Source code
Description: source: package: org.apache.commons.logging.impl, class: WeakHashtable

    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 */
    017
    018package org.apache.commons.logging.impl;
    019
    020import java.lang.ref.ReferenceQueue;
    021import java.lang.ref.WeakReference;
    022import java.util.ArrayList;
    023import java.util.Collection;
    024import java.util.Enumeration;
    025import java.util.HashSet;
    026import java.util.Hashtable;
    027import java.util.List;
    028import java.util.Map;
    029import java.util.Objects;
    030import java.util.Set;
    031
    032/**
    033 * Implements {@code Hashtable} to use {@code WeakReference}'s
    034 * to hold its keys thus allowing them to be reclaimed by the garbage collector.
    035 * The associated values are retained using strong references.
    036 * <p>
    037 * This class follows the semantics of {@code Hashtable} as closely as
    038 * possible. It therefore does not accept null values or keys.
    039 * </p>
    040 * <p>
    041 * <strong>Note:</strong>
    042 * This is <em>not</em> intended to be a general purpose hash table replacement.
    043 * This implementation is also tuned towards a particular purpose: for use as a replacement
    044 * for {@code Hashtable} in {@code LogFactory}. This application requires
    045 * good liveliness for {@code get} and {@code put}. Various tradeoffs
    046 * have been made with this in mind.
    047 * </p>
    048 * <p>
    049 * <strong>Usage:</strong> typical use case is as a drop-in replacement
    050 * for the {@code Hashtable} used in {@code LogFactory} for J2EE environments
    051 * running 1.3+ JVMs. Use of this class <em>in most cases</em> (see below) will
    052 * allow class loaders to be collected by the garbage collector without the need
    053 * to call {@link org.apache.commons.logging.LogFactory#release(ClassLoader) LogFactory.release(ClassLoader)}.
    054 * </p>
    055 * <p>
    056 * {@code org.apache.commons.logging.LogFactory} checks whether this class
    057 * can be supported by the current JVM, and if so then uses it to store
    058 * references to the {@code LogFactory} implementation it loads
    059 * (rather than using a standard Hashtable instance).
    060 * Having this class used instead of {@code Hashtable} solves
    061 * certain issues related to dynamic reloading of applications in J2EE-style
    062 * environments. However this class requires Java 1.3 or later (due to its use
    063 * of {@link java.lang.ref.WeakReference} and associates).
    064 * And by the way, this extends {@code Hashtable} rather than {@code HashMap}
    065 * for backwards compatibility reasons. See the documentation
    066 * for method {@code LogFactory.createFactoryStore} for more details.
    067 * </p>
    068 * <p>
    069 * The reason all this is necessary is due to a issue which
    070 * arises during hot deploy in a J2EE-like containers.
    071 * Each component running in the container owns one or more class loaders; when
    072 * the component loads a LogFactory instance via the component class loader
    073 * a reference to it gets stored in the static LogFactory.factories member,
    074 * keyed by the component's class loader so different components don't
    075 * stomp on each other. When the component is later unloaded, the container
    076 * sets the component's class loader to null with the intent that all the
    077 * component's classes get garbage-collected. However there's still a
    078 * reference to the component's class loader from a key in the "global"
    079 * {@code LogFactory}'s factories member! If {@code LogFactory.release()}
    080 * is called whenever component is unloaded, the class loaders will be correctly
    081 * garbage collected; this <em>should</em> be done by any container that
    082 * bundles commons-logging by default. However, holding the class loader
    083 * references weakly ensures that the class loader will be garbage collected
    084 * without the container performing this step.
    085 * </p>
    086 * <p>
    087 * <strong>Limitations:</strong>
    088 * There is still one (unusual) scenario in which a component will not
    089 * be correctly unloaded without an explicit release. Though weak references
    090 * are used for its keys, it is necessary to use strong references for its values.
    091 * </p>
    092 * <p>
    093 * If the abstract class {@code LogFactory} is
    094 * loaded by the container class loader but a subclass of
    095 * {@code LogFactory} [LogFactory1] is loaded by the component's
    096 * class loader and an instance stored in the static map associated with the
    097 * base LogFactory class, then there is a strong reference from the LogFactory
    098 * class to the LogFactory1 instance (as normal) and a strong reference from
    099 * the LogFactory1 instance to the component class loader via
    100 * {@code getClass().getClassLoader()}. This chain of references will prevent
    101 * collection of the child class loader.
    102 * </p>
    103 * <p>
    104 * Such a situation occurs when the commons-logging.jar is
    105 * loaded by a parent class loader (e.g. a server level class loader in a
    106 * servlet container) and a custom {@code LogFactory} implementation is
    107 * loaded by a child class loader (e.g. a web app class loader).
    108 * </p>
    109 * <p>
    110 * To avoid this scenario, ensure
    111 * that any custom LogFactory subclass is loaded by the same class loader as
    112 * the base {@code LogFactory}. Creating custom LogFactory subclasses is,
    113 * however, rare. The standard LogFactoryImpl class should be sufficient
    114 * for most or all users.
    115 * </p>
    116 *
    117 * @since 1.1
    118 * @deprecated No longer used, will be removed in 2.0.
    119 */
    120@Deprecated
    121public final class WeakHashtable extends Hashtable {
    122
    123    /** Entry implementation */
    124    private final static class Entry implements Map.Entry {
    125
    126        private final Object key;
    127        private final Object value;
    128
    129        private Entry(final Object key, final Object value) {
    130            this.key = key;
    131            this.value = value;
    132        }
    133
    134        @Override
    135        public boolean equals(final Object o) {
    136            boolean result = false;
    137            if (o instanceof Map.Entry) {
    138                final Map.Entry entry = (Map.Entry) o;
    139                result = (getKey() == null ? entry.getKey() == null : getKey().equals(entry.getKey()))
    140                        && (getValue() == null ? entry.getValue() == null : getValue().equals(entry.getValue()));
    141            }
    142            return result;
    143        }
    144
    145        @Override
    146        public Object getKey() {
    147            return key;
    148        }
    149
    150        @Override
    151        public Object getValue() {
    152            return value;
    153        }
    154
    155        @Override
    156        public int hashCode() {
    157            return (getKey() == null ? 0 : getKey().hashCode()) ^ (getValue() == null ? 0 : getValue().hashCode());
    158        }
    159
    160        @Override
    161        public Object setValue(final Object value) {
    162            throw new UnsupportedOperationException("Entry.setValue is not supported.");
    163        }
    164    }
    165
    166    /** Wrapper giving correct symantics for equals and hash code */
    167    private final static class Referenced {
    168
    169        private final WeakReference reference;
    170        private final int hashCode;
    171
    172        /**
    173         *
    174         * @throws NullPointerException if referant is {@code null}
    175         */
    176        private Referenced(final Object referant) {
    177            reference = new WeakReference(referant);
    178            // Calc a permanent hashCode so calls to Hashtable.remove()
    179            // work if the WeakReference has been cleared
    180            hashCode = referant.hashCode();
    181        }
    182
    183        /**
    184         *
    185         * @throws NullPointerException if key is {@code null}
    186         */
    187        private Referenced(final Object key, final ReferenceQueue queue) {
    188            reference = new WeakKey(key, queue, this);
    189            // Calc a permanent hashCode so calls to Hashtable.remove()
    190            // work if the WeakReference has been cleared
    191            hashCode = key.hashCode();
    192        }
    193
    194        @Override
    195        public boolean equals(final Object o) {
    196            boolean result = false;
    197            if (o instanceof Referenced) {
    198                final Referenced otherKey = (Referenced) o;
    199                final Object thisKeyValue = getValue();
    200                final Object otherKeyValue = otherKey.getValue();
    201                if (thisKeyValue == null) {
    202                    result = otherKeyValue == null;
    203                    // Since our hash code was calculated from the original
    204                    // non-null referant, the above check breaks the
    205                    // hash code/equals contract, as two cleared Referenced
    206                    // objects could test equal but have different hash codes.
    207                    // We can reduce (not eliminate) the chance of this
    208                    // happening by comparing hash codes.
    209                    result = result && hashCode() == otherKey.hashCode();
    210                    // In any case, as our constructor does not allow null referants
    211                    // and Hashtable does not do equality checks between
    212                    // existing keys, normal hash table operations should never
    213                    // result in an equals comparison between null referants
    214                } else {
    215                    result = thisKeyValue.equals(otherKeyValue);
    216                }
    217            }
    218            return result;
    219        }
    220
    221        private Object getValue() {
    222            return reference.get();
    223        }
    224
    225        @Override
    226        public int hashCode() {
    227            return hashCode;
    228        }
    229    }
    230
    231    /**
    232     * WeakReference subclass that holds a hard reference to an
    233     * associated {@code value} and also makes accessible
    234     * the Referenced object holding it.
    235     */
    236    private final static class WeakKey extends WeakReference {
    237
    238        private final Referenced referenced;
    239
    240        private WeakKey(final Object key, final ReferenceQueue queue, final Referenced referenced) {
    241            super(key, queue);
    242            this.referenced = referenced;
    243        }
    244
    245        private Referenced getReferenced() {
    246            return referenced;
    247        }
    248    }
    249
    250    /** Serializable version identifier. */
    251    private static final long serialVersionUID = -1546036869799732453L;
    252
    253    /**
    254     * The maximum number of times put() or remove() can be called before
    255     * the map will be purged of all cleared entries.
    256     */
    257    private static final int MAX_CHANGES_BEFORE_PURGE = 100;
    258
    259    /**
    260     * The maximum number of times put() or remove() can be called before
    261     * the map will be purged of one cleared entry.
    262     */
    263    private static final int PARTIAL_PURGE_COUNT = 10;
    264
    265    /** ReferenceQueue we check for GC'd keys. */
    266    private final transient ReferenceQueue queue = new ReferenceQueue();
    267
    268    /** Counter used to control how often we purge gc'd entries. */
    269    private int changeCount;
    270
    271    /**
    272     * Constructs a WeakHashtable with the Hashtable default
    273     * capacity and load factor.
    274     */
    275    public WeakHashtable() {
    276        // empty
    277    }
    278
    279    /**
    280     *@see Hashtable
    281     */
    282    @Override
    283    public boolean containsKey(final Object key) {
    284        // purge should not be required
    285        return super.containsKey(new Referenced(key));
    286    }
    287
    288    /**
    289     *@see Hashtable
    290     */
    291    @Override
    292    public Enumeration elements() {
    293        purge();
    294        return super.elements();
    295    }
    296
    297    /**
    298     *@see Hashtable
    299     */
    300    @Override
    301    public Set entrySet() {
    302        purge();
    303        final Set referencedEntries = super.entrySet();
    304        final Set unreferencedEntries = new HashSet();
    305        for (final Object referencedEntry : referencedEntries) {
    306            final Map.Entry entry = (Map.Entry) referencedEntry;
    307            final Referenced referencedKey = (Referenced) entry.getKey();
    308            final Object key = referencedKey.getValue();
    309            final Object value = entry.getValue();
    310            if (key != null) {
    311                final Entry dereferencedEntry = new Entry(key, value);
    312                unreferencedEntries.add(dereferencedEntry);
    313            }
    314        }
    315        return unreferencedEntries;
    316    }
    317
    318    /**
    319     *@see Hashtable
    320     */
    321    @Override
    322    public Object get(final Object key) {
    323        // for performance reasons, no purge
    324        return super.get(new Referenced(key));
    325    }
    326
    327    /**
    328     *@see Hashtable
    329     */
    330    @Override
    331    public boolean isEmpty() {
    332        purge();
    333        return super.isEmpty();
    334    }
    335
    336    /**
    337     *@see Hashtable
    338     */
    339    @Override
    340    public Enumeration keys() {
    341        purge();
    342        final Enumeration enumer = super.keys();
    343        return new Enumeration() {
    344            @Override
    345            public boolean hasMoreElements() {
    346                return enumer.hasMoreElements();
    347            }
    348            @Override
    349            public Object nextElement() {
    350                 final Referenced nextReference = (Referenced) enumer.nextElement();
    351                 return nextReference.getValue();
    352            }
    353        };
    354    }
    355
    356    /**
    357     *@see Hashtable
    358     */
    359    @Override
    360    public Set keySet() {
    361        purge();
    362        final Set referencedKeys = super.keySet();
    363        final Set unreferencedKeys = new HashSet();
    364        for (final Object referencedKey : referencedKeys) {
    365            final Referenced referenceKey = (Referenced) referencedKey;
    366            final Object keyValue = referenceKey.getValue();
    367            if (keyValue != null) {
    368                unreferencedKeys.add(keyValue);
    369            }
    370        }
    371        return unreferencedKeys;
    372    }
    373
    374    /**
    375     * Purges all entries whose wrapped keys
    376     * have been garbage collected.
    377     */
    378    private void purge() {
    379        final List toRemove = new ArrayList<>();
    380        synchronized (queue) {
    381            WeakKey key;
    382            while ((key = (WeakKey) queue.poll()) != null) {
    383                toRemove.add(key.getReferenced());
    384            }
    385        }
    386        // LOGGING-119: do the actual removal of the keys outside the sync block
    387        // to prevent deadlock scenarios as purge() may be called from
    388        // non-synchronized methods too
    389        final int size = toRemove.size();
    390        for (int i = 0; i < size; i++) {
    391            super.remove(toRemove.get(i));
    392        }
    393    }
    394
    395    /**
    396     * Purges one entry whose wrapped key
    397     * has been garbage collected.
    398     */
    399    private void purgeOne() {
    400        synchronized (queue) {
    401            final WeakKey key = (WeakKey) queue.poll();
    402            if (key != null) {
    403                super.remove(key.getReferenced());
    404            }
    405        }
    406    }
    407
    408    /**
    409     *@see Hashtable
    410     */
    411    @Override
    412    public synchronized Object put(final Object key, final Object value) {
    413        // check for nulls, ensuring semantics match superclass
    414        Objects.requireNonNull(key, "key");
    415        Objects.requireNonNull(value, "value");
    416        // for performance reasons, only purge every
    417        // MAX_CHANGES_BEFORE_PURGE times
    418        if (changeCount++ > MAX_CHANGES_BEFORE_PURGE) {
    419            purge();
    420            changeCount = 0;
    421        }
    422        // do a partial purge more often
    423        else if (changeCount % PARTIAL_PURGE_COUNT == 0) {
    424            purgeOne();
    425        }
    426        final Referenced keyRef = new Referenced(key, queue);
    427        return super.put(keyRef, value);
    428    }
    429
    430    /**
    431     *@see Hashtable
    432     */
    433    @Override
    434    public void putAll(final Map t) {
    435        if (t != null) {
    436            final Set entrySet = t.entrySet();
    437            for (final Object element : entrySet) {
    438                final Map.Entry entry = (Map.Entry) element;
    439                put(entry.getKey(), entry.getValue());
    440            }
    441        }
    442    }
    443
    444    /**
    445     * @see Hashtable
    446     */
    447    @Override
    448    protected void rehash() {
    449        // purge here to save the effort of rehashing dead entries
    450        purge();
    451        super.rehash();
    452    }
    453
    454    /**
    455     *@see Hashtable
    456     */
    457    @Override
    458    public synchronized Object remove(final Object key) {
    459        // for performance reasons, only purge every
    460        // MAX_CHANGES_BEFORE_PURGE times
    461        if (changeCount++ > MAX_CHANGES_BEFORE_PURGE) {
    462            purge();
    463            changeCount = 0;
    464        }
    465        // do a partial purge more often
    466        else if (changeCount % PARTIAL_PURGE_COUNT == 0) {
    467            purgeOne();
    468        }
    469        return super.remove(new Referenced(key));
    470    }
    471
    472    /**
    473     *@see Hashtable
    474     */
    475    @Override
    476    public int size() {
    477        purge();
    478        return super.size();
    479    }
    480
    481    /**
    482     *@see Hashtable
    483     */
    484    @Override
    485    public String toString() {
    486        purge();
    487        return super.toString();
    488    }
    489
    490    /**
    491     *@see Hashtable
    492     */
    493    @Override
    494    public Collection values() {
    495        purge();
    496        return super.values();
    497    }
    498}