    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.util.Map;
    024
    025import org.apache.commons.collections4.BoundedMap;
    026
    027/**
    028 * A <code>Map</code> implementation with a fixed maximum size which removes
    029 * the least recently used entry if an entry is added when full.
    030 * <p>
    031 * The least recently used algorithm works on the get and put operations only.
    032 * Iteration of any kind, including setting the value by iteration, does not
    033 * change the order. Queries such as containsKey and containsValue or access
    034 * via views also do not change the order.
    035 * </p>
    036 * <p>
    037 * A somewhat subtle ramification of the least recently used
    038 * algorithm is that calls to {@link #get(Object)} stand a very good chance
    039 * of modifying the map's iteration order and thus invalidating any
    040 * iterators currently in use.  It is therefore suggested that iterations
    041 * over an {@link LRUMap} instance access entry values only through a
    042 * {@link org.apache.commons.collections4.MapIterator MapIterator} or {@link #entrySet()} iterator.
    043 * </p>
    044 * <p>
    045 * The map implements <code>OrderedMap</code> and entries may be queried using
    046 * the bidirectional <code>OrderedMapIterator</code>. The order returned is
    047 * least recently used to most recently used. Iterators from map views can
    048 * also be cast to <code>OrderedIterator</code> if required.
    049 * </p>
    050 * <p>
    051 * All the available iterators can be reset back to the start by casting to
    052 * <code>ResettableIterator</code> and calling <code>reset()</code>.
    053 * </p>
    054 * <p>
    055 * <strong>Note that LRUMap is not synchronized and is not thread-safe.</strong>
    056 * If you wish to use this map from multiple threads concurrently, you must use
    057 * appropriate synchronization. The simplest approach is to wrap this map
    058 * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
    059 * <code>NullPointerException</code>'s when accessed by concurrent threads.
    060 * </p>
    061 *
    062 * @param <K> the type of the keys in this map
    063 * @param <V> the type of the values in this map
    064 * @since 3.0 (previously in main package v1.0)
    065 */
    066public class LRUMap<K, V>
    067        extends AbstractLinkedMap<K, V> implements BoundedMap<K, V>, Serializable, Cloneable {
    068
    069    /** Serialisation version */
    070    private static final long serialVersionUID = -612114643488955218L;
    071    /** Default maximum size */
    072    protected static final int DEFAULT_MAX_SIZE = 100;
    073
    074    /** Maximum size */
    075    private transient int maxSize;
    076    /** Scan behaviour */
    077    private boolean scanUntilRemovable;
    078
    079    /**
    080     * Constructs a new empty map with a maximum size of 100.
    081     */
    082    public LRUMap() {
    083        this(DEFAULT_MAX_SIZE, DEFAULT_LOAD_FACTOR, false);
    084    }
    085
    086    /**
    087     * Constructs a new, empty map with the specified maximum size.
    088     *
    089     * @param maxSize  the maximum size of the map
    090     * @throws IllegalArgumentException if the maximum size is less than one
    091     */
    092    public LRUMap(final int maxSize) {
    093        this(maxSize, DEFAULT_LOAD_FACTOR);
    094    }
    095
    096    /**
    097     * Constructs a new, empty map with the specified maximum size.
    098     *
    099     * @param maxSize  the maximum size of the map
    100     * @param initialSize  the initial size of the map
    101     * @throws IllegalArgumentException if the maximum size is less than one
    102     * @throws IllegalArgumentException if the initial size is negative or larger than the maximum size
    103     * @since 4.1
    104     */
    105    public LRUMap(final int maxSize, final int initialSize) {
    106        this(maxSize, initialSize, DEFAULT_LOAD_FACTOR);
    107    }
    108
    109    /**
    110     * Constructs a new, empty map with the specified maximum size.
    111     *
    112     * @param maxSize  the maximum size of the map
    113     * @param scanUntilRemovable  scan until a removeable entry is found, default false
    114     * @throws IllegalArgumentException if the maximum size is less than one
    115     * @since 3.1
    116     */
    117    public LRUMap(final int maxSize, final boolean scanUntilRemovable) {
    118        this(maxSize, DEFAULT_LOAD_FACTOR, scanUntilRemovable);
    119    }
    120
    121    /**
    122     * Constructs a new, empty map with the specified max capacity and
    123     * load factor.
    124     *
    125     * @param maxSize  the maximum size of the map
    126     * @param loadFactor  the load factor
    127     * @throws IllegalArgumentException if the maximum size is less than one
    128     * @throws IllegalArgumentException if the load factor is less than zero
    129     */
    130    public LRUMap(final int maxSize, final float loadFactor) {
    131        this(maxSize, loadFactor, false);
    132    }
    133
    134    /**
    135     * Constructs a new, empty map with the specified max / initial capacity and
    136     * load factor.
    137     *
    138     * @param maxSize  the maximum size of the map
    139     * @param initialSize  the initial size of the map
    140     * @param loadFactor  the load factor
    141     * @throws IllegalArgumentException if the maximum size is less than one
    142     * @throws IllegalArgumentException if the initial size is negative or larger than the maximum size
    143     * @throws IllegalArgumentException if the load factor is less than zero
    144     * @since 4.1
    145     */
    146    public LRUMap(final int maxSize, final int initialSize, final float loadFactor) {
    147        this(maxSize, initialSize, loadFactor, false);
    148    }
    149
    150    /**
    151     * Constructs a new, empty map with the specified max capacity and load factor.
    152     *
    153     * @param maxSize  the maximum size of the map
    154     * @param loadFactor  the load factor
    155     * @param scanUntilRemovable  scan until a removeable entry is found, default false
    156     * @throws IllegalArgumentException if the maximum size is less than one
    157     * @throws IllegalArgumentException if the load factor is less than zero
    158     * @since 3.1
    159     */
    160    public LRUMap(final int maxSize, final float loadFactor, final boolean scanUntilRemovable) {
    161        this(maxSize, maxSize, loadFactor, scanUntilRemovable);
    162    }
    163
    164    /**
    165     * Constructs a new, empty map with the specified max / initial capacity and load factor.
    166     *
    167     * @param maxSize  the maximum size of the map
    168     * @param initialSize  the initial size of the map
    169     * @param loadFactor  the load factor
    170     * @param scanUntilRemovable  scan until a removeable entry is found, default false
    171     * @throws IllegalArgumentException if the maximum size is less than one
    172     * @throws IllegalArgumentException if the initial size is negative or larger than the maximum size
    173     * @throws IllegalArgumentException if the load factor is less than zero
    174     * @since 4.1
    175     */
    176    public LRUMap(final int maxSize,
    177                  final int initialSize,
    178                  final float loadFactor,
    179                  final boolean scanUntilRemovable) {
    180
    181        super(initialSize, loadFactor);
    182        if (maxSize < 1) {
    183            throw new IllegalArgumentException("LRUMap max size must be greater than 0");
    184        }
    185        if (initialSize > maxSize) {
    186            throw new IllegalArgumentException("LRUMap initial size must not be greather than max size");
    187        }
    188        this.maxSize = maxSize;
    189        this.scanUntilRemovable = scanUntilRemovable;
    190    }
    191
    192    /**
    193     * Constructor copying elements from another map.
    194     * <p>
    195     * The maximum size is set from the map's size.
    196     *
    197     * @param map  the map to copy
    198     * @throws NullPointerException if the map is null
    199     * @throws IllegalArgumentException if the map is empty
    200     */
    201    public LRUMap(final Map<? extends K, ? extends V> map) {
    202        this(map, false);
    203    }
    204
    205    /**
    206     * Constructor copying elements from another map.
    207     *
    208     * <p>The maximum size is set from the map's size.</p>
    209     *
    210     * @param map  the map to copy
    211     * @param scanUntilRemovable  scan until a removeable entry is found, default false
    212     * @throws NullPointerException if the map is null
    213     * @throws IllegalArgumentException if the map is empty
    214     * @since 3.1
    215     */
    216    public LRUMap(final Map<? extends K, ? extends V> map, final boolean scanUntilRemovable) {
    217        this(map.size(), DEFAULT_LOAD_FACTOR, scanUntilRemovable);
    218        putAll(map);
    219    }
    220
    221    //-----------------------------------------------------------------------
    222    /**
    223     * Gets the value mapped to the key specified.
    224     * <p>
    225     * This operation changes the position of the key in the map to the
    226     * most recently used position (last).
    227     *
    228     * @param key  the key
    229     * @return the mapped value, null if no match
    230     */
    231    @Override
    232    public V get(final Object key) {
    233        return get(key, true);
    234    }
    235
    236    /**
    237     * Gets the value mapped to the key specified.
    238     * <p>
    239     * If {@code updateToMRU} is {@code true}, the position of the key in the map
    240     * is changed to the most recently used position (last), otherwise the iteration
    241     * order is not changed by this operation.
    242     *
    243     * @param key  the key
    244     * @param updateToMRU  whether the key shall be updated to the
    245     *   most recently used position
    246     * @return the mapped value, null if no match
    247     * @since 4.1
    248     */
    249    public V get(final Object key, final boolean updateToMRU) {
    250        final LinkEntry<K, V> entry = getEntry(key);
    251        if (entry == null) {
    252            return null;
    253        }
    254        if (updateToMRU) {
    255            moveToMRU(entry);
    256        }
    257        return entry.getValue();
    258    }
    259
    260    //-----------------------------------------------------------------------
    261    /**
    262     * Moves an entry to the MRU position at the end of the list.
    263     * <p>
    264     * This implementation moves the updated entry to the end of the list.
    265     *
    266     * @param entry  the entry to update
    267     */
    268    protected void moveToMRU(final LinkEntry<K, V> entry) {
    269        if (entry.after != header) {
    270            modCount++;
    271            // remove
    272            if(entry.before == null) {
    273                throw new IllegalStateException("Entry.before is null." +
    274                    " This should not occur if your keys are immutable, and you have used synchronization properly.");
    275            }
    276            entry.before.after = entry.after;
    277            entry.after.before = entry.before;
    278            // add first
    279            entry.after = header;
    280            entry.before = header.before;
    281            header.before.after = entry;
    282            header.before = entry;
    283        } else if (entry == header) {
    284            throw new IllegalStateException("Can't move header to MRU" +
    285                    " This should not occur if your keys are immutable, and you have used synchronization properly.");
    286        }
    287    }
    288
    289    /**
    290     * Updates an existing key-value mapping.
    291     * <p>
    292     * This implementation moves the updated entry to the end of the list
    293     * using {@link #moveToMRU(AbstractLinkedMap.LinkEntry)}.
    294     *
    295     * @param entry  the entry to update
    296     * @param newValue  the new value to store
    297     */
    298    @Override
    299    protected void updateEntry(final HashEntry<K, V> entry, final V newValue) {
    300        moveToMRU((LinkEntry<K, V>) entry);  // handles modCount
    301        entry.setValue(newValue);
    302    }
    303
    304    /**
    305     * Adds a new key-value mapping into this map.
    306     * <p>
    307     * This implementation checks the LRU size and determines whether to
    308     * discard an entry or not using {@link #removeLRU(AbstractLinkedMap.LinkEntry)}.
    309     * <p>
    310     * From Commons Collections 3.1 this method uses {@link #isFull()} rather
    311     * than accessing <code>size</code> and <code>maxSize</code> directly.
    312     * It also handles the scanUntilRemovable functionality.
    313     *
    314     * @param hashIndex  the index into the data array to store at
    315     * @param hashCode  the hash code of the key to add
    316     * @param key  the key to add
    317     * @param value  the value to add
    318     */
    319    @Override
    320    protected void addMapping(final int hashIndex, final int hashCode, final K key, final V value) {
    321        if (isFull()) {
    322            LinkEntry<K, V> reuse = header.after;
    323            boolean removeLRUEntry = false;
    324            if (scanUntilRemovable) {
    325                while (reuse != header && reuse != null) {
    326                    if (removeLRU(reuse)) {
    327                        removeLRUEntry = true;
    328                        break;
    329                    }
    330                    reuse = reuse.after;
    331                }
    332                if (reuse == null) {
    333                    throw new IllegalStateException(
    334                        "Entry.after=null, header.after=" + header.after + " header.before=" + header.before +
    335                        " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
    336                        " This should not occur if your keys are immutable, and you have used synchronization properly.");
    337                }
    338            } else {
    339                removeLRUEntry = removeLRU(reuse);
    340            }
    341
    342            if (removeLRUEntry) {
    343                if (reuse == null) {
    344                    throw new IllegalStateException(
    345                        "reuse=null, header.after=" + header.after + " header.before=" + header.before +
    346                        " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
    347                        " This should not occur if your keys are immutable, and you have used synchronization properly.");
    348            }
    349                reuseMapping(reuse, hashIndex, hashCode, key, value);
    350            } else {
    351                super.addMapping(hashIndex, hashCode, key, value);
    352            }
    353        } else {
    354            super.addMapping(hashIndex, hashCode, key, value);
    355        }
    356    }
    357
    358    /**
    359     * Reuses an entry by removing it and moving it to a new place in the map.
    360     * <p>
    361     * This method uses {@link #removeEntry}, {@link #reuseEntry} and {@link #addEntry}.
    362     *
    363     * @param entry  the entry to reuse
    364     * @param hashIndex  the index into the data array to store at
    365     * @param hashCode  the hash code of the key to add
    366     * @param key  the key to add
    367     * @param value  the value to add
    368     */
    369    protected void reuseMapping(final LinkEntry<K, V> entry, final int hashIndex, final int hashCode,
    370                                final K key, final V value) {
    371        // find the entry before the entry specified in the hash table
    372        // remember that the parameters (except the first) refer to the new entry,
    373        // not the old one
    374        try {
    375            final int removeIndex = hashIndex(entry.hashCode, data.length);
    376            final HashEntry<K, V>[] tmp = data;  // may protect against some sync issues
    377            HashEntry<K, V> loop = tmp[removeIndex];
    378            HashEntry<K, V> previous = null;
    379            while (loop != entry && loop != null) {
    380                previous = loop;
    381                loop = loop.next;
    382            }
    383            if (loop == null) {
    384                throw new IllegalStateException(
    385                    "Entry.next=null, data[removeIndex]=" + data[removeIndex] + " previous=" + previous +
    386                    " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
    387                    " This should not occur if your keys are immutable, and you have used synchronization properly.");
    388            }
    389
    390            // reuse the entry
    391            modCount++;
    392            removeEntry(entry, removeIndex, previous);
    393            reuseEntry(entry, hashIndex, hashCode, key, value);
    394            addEntry(entry, hashIndex);
    395        } catch (final NullPointerException ex) {
    396            throw new IllegalStateException(
    397                    "NPE, entry=" + entry + " entryIsHeader=" + (entry==header) +
    398                    " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
    399                    " This should not occur if your keys are immutable, and you have used synchronization properly.");
    400        }
    401    }
    402
    403    /**
    404     * Subclass method to control removal of the least recently used entry from the map.
    405     * <p>
    406     * This method exists for subclasses to override. A subclass may wish to
    407     * provide cleanup of resources when an entry is removed. For example:
    408     * <pre>
    409     * protected boolean removeLRU(LinkEntry entry) {
    410     *   releaseResources(entry.getValue());  // release resources held by entry
    411     *   return true;  // actually delete entry
    412     * }
    413     * </pre>
    414     * <p>
    415     * Alternatively, a subclass may choose to not remove the entry or selectively
    416     * keep certain LRU entries. For example:
    417     * <pre>
    418     * protected boolean removeLRU(LinkEntry entry) {
    419     *   if (entry.getKey().toString().startsWith("System.")) {
    420     *     return false;  // entry not removed from LRUMap
    421     *   } else {
    422     *     return true;  // actually delete entry
    423     *   }
    424     * }
    425     * </pre>
    426     * The effect of returning false is dependent on the scanUntilRemovable flag.
    427     * If the flag is true, the next LRU entry will be passed to this method and so on
    428     * until one returns false and is removed, or every entry in the map has been passed.
    429     * If the scanUntilRemovable flag is false, the map will exceed the maximum size.
    430     * <p>
    431     * NOTE: Commons Collections 3.0 passed the wrong entry to this method.
    432     * This is fixed in version 3.1 onwards.
    433     *
    434     * @param entry  the entry to be removed
    435     * @return {@code true}
    436     */
    437    protected boolean removeLRU(final LinkEntry<K, V> entry) {
    438        return true;
    439    }
    440
    441    //-----------------------------------------------------------------------
    442    /**
    443     * Returns true if this map is full and no new mappings can be added.
    444     *
    445     * @return <code>true</code> if the map is full
    446     */
    447    @Override
    448    public boolean isFull() {
    449        return size >= maxSize;
    450    }
    451
    452    /**
    453     * Gets the maximum size of the map (the bound).
    454     *
    455     * @return the maximum number of elements the map can hold
    456     */
    457    @Override
    458    public int maxSize() {
    459        return maxSize;
    460    }
    461
    462    /**
    463     * Whether this LRUMap will scan until a removable entry is found when the
    464     * map is full.
    465     *
    466     * @return true if this map scans
    467     * @since 3.1
    468     */
    469    public boolean isScanUntilRemovable() {
    470        return scanUntilRemovable;
    471    }
    472
    473    //-----------------------------------------------------------------------
    474    /**
    475     * Clones the map without cloning the keys or values.
    476     *
    477     * @return a shallow clone
    478     */
    479    @Override
    480    public LRUMap<K, V> clone() {
    481        return (LRUMap<K, V>) super.clone();
    482    }
    483
    484    /**
    485     * Write the map out using a custom routine.
    486     *
    487     * @param out  the output stream
    488     * @throws IOException if an error occurs while writing to the stream
    489     */
    490    private void writeObject(final ObjectOutputStream out) throws IOException {
    491        out.defaultWriteObject();
    492        doWriteObject(out);
    493    }
    494
    495    /**
    496     * Read the map in using a custom routine.
    497     *
    498     * @param in the input stream
    499     * @throws IOException if an error occurs while reading from the stream
    500     * @throws ClassNotFoundException if an object read from the stream can not be loaded
    501     */
    502    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    503        in.defaultReadObject();
    504        doReadObject(in);
    505    }
    506
    507    /**
    508     * Writes the data necessary for <code>put()</code> to work in deserialization.
    509     *
    510     * @param out  the output stream
    511     * @throws IOException if an error occurs while writing to the stream
    512     */
    513    @Override
    514    protected void doWriteObject(final ObjectOutputStream out) throws IOException {
    515        out.writeInt(maxSize);
    516        super.doWriteObject(out);
    517    }
    518
    519    /**
    520     * Reads the data necessary for <code>put()</code> to work in the superclass.
    521     *
    522     * @param in  the input stream
    523     * @throws IOException if an error occurs while reading from the stream
    524     * @throws ClassNotFoundException if an object read from the stream can not be loaded
    525     */
    526    @Override
    527    protected void doReadObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    528        maxSize = in.readInt();
    529        super.doReadObject(in);
    530    }
    531
    532}