    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.pool2.impl;
    018
    019import java.lang.ref.Reference;
    020import java.lang.ref.ReferenceQueue;
    021import java.lang.ref.SoftReference;
    022import java.util.ArrayList;
    023import java.util.Iterator;
    024import java.util.NoSuchElementException;
    025
    026import org.apache.commons.pool2.BaseObjectPool;
    027import org.apache.commons.pool2.ObjectPool;
    028import org.apache.commons.pool2.PoolUtils;
    029import org.apache.commons.pool2.PooledObjectFactory;
    030
    031/**
    032 * A {@link java.lang.ref.SoftReference SoftReference} based {@link ObjectPool}.
    033 * <p>
    034 * This class is intended to be thread-safe.
    035 * </p>
    036 *
    037 * @param <T>
    038 *            Type of element pooled in this pool.
    039 *
    040 * @since 2.0
    041 */
    042public class SoftReferenceObjectPool<T> extends BaseObjectPool<T> {
    043
    044    /** Factory to source pooled objects */
    045    private final PooledObjectFactory<T> factory;
    046
    047    /**
    048     * Queue of broken references that might be able to be removed from
    049     * {@code _pool}. This is used to help {@link #getNumIdle()} be more
    050     * accurate with minimal performance overhead.
    051     */
    052    private final ReferenceQueue<T> refQueue = new ReferenceQueue<>();
    053
    054    /** Count of instances that have been checkout out to pool clients */
    055    private int numActive; // @GuardedBy("this")
    056
    057    /** Total number of instances that have been destroyed */
    058    private long destroyCount; // @GuardedBy("this")
    059
    060
    061    /** Total number of instances that have been created */
    062    private long createCount; // @GuardedBy("this")
    063
    064    /** Idle references - waiting to be borrowed */
    065    private final LinkedBlockingDeque<PooledSoftReference<T>> idleReferences =
    066        new LinkedBlockingDeque<>();
    067
    068    /** All references - checked out or waiting to be borrowed. */
    069    private final ArrayList<PooledSoftReference<T>> allReferences =
    070        new ArrayList<>();
    071
    072    /**
    073     * Create a {@code SoftReferenceObjectPool} with the specified factory.
    074     *
    075     * @param factory object factory to use.
    076     */
    077    public SoftReferenceObjectPool(final PooledObjectFactory<T> factory) {
    078        this.factory = factory;
    079    }
    080
    081    /**
    082     * Creates an object, and places it into the pool. addObject() is useful for
    083     * "pre-loading" a pool with idle objects.
    084     * <p>
    085     * Before being added to the pool, the newly created instance is
    086     * {@link PooledObjectFactory#validateObject(
    087     * org.apache.commons.pool2.PooledObject) validated} and
    088     * {@link PooledObjectFactory#passivateObject(
    089     * org.apache.commons.pool2.PooledObject) passivated}. If
    090     * validation fails, the new instance is
    091     * {@link PooledObjectFactory#destroyObject(
    092     * org.apache.commons.pool2.PooledObject) destroyed}. Exceptions
    093     * generated by the factory {@code makeObject} or
    094     * {@code passivate} are propagated to the caller. Exceptions
    095     * destroying instances are silently swallowed.
    096     *
    097     * @throws IllegalStateException
    098     *             if invoked on a {@link #close() closed} pool
    099     * @throws Exception
    100     *             when the {@link #getFactory() factory} has a problem creating
    101     *             or passivating an object.
    102     */
    103    @Override
    104    public synchronized void addObject() throws Exception {
    105        assertOpen();
    106        if (factory == null) {
    107            throw new IllegalStateException(
    108                    "Cannot add objects without a factory.");
    109        }
    110        final T obj = factory.makeObject().getObject();
    111        createCount++;
    112        // Create and register with the queue
    113        final PooledSoftReference<T> ref = new PooledSoftReference<>(
    114                new SoftReference<>(obj, refQueue));
    115        allReferences.add(ref);
    116
    117        boolean success = true;
    118        if (!factory.validateObject(ref)) {
    119            success = false;
    120        } else {
    121            factory.passivateObject(ref);
    122        }
    123
    124        final boolean shouldDestroy = !success;
    125        if (success) {
    126            idleReferences.add(ref);
    127            notifyAll(); // numActive has changed
    128        }
    129
    130        if (shouldDestroy) {
    131            try {
    132                destroy(ref);
    133            } catch (final Exception e) {
    134                // ignored
    135            }
    136        }
    137    }
    138
    139    /**
    140     * Borrows an object from the pool. If there are no idle instances available
    141     * in the pool, the configured factory's
    142     * {@link PooledObjectFactory#makeObject()} method is invoked to create a
    143     * new instance.
    144     * <p>
    145     * All instances are {@link PooledObjectFactory#activateObject(
    146     * org.apache.commons.pool2.PooledObject) activated}
    147     * and {@link PooledObjectFactory#validateObject(
    148     * org.apache.commons.pool2.PooledObject)
    149     * validated} before being returned by this method. If validation fails or
    150     * an exception occurs activating or validating an idle instance, the
    151     * failing instance is {@link PooledObjectFactory#destroyObject(
    152     * org.apache.commons.pool2.PooledObject)
    153     * destroyed} and another instance is retrieved from the pool, validated and
    154     * activated. This process continues until either the pool is empty or an
    155     * instance passes validation. If the pool is empty on activation or it does
    156     * not contain any valid instances, the factory's {@code makeObject}
    157     * method is used to create a new instance. If the created instance either
    158     * raises an exception on activation or fails validation,
    159     * {@code NoSuchElementException} is thrown. Exceptions thrown by
    160     * {@code MakeObject} are propagated to the caller; but other than
    161     * {@code ThreadDeath} or {@code VirtualMachineError}, exceptions
    162     * generated by activation, validation or destroy methods are swallowed
    163     * silently.
    164     *
    165     * @throws NoSuchElementException
    166     *             if a valid object cannot be provided
    167     * @throws IllegalStateException
    168     *             if invoked on a {@link #close() closed} pool
    169     * @throws Exception
    170     *             if an exception occurs creating a new instance
    171     * @return a valid, activated object instance
    172     */
    173    @SuppressWarnings("null") // ref cannot be null
    174    @Override
    175    public synchronized T borrowObject() throws Exception {
    176        assertOpen();
    177        T obj = null;
    178        boolean newlyCreated = false;
    179        PooledSoftReference<T> ref = null;
    180        while (null == obj) {
    181            if (idleReferences.isEmpty()) {
    182                if (null == factory) {
    183                    throw new NoSuchElementException();
    184                }
    185                newlyCreated = true;
    186                obj = factory.makeObject().getObject();
    187                createCount++;
    188                // Do not register with the queue
    189                ref = new PooledSoftReference<>(new SoftReference<>(obj));
    190                allReferences.add(ref);
    191            } else {
    192                ref = idleReferences.pollFirst();
    193                obj = ref.getObject();
    194                // Clear the reference so it will not be queued, but replace with a
    195                // a new, non-registered reference so we can still track this object
    196                // in allReferences
    197                ref.getReference().clear();
    198                ref.setReference(new SoftReference<>(obj));
    199            }
    200            if (null != factory && null != obj) {
    201                try {
    202                    factory.activateObject(ref);
    203                    if (!factory.validateObject(ref)) {
    204                        throw new Exception("ValidateObject failed");
    205                    }
    206                } catch (final Throwable t) {
    207                    PoolUtils.checkRethrow(t);
    208                    try {
    209                        destroy(ref);
    210                    } catch (final Throwable t2) {
    211                        PoolUtils.checkRethrow(t2);
    212                        // Swallowed
    213                    } finally {
    214                        obj = null;
    215                    }
    216                    if (newlyCreated) {
    217                        throw new NoSuchElementException(
    218                                "Could not create a validated object, cause: " +
    219                                        t.getMessage());
    220                    }
    221                }
    222            }
    223        }
    224        numActive++;
    225        ref.allocate();
    226        return obj;
    227    }
    228
    229    /**
    230     * Clears any objects sitting idle in the pool.
    231     */
    232    @Override
    233    public synchronized void clear() {
    234        if (null != factory) {
    235            final Iterator<PooledSoftReference<T>> iter = idleReferences.iterator();
    236            while (iter.hasNext()) {
    237                try {
    238                    final PooledSoftReference<T> ref = iter.next();
    239                    if (null != ref.getObject()) {
    240                        factory.destroyObject(ref);
    241                    }
    242                } catch (final Exception e) {
    243                    // ignore error, keep destroying the rest
    244                }
    245            }
    246        }
    247        idleReferences.clear();
    248        pruneClearedReferences();
    249    }
    250
    251    /**
    252     * Closes this pool, and frees any resources associated with it. Invokes
    253     * {@link #clear()} to destroy and remove instances in the pool.
    254     * <p>
    255     * Calling {@link #addObject} or {@link #borrowObject} after invoking this
    256     * method on a pool will cause them to throw an
    257     * {@link IllegalStateException}.
    258     */
    259    @Override
    260    public void close() {
    261        super.close();
    262        clear();
    263    }
    264
    265    /**
    266     * Destroys a {@code PooledSoftReference} and removes it from the idle and all
    267     * references pools.
    268     *
    269     * @param toDestroy PooledSoftReference to destroy
    270     *
    271     * @throws Exception If an error occurs while trying to destroy the object
    272     */
    273    private void destroy(final PooledSoftReference<T> toDestroy) throws Exception {
    274        toDestroy.invalidate();
    275        idleReferences.remove(toDestroy);
    276        allReferences.remove(toDestroy);
    277        try {
    278            factory.destroyObject(toDestroy);
    279        } finally {
    280            destroyCount++;
    281            toDestroy.getReference().clear();
    282        }
    283    }
    284
    285    /**
    286     * Finds the PooledSoftReference in allReferences that points to obj.
    287     *
    288     * @param obj returning object
    289     * @return PooledSoftReference wrapping a soft reference to obj
    290     */
    291    private PooledSoftReference<T> findReference(final T obj) {
    292        final Iterator<PooledSoftReference<T>> iterator = allReferences.iterator();
    293        while (iterator.hasNext()) {
    294            final PooledSoftReference<T> reference = iterator.next();
    295            if (reference.getObject() != null && reference.getObject().equals(obj)) {
    296                return reference;
    297            }
    298        }
    299        return null;
    300    }
    301
    302    /**
    303     * Gets the {@link PooledObjectFactory} used by this pool to create and
    304     * manage object instances.
    305     *
    306     * @return the factory
    307     */
    308    public synchronized PooledObjectFactory<T> getFactory() {
    309        return factory;
    310    }
    311
    312    /**
    313     * Gets the number of instances currently borrowed from this pool.
    314     *
    315     * @return the number of instances currently borrowed from this pool
    316     */
    317    @Override
    318    public synchronized int getNumActive() {
    319        return numActive;
    320    }
    321
    322    /**
    323     * Gets an approximation not less than the of the number of idle
    324     * instances in the pool.
    325     *
    326     * @return estimated number of idle instances in the pool
    327     */
    328    @Override
    329    public synchronized int getNumIdle() {
    330        pruneClearedReferences();
    331        return idleReferences.size();
    332    }
    333
    334    /**
    335     * {@inheritDoc}
    336     */
    337    @Override
    338    public synchronized void invalidateObject(final T obj) throws Exception {
    339        final PooledSoftReference<T> ref = findReference(obj);
    340        if (ref == null) {
    341            throw new IllegalStateException(
    342                "Object to invalidate is not currently part of this pool");
    343        }
    344        if (factory != null) {
    345            destroy(ref);
    346        }
    347        numActive--;
    348        notifyAll(); // numActive has changed
    349    }
    350
    351    /**
    352     * If any idle objects were garbage collected, remove their
    353     * {@link Reference} wrappers from the idle object pool.
    354     */
    355    private void pruneClearedReferences() {
    356        // Remove wrappers for enqueued references from idle and allReferences lists
    357        removeClearedReferences(idleReferences.iterator());
    358        removeClearedReferences(allReferences.iterator());
    359        while (refQueue.poll() != null) {
    360            // empty
    361        }
    362    }
    363
    364    /**
    365     * Clears cleared references from iterator's collection
    366     * @param iterator iterator over idle/allReferences
    367     */
    368    private void removeClearedReferences(final Iterator<PooledSoftReference<T>> iterator) {
    369        PooledSoftReference<T> ref;
    370        while (iterator.hasNext()) {
    371            ref = iterator.next();
    372            if (ref.getReference() == null || ref.getReference().isEnqueued()) {
    373                iterator.remove();
    374            }
    375        }
    376    }
    377
    378    /**
    379     * Returns an instance to the pool after successful validation and
    380     * passivation. The returning instance is destroyed if any of the following
    381     * are true:
    382     * <ul>
    383     * <li>the pool is closed</li>
    384     * <li>{@link PooledObjectFactory#validateObject(
    385     * org.apache.commons.pool2.PooledObject) validation} fails
    386     * </li>
    387     * <li>{@link PooledObjectFactory#passivateObject(
    388     * org.apache.commons.pool2.PooledObject) passivation}
    389     * throws an exception</li>
    390     * </ul>
    391     * Exceptions passivating or destroying instances are silently swallowed.
    392     * Exceptions validating instances are propagated to the client.
    393     *
    394     * @param obj
    395     *            instance to return to the pool
    396     * @throws IllegalArgumentException
    397     *            if obj is not currently part of this pool
    398     */
    399    @Override
    400    public synchronized void returnObject(final T obj) throws Exception {
    401        boolean success = !isClosed();
    402        final PooledSoftReference<T> ref = findReference(obj);
    403        if (ref == null) {
    404            throw new IllegalStateException(
    405                "Returned object not currently part of this pool");
    406        }
    407        if (factory != null) {
    408            if (!factory.validateObject(ref)) {
    409                success = false;
    410            } else {
    411                try {
    412                    factory.passivateObject(ref);
    413                } catch (final Exception e) {
    414                    success = false;
    415                }
    416            }
    417        }
    418
    419        final boolean shouldDestroy = !success;
    420        numActive--;
    421        if (success) {
    422
    423            // Deallocate and add to the idle instance pool
    424            ref.deallocate();
    425            idleReferences.add(ref);
    426        }
    427        notifyAll(); // numActive has changed
    428
    429        if (shouldDestroy && factory != null) {
    430            try {
    431                destroy(ref);
    432            } catch (final Exception e) {
    433                // ignored
    434            }
    435        }
    436    }
    437
    438    @Override
    439    protected void toStringAppendFields(final StringBuilder builder) {
    440        super.toStringAppendFields(builder);
    441        builder.append(", factory=");
    442        builder.append(factory);
    443        builder.append(", refQueue=");
    444        builder.append(refQueue);
    445        builder.append(", numActive=");
    446        builder.append(numActive);
    447        builder.append(", destroyCount=");
    448        builder.append(destroyCount);
    449        builder.append(", createCount=");
    450        builder.append(createCount);
    451        builder.append(", idleReferences=");
    452        builder.append(idleReferences);
    453        builder.append(", allReferences=");
    454        builder.append(allReferences);
    455    }
    456}