maps.
    051 * </p>
    052 * <p>
    053 * This {@link Map Map} implementation does <em>not</em> allow null elements.
    054 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
    055 * </p>
    056 * <p>
    057 * This implementation is not synchronized.
    058 * You can use {@link java.util.Collections#synchronizedMap} to
    059 * provide synchronized access to a {@code ReferenceIdentityMap}.
    060 * Remember that synchronization will not stop the garbage collector removing entries.
    061 * </p>
    062 * <p>
    063 * All the available iterators can be reset back to the start by casting to
    064 * {@code ResettableIterator} and calling {@code reset()}.
    065 * </p>
    066 * <p>
    067 * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
    068 * If you wish to use this map from multiple threads concurrently, you must use
    069 * appropriate synchronization. The simplest approach is to wrap this map
    070 * using {@link java.util.Collections#synchronizedMap}. This class may throw
    071 * exceptions when accessed by concurrent threads without synchronization.
    072 * </p>
    073 *
    074 * @param <K> The type of the keys in this map
    075 * @param <V> The type of the values in this map
    076 * @see java.lang.ref.Reference
    077 * @since 3.0 (previously in main package v2.1)
    078 */
    079public class ReferenceIdentityMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable {
    080
    081    /** Serialization version */
    082    private static final long serialVersionUID = -1266190134568365852L;
    083
    084    /**
    085     * Constructs a new {@code ReferenceIdentityMap} that will
    086     * use hard references to keys and soft references to values.
    087     */
    088    public ReferenceIdentityMap() {
    089        super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY,
    090                DEFAULT_LOAD_FACTOR, false);
    091    }
    092
    093    /**
    094     * Constructs a new {@code ReferenceIdentityMap} that will
    095     * use the specified types of references.
    096     *
    097     * @param keyType  The type of reference to use for keys;
    098     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    099     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    100     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    101     * @param valueType  The type of reference to use for values;
    102     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    103     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    104     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    105     */
    106    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
    107        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    108    }
    109
    110    /**
    111     * Constructs a new {@code ReferenceIdentityMap} that will
    112     * use the specified types of references.
    113     *
    114     * @param keyType  The type of reference to use for keys;
    115     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    116     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    117     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    118     * @param valueType  The type of reference to use for values;
    119     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    120     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    121     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    122     * @param purgeValues should the value be automatically purged when the
    123     *   key is garbage collected
    124     */
    125    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
    126            final boolean purgeValues) {
    127        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
    128    }
    129
    130    /**
    131     * Constructs a new {@code ReferenceIdentityMap} with the
    132     * specified reference types, load factor and initial capacity.
    133     *
    134     * @param keyType  The type of reference to use for keys;
    135     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    136     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    137     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    138     * @param valueType  The type of reference to use for values;
    139     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    140     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    141     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    142     * @param capacity  The initial capacity for the map
    143     * @param loadFactor  The load factor for the map
    144     */
    145    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
    146            final int capacity, final float loadFactor) {
    147        super(keyType, valueType, capacity, loadFactor, false);
    148    }
    149
    150    /**
    151     * Constructs a new {@code ReferenceIdentityMap} with the
    152     * specified reference types, load factor and initial capacity.
    153     *
    154     * @param keyType  The type of reference to use for keys;
    155     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    156     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    157     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    158     * @param valueType  The type of reference to use for values;
    159     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
    160     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
    161     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
    162     * @param capacity  The initial capacity for the map
    163     * @param loadFactor  The load factor for the map
    164     * @param purgeValues  should the value be automatically purged when the
    165     *   key is garbage collected
    166     */
    167    public ReferenceIdentityMap(final ReferenceStrength keyType, final ReferenceStrength valueType,
    168            final int capacity, final float loadFactor, final boolean purgeValues) {
    169        super(keyType, valueType, capacity, loadFactor, purgeValues);
    170    }
    171
    172    /**
    173     * Gets the hash code for the key specified.
    174     * <p>
    175     * This implementation uses the identity hash code.
    176     * </p>
    177     *
    178     * @param key  The key to get a hash code for
    179     * @return The hash code
    180     */
    181    @Override
    182    protected int hash(final Object key) {
    183        return System.identityHashCode(key);
    184    }
    185
    186    /**
    187     * Gets the hash code for a MapEntry.
    188     * <p>
    189     * This implementation uses the identity hash code.
    190     * </p>
    191     *
    192     * @param key  The key to get a hash code for, may be null
    193     * @param value  The value to get a hash code for, may be null
    194     * @return The hash code, as per the MapEntry specification
    195     */
    196    @Override
    197    protected int hashEntry(final Object key, final Object value) {
    198        return System.identityHashCode(key) ^
    199               System.identityHashCode(value);
    200    }
    201
    202    /**
    203     * Compares two keys for equals.
    204     * <p>
    205     * This implementation converts the key from the entry to a real reference
    206     * before comparison and uses {@code ==}.
    207     * </p>
    208     *
    209     * @param key1  The first key to compare passed in from outside
    210     * @param key2  The second key extracted from the entry via {@code entry.key}
    211     * @return true if equal by identity
    212     */
    213    @Override
    214    protected boolean isEqualKey(final Object key1, Object key2) {
    215        key2 = isKeyType(ReferenceStrength.HARD) ? key2 : ((Reference<?>) key2).get();
    216        return key1 == key2;
    217    }
    218
    219    /**
    220     * Compares two values for equals.
    221     * <p>
    222     * This implementation uses {@code ==}.
    223     * </p>
    224     *
    225     * @param value1  The first value to compare passed in from outside
    226     * @param value2  The second value extracted from the entry via {@code getValue()}
    227     * @return true if equal by identity
    228     */
    229    @Override
    230    protected boolean isEqualValue(final Object value1, final Object value2) {
    231        return value1 == value2;
    232    }
    233
    234    /**
    235     * Deserializes the map in using a custom routine.
    236     *
    237     * @param in The input stream
    238     * @throws IOException Thrown if an error occurs while reading from the stream
    239     * @throws ClassNotFoundException if an object read from the stream cannot be loaded
    240     */
    241    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    242        in.defaultReadObject();
    243        doReadObject(in);
    244    }
    245
    246    /**
    247     * Serializes this object to an ObjectOutputStream.
    248     *
    249     * @param out The target ObjectOutputStream.
    250     * @throws IOException thrown when an I/O errors occur writing to the target stream.
    251     */
    252    private void writeObject(final ObjectOutputStream out) throws IOException {
    253        out.defaultWriteObject();
    254        doWriteObject(out);
    255    }
    256
    257}