ReferenceMap.java

/*
 * 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.commons.collections4.map;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

/**
 * A {@code Map} implementation that allows mappings to be
 * removed by the garbage collector.
 * <p>
 * When you construct a {@code 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}.
 * </p>
 * <p>
 * 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
 * <a href="https://docs.oracle.com/javase/8/docs/api/java/util/WeakHashMap.html">
 * {@code WeakHashMap}</a>. 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.
 * </p>
 * <p>
 * This map is similar to
 * {@link org.apache.commons.collections4.map.ReferenceIdentityMap ReferenceIdentityMap}.
 * It differs in that keys and values in this class are compared using {@code equals()}.
 * </p>
 * <p>
 * This {@link java.util.Map Map} implementation does <em>not</em> allow null elements.
 * Attempting to add a null key or value to the map will raise a {@code NullPointerException}.
 * </p>
 * <p>
 * This implementation is not synchronized.
 * You can use {@link java.util.Collections#synchronizedMap} to
 * provide synchronized access to a {@code ReferenceMap}.
 * Remember that synchronization will not stop the garbage collector removing entries.
 * </p>
 * <p>
 * All the available iterators can be reset back to the start by casting to
 * {@code ResettableIterator} and calling {@code reset()}.
 * </p>
 * <p>
 * <strong>Note that ReferenceMap 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.
 * </p>
 * <p>
 * NOTE: As from Commons Collections 3.1 this map extends {@code AbstractReferenceMap}
 * (previously it extended AbstractMap). As a result, the implementation is now
 * extensible and provides a {@code MapIterator}.
 * </p>
 *
 * @param <K> the type of the keys in the map
 * @param <V> 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<K, V> extends AbstractReferenceMap<K, V> implements Serializable {

    /** Serialization version */
    private static final long serialVersionUID = 1555089888138299607L;

    /**
     * Constructs a new {@code 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 {@code ReferenceMap} that will
     * use the specified types of references.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
     */
    public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) {
        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    }

    /**
     * Constructs a new {@code ReferenceMap} that will
     * use the specified types of references.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.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 {@code ReferenceMap} with the
     * specified reference types, load factor and initial
     * capacity.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.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 {@code ReferenceMap} with the
     * specified reference types, load factor and initial
     * capacity.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD},
     *   {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT},
     *   {@link AbstractReferenceMap.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);
    }

    /**
     * Deserializes 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 cannot be loaded
     */
    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        doReadObject(in);
    }

    /**
     * Serializes this object to an ObjectOutputStream.
     *
     * @param out the target ObjectOutputStream.
     * @throws IOException thrown when an I/O errors occur writing to the target stream.
     */
    private void writeObject(final ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        doWriteObject(out);
    }

}