    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.list;
    018
    019import java.io.IOException;
    020import java.io.ObjectInputStream;
    021import java.io.ObjectOutputStream;
    022import java.io.Serializable;
    023import java.util.Collection;
    024
    025/**
    026 * A <code>List</code> implementation that stores a cache of internal Node objects
    027 * in an effort to reduce wasteful object creation.
    028 * <p>
    029 * A linked list creates one Node for each item of data added. This can result in
    030 * a lot of object creation and garbage collection. This implementation seeks to
    031 * avoid that by maintaining a store of cached nodes.
    032 * <p>
    033 * This implementation is suitable for long-lived lists where both add and remove
    034 * are used. Short-lived lists, or lists which only grow will have worse performance
    035 * using this class.
    036 * <p>
    037 * <b>Note that this implementation is not synchronized.</b>
    038 *
    039 * @since 3.0
    040 */
    041public class NodeCachingLinkedList<E> extends AbstractLinkedList<E> implements Serializable {
    042
    043    /** Serialization version */
    044    private static final long serialVersionUID = 6897789178562232073L;
    045
    046    /**
    047     * The default value for {@link #maximumCacheSize}.
    048     */
    049    private static final int DEFAULT_MAXIMUM_CACHE_SIZE = 20;
    050
    051    /**
    052     * The first cached node, or <code>null</code> if no nodes are cached.
    053     * Cached nodes are stored in a singly-linked list with
    054     * <code>next</code> pointing to the next element.
    055     */
    056    private transient Node<E> firstCachedNode;
    057
    058    /**
    059     * The size of the cache.
    060     */
    061    private transient int cacheSize;
    062
    063    /**
    064     * The maximum size of the cache.
    065     */
    066    private int maximumCacheSize;
    067
    068    //-----------------------------------------------------------------------
    069    /**
    070     * Constructor that creates.
    071     */
    072    public NodeCachingLinkedList() {
    073        this(DEFAULT_MAXIMUM_CACHE_SIZE);
    074    }
    075
    076    /**
    077     * Constructor that copies the specified collection
    078     *
    079     * @param coll  the collection to copy
    080     */
    081    public NodeCachingLinkedList(final Collection<? extends E> coll) {
    082        super(coll);
    083        this.maximumCacheSize = DEFAULT_MAXIMUM_CACHE_SIZE;
    084    }
    085
    086    /**
    087     * Constructor that species the maximum cache size.
    088     *
    089     * @param maximumCacheSize  the maximum cache size
    090     */
    091    public NodeCachingLinkedList(final int maximumCacheSize) {
    092        super();
    093        this.maximumCacheSize = maximumCacheSize;
    094        init();  // must call init() as use super();
    095    }
    096
    097    //-----------------------------------------------------------------------
    098    /**
    099     * Gets the maximum size of the cache.
    100     *
    101     * @return the maximum cache size
    102     */
    103    protected int getMaximumCacheSize() {
    104        return maximumCacheSize;
    105    }
    106
    107    /**
    108     * Sets the maximum size of the cache.
    109     *
    110     * @param maximumCacheSize  the new maximum cache size
    111     */
    112    protected void setMaximumCacheSize(final int maximumCacheSize) {
    113        this.maximumCacheSize = maximumCacheSize;
    114        shrinkCacheToMaximumSize();
    115    }
    116
    117    /**
    118     * Reduce the size of the cache to the maximum, if necessary.
    119     */
    120    protected void shrinkCacheToMaximumSize() {
    121        // Rich Dougherty: This could be more efficient.
    122        while (cacheSize > maximumCacheSize) {
    123            getNodeFromCache();
    124        }
    125    }
    126
    127    /**
    128     * Gets a node from the cache. If a node is returned, then the value of
    129     * {@link #cacheSize} is decreased accordingly. The node that is returned
    130     * will have <code>null</code> values for next, previous and element.
    131     *
    132     * @return a node, or <code>null</code> if there are no nodes in the cache.
    133     */
    134    protected Node<E> getNodeFromCache() {
    135        if (cacheSize == 0) {
    136            return null;
    137        }
    138        final Node<E> cachedNode = firstCachedNode;
    139        firstCachedNode = cachedNode.next;
    140        cachedNode.next = null; // This should be changed anyway, but defensively
    141                                // set it to null.
    142        cacheSize--;
    143        return cachedNode;
    144    }
    145
    146    /**
    147     * Checks whether the cache is full.
    148     *
    149     * @return true if the cache is full
    150     */
    151    protected boolean isCacheFull() {
    152        return cacheSize >= maximumCacheSize;
    153    }
    154
    155    /**
    156     * Adds a node to the cache, if the cache isn't full.
    157     * The node's contents are cleared to so they can be garbage collected.
    158     *
    159     * @param node  the node to add to the cache
    160     */
    161    protected void addNodeToCache(final Node<E> node) {
    162        if (isCacheFull()) {
    163            // don't cache the node.
    164            return;
    165        }
    166        // clear the node's contents and add it to the cache.
    167        final Node<E> nextCachedNode = firstCachedNode;
    168        node.previous = null;
    169        node.next = nextCachedNode;
    170        node.setValue(null);
    171        firstCachedNode = node;
    172        cacheSize++;
    173    }
    174
    175    //-----------------------------------------------------------------------
    176    /**
    177     * Creates a new node, either by reusing one from the cache or creating
    178     * a new one.
    179     *
    180     * @param value  value of the new node
    181     * @return the newly created node
    182     */
    183    @Override
    184    protected Node<E> createNode(final E value) {
    185        final Node<E> cachedNode = getNodeFromCache();
    186        if (cachedNode == null) {
    187            return super.createNode(value);
    188        }
    189        cachedNode.setValue(value);
    190        return cachedNode;
    191    }
    192
    193    /**
    194     * Removes the node from the list, storing it in the cache for reuse
    195     * if the cache is not yet full.
    196     *
    197     * @param node  the node to remove
    198     */
    199    @Override
    200    protected void removeNode(final Node<E> node) {
    201        super.removeNode(node);
    202        addNodeToCache(node);
    203    }
    204
    205    /**
    206     * Removes all the nodes from the list, storing as many as required in the
    207     * cache for reuse.
    208     *
    209     */
    210    @Override
    211    protected void removeAllNodes() {
    212        // Add the removed nodes to the cache, then remove the rest.
    213        // We can add them to the cache before removing them, since
    214        // {@link AbstractLinkedList.removeAllNodes()} removes the
    215        // nodes by removing references directly from {@link #header}.
    216        final int numberOfNodesToCache = Math.min(size, maximumCacheSize - cacheSize);
    217        Node<E> node = header.next;
    218        for (int currentIndex = 0; currentIndex < numberOfNodesToCache; currentIndex++) {
    219            final Node<E> oldNode = node;
    220            node = node.next;
    221            addNodeToCache(oldNode);
    222        }
    223        super.removeAllNodes();
    224    }
    225
    226    //-----------------------------------------------------------------------
    227    /**
    228     * Serializes the data held in this object to the stream specified.
    229     *
    230     * @param out  the output stream
    231     * @throws IOException if an error occurs while writing to the stream
    232     */
    233    private void writeObject(final ObjectOutputStream out) throws IOException {
    234        out.defaultWriteObject();
    235        doWriteObject(out);
    236    }
    237
    238    /**
    239     * Deserializes the data held in this object to the stream specified.
    240     *
    241     * @param in  the input stream
    242     * @throws IOException if an error occurs while reading from the stream
    243     * @throws ClassNotFoundException if an object read from the stream can not be loaded
    244     */
    245    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    246        in.defaultReadObject();
    247        doReadObject(in);
    248    }
    249
    250}