/*
 * 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.tomcat.dbcp.pool2.impl;

import java.io.PrintWriter;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.management.ManagementFactory;
import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.TimerTask;
import java.util.concurrent.atomic.AtomicLong;

import javax.management.InstanceAlreadyExistsException;
import javax.management.InstanceNotFoundException;
import javax.management.MBeanRegistrationException;
import javax.management.MBeanServer;
import javax.management.MalformedObjectNameException;
import javax.management.NotCompliantMBeanException;
import javax.management.ObjectName;

import org.apache.tomcat.dbcp.pool2.PooledObject;
import org.apache.tomcat.dbcp.pool2.SwallowedExceptionListener;

/**
 * Base class that provides common functionality for {@link GenericObjectPool}
 * and {@link GenericKeyedObjectPool}. The primary reason this class exists is
 * reduce code duplication between the two pool implementations.
 *
 * @param <T> Type of element pooled in this pool.
 *
 * This class is intended to be thread-safe.
 *
 * @since 2.0
 */
public abstract class BaseGenericObjectPool<T> {

    // Constants
    /**
     * The size of the caches used to store historical data for some attributes
     * so that rolling means may be calculated.
     */
    public static final int MEAN_TIMING_STATS_CACHE_SIZE = 100;

    // Configuration attributes
    private volatile int maxTotal =
            GenericKeyedObjectPoolConfig.DEFAULT_MAX_TOTAL;
    private volatile boolean blockWhenExhausted =
            BaseObjectPoolConfig.DEFAULT_BLOCK_WHEN_EXHAUSTED;
    private volatile long maxWaitMillis =
            BaseObjectPoolConfig.DEFAULT_MAX_WAIT_MILLIS;
    private volatile boolean lifo = BaseObjectPoolConfig.DEFAULT_LIFO;
    private final boolean fairness;
    private volatile boolean testOnCreate =
            BaseObjectPoolConfig.DEFAULT_TEST_ON_CREATE;
    private volatile boolean testOnBorrow =
            BaseObjectPoolConfig.DEFAULT_TEST_ON_BORROW;
    private volatile boolean testOnReturn =
            BaseObjectPoolConfig.DEFAULT_TEST_ON_RETURN;
    private volatile boolean testWhileIdle =
            BaseObjectPoolConfig.DEFAULT_TEST_WHILE_IDLE;
    private volatile long timeBetweenEvictionRunsMillis =
            BaseObjectPoolConfig.DEFAULT_TIME_BETWEEN_EVICTION_RUNS_MILLIS;
    private volatile int numTestsPerEvictionRun =
            BaseObjectPoolConfig.DEFAULT_NUM_TESTS_PER_EVICTION_RUN;
    private volatile long minEvictableIdleTimeMillis =
            BaseObjectPoolConfig.DEFAULT_MIN_EVICTABLE_IDLE_TIME_MILLIS;
    private volatile long softMinEvictableIdleTimeMillis =
            BaseObjectPoolConfig.DEFAULT_SOFT_MIN_EVICTABLE_IDLE_TIME_MILLIS;
    private volatile EvictionPolicy<T> evictionPolicy;


    // Internal (primarily state) attributes
    final Object closeLock = new Object();
    volatile boolean closed = false;
    final Object evictionLock = new Object();
    private Evictor evictor = null; // @GuardedBy("evictionLock")
    Iterator<PooledObject<T>> evictionIterator = null; // @GuardedBy("evictionLock")
    /*
     * Class loader for evictor thread to use since, in a JavaEE or similar
     * environment, the context class loader for the evictor thread may not have
     * visibility of the correct factory. See POOL-161. Uses a weak reference to
     * avoid potential memory leaks if the Pool is discarded rather than closed.
     */
    private final WeakReference<ClassLoader> factoryClassLoader;


    // Monitoring (primarily JMX) attributes
    private final ObjectName oname;
    private final String creationStackTrace;
    private final AtomicLong borrowedCount = new AtomicLong(0);
    private final AtomicLong returnedCount = new AtomicLong(0);
    final AtomicLong createdCount = new AtomicLong(0);
    final AtomicLong destroyedCount = new AtomicLong(0);
    final AtomicLong destroyedByEvictorCount = new AtomicLong(0);
    final AtomicLong destroyedByBorrowValidationCount = new AtomicLong(0);
    private final StatsStore activeTimes = new StatsStore(MEAN_TIMING_STATS_CACHE_SIZE);
    private final StatsStore idleTimes = new StatsStore(MEAN_TIMING_STATS_CACHE_SIZE);
    private final StatsStore waitTimes = new StatsStore(MEAN_TIMING_STATS_CACHE_SIZE);
    private final Object maxBorrowWaitTimeMillisLock = new Object();
    private volatile long maxBorrowWaitTimeMillis = 0; // @GuardedBy("maxBorrowWaitTimeMillisLock")
    private volatile SwallowedExceptionListener swallowedExceptionListener = null;


    /**
     * Handles JMX registration (if required) and the initialization required for
     * monitoring.
     *
     * @param config        Pool configuration
     * @param jmxNameBase   The default base JMX name for the new pool unless
     *                      overridden by the config
     * @param jmxNamePrefix Prefix to be used for JMX name for the new pool
     */
    public BaseGenericObjectPool(BaseObjectPoolConfig config,
            String jmxNameBase, String jmxNamePrefix) {
        if (config.getJmxEnabled()) {
            this.oname = jmxRegister(config, jmxNameBase, jmxNamePrefix);
        } else {
            this.oname = null;
        }

        // Populate the creation stack trace
        this.creationStackTrace = getStackTrace(new Exception());

        // save the current TCCL (if any) to be used later by the evictor Thread
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        if (cl == null) {
            factoryClassLoader = null;
        } else {
            factoryClassLoader = new WeakReference<>(cl);
        }

        fairness = config.getFairness();
    }


    /**
     * Returns the maximum number of objects that can be allocated by the pool
     * (checked out to clients, or idle awaiting checkout) at a given time. When
     * negative, there is no limit to the number of objects that can be
     * managed by the pool at one time.
     *
     * @return the cap on the total number of object instances managed by the
     *         pool.
     *
     * @see #setMaxTotal
     */
    public final int getMaxTotal() {
        return maxTotal;
    }

    /**
     * Sets the cap on the number of objects that can be allocated by the pool
     * (checked out to clients, or idle awaiting checkout) at a given time. Use
     * a negative value for no limit.
     *
     * @param maxTotal  The cap on the total number of object instances managed
     *                  by the pool. Negative values mean that there is no limit
     *                  to the number of objects allocated by the pool.
     *
     * @see #getMaxTotal
     */
    public final void setMaxTotal(int maxTotal) {
        this.maxTotal = maxTotal;
    }

    /**
     * Returns whether to block when the <code>borrowObject()</code> method is
     * invoked when the pool is exhausted (the maximum number of "active"
     * objects has been reached).
     *
     * @return <code>true</code> if <code>borrowObject()</code> should block
     *         when the pool is exhausted
     *
     * @see #setBlockWhenExhausted
     */
    public final boolean getBlockWhenExhausted() {
        return blockWhenExhausted;
    }

    /**
     * Sets whether to block when the <code>borrowObject()</code> method is
     * invoked when the pool is exhausted (the maximum number of "active"
     * objects has been reached).
     *
     * @param blockWhenExhausted    <code>true</code> if
     *                              <code>borrowObject()</code> should block
     *                              when the pool is exhausted
     *
     * @see #getBlockWhenExhausted
     */
    public final void setBlockWhenExhausted(boolean blockWhenExhausted) {
        this.blockWhenExhausted = blockWhenExhausted;
    }

    /**
     * Returns the maximum amount of time (in milliseconds) the
     * <code>borrowObject()</code> method should block before throwing an
     * exception when the pool is exhausted and
     * {@link #getBlockWhenExhausted} is true. When less than 0, the
     * <code>borrowObject()</code> method may block indefinitely.
     *
     * @return the maximum number of milliseconds <code>borrowObject()</code>
     *         will block.
     *
     * @see #setMaxWaitMillis
     * @see #setBlockWhenExhausted
     */
    public final long getMaxWaitMillis() {
        return maxWaitMillis;
    }

    /**
     * Sets the maximum amount of time (in milliseconds) the
     * <code>borrowObject()</code> method should block before throwing an
     * exception when the pool is exhausted and
     * {@link #getBlockWhenExhausted} is true. When less than 0, the
     * <code>borrowObject()</code> method may block indefinitely.
     *
     * @param maxWaitMillis the maximum number of milliseconds
     *                      <code>borrowObject()</code> will block or negative
     *                      for indefinitely.
     *
     * @see #getMaxWaitMillis
     * @see #setBlockWhenExhausted
     */
    public final void setMaxWaitMillis(long maxWaitMillis) {
        this.maxWaitMillis = maxWaitMillis;
    }

    /**
     * Returns whether the pool has LIFO (last in, first out) behaviour with
     * respect to idle objects - always returning the most recently used object
     * from the pool, or as a FIFO (first in, first out) queue, where the pool
     * always returns the oldest object in the idle object pool.
     *
     * @return <code>true</code> if the pool is configured with LIFO behaviour
     *         or <code>false</code> if the pool is configured with FIFO
     *         behaviour
     *
     * @see #setLifo
     */
    public final boolean getLifo() {
        return lifo;
    }

    /**
     * Returns whether or not the pool serves threads waiting to borrow objects fairly.
     * True means that waiting threads are served as if waiting in a FIFO queue.
     *
     * @return <code>true</code> if waiting threads are to be served
     *             by the pool in arrival order
     */
    public final boolean getFairness() {
        return fairness;
    }

    /**
     * Sets whether the pool has LIFO (last in, first out) behaviour with
     * respect to idle objects - always returning the most recently used object
     * from the pool, or as a FIFO (first in, first out) queue, where the pool
     * always returns the oldest object in the idle object pool.
     *
     * @param lifo  <code>true</code> if the pool is to be configured with LIFO
     *              behaviour or <code>false</code> if the pool is to be
     *              configured with FIFO behaviour
     *
     * @see #getLifo()
     */
    public final void setLifo(boolean lifo) {
        this.lifo = lifo;
    }

    /**
     * Returns whether objects created for the pool will be validated before
     * being returned from the <code>borrowObject()</code> method. Validation is
     * performed by the <code>validateObject()</code> method of the factory
     * associated with the pool. If the object fails to validate, then
     * <code>borrowObject()</code> will fail.
     *
     * @return <code>true</code> if newly created objects are validated before
     *         being returned from the <code>borrowObject()</code> method
     *
     * @see #setTestOnCreate
     *
     * @since 2.2
     */
    public final boolean getTestOnCreate() {
        return testOnCreate;
    }

    /**
     * Sets whether objects created for the pool will be validated before
     * being returned from the <code>borrowObject()</code> method. Validation is
     * performed by the <code>validateObject()</code> method of the factory
     * associated with the pool. If the object fails to validate, then
     * <code>borrowObject()</code> will fail.
     *
     * @param testOnCreate  <code>true</code> if newly created objects should be
     *                      validated before being returned from the
     *                      <code>borrowObject()</code> method
     *
     * @see #getTestOnCreate
     *
     * @since 2.2
     */
    public final void setTestOnCreate(boolean testOnCreate) {
        this.testOnCreate = testOnCreate;
    }

    /**
     * Returns whether objects borrowed from the pool will be validated before
     * being returned from the <code>borrowObject()</code> method. Validation is
     * performed by the <code>validateObject()</code> method of the factory
     * associated with the pool. If the object fails to validate, it will be
     * removed from the pool and destroyed, and a new attempt will be made to
     * borrow an object from the pool.
     *
     * @return <code>true</code> if objects are validated before being returned
     *         from the <code>borrowObject()</code> method
     *
     * @see #setTestOnBorrow
     */
    public final boolean getTestOnBorrow() {
        return testOnBorrow;
    }

    /**
     * Sets whether objects borrowed from the pool will be validated before
     * being returned from the <code>borrowObject()</code> method. Validation is
     * performed by the <code>validateObject()</code> method of the factory
     * associated with the pool. If the object fails to validate, it will be
     * removed from the pool and destroyed, and a new attempt will be made to
     * borrow an object from the pool.
     *
     * @param testOnBorrow  <code>true</code> if objects should be validated
     *                      before being returned from the
     *                      <code>borrowObject()</code> method
     *
     * @see #getTestOnBorrow
     */
    public final void setTestOnBorrow(boolean testOnBorrow) {
        this.testOnBorrow = testOnBorrow;
    }

    /**
     * Returns whether objects borrowed from the pool will be validated when
     * they are returned to the pool via the <code>returnObject()</code> method.
     * Validation is performed by the <code>validateObject()</code> method of
     * the factory associated with the pool. Returning objects that fail validation
     * are destroyed rather then being returned the pool.
     *
     * @return <code>true</code> if objects are validated on return to
     *         the pool via the <code>returnObject()</code> method
     *
     * @see #setTestOnReturn
     */
    public final boolean getTestOnReturn() {
        return testOnReturn;
    }

    /**
     * Sets whether objects borrowed from the pool will be validated when
     * they are returned to the pool via the <code>returnObject()</code> method.
     * Validation is performed by the <code>validateObject()</code> method of
     * the factory associated with the pool. Returning objects that fail validation
     * are destroyed rather then being returned the pool.
     *
     * @param testOnReturn <code>true</code> if objects are validated on
     *                     return to the pool via the
     *                     <code>returnObject()</code> method
     *
     * @see #getTestOnReturn
     */
    public final void setTestOnReturn(boolean testOnReturn) {
        this.testOnReturn = testOnReturn;
    }

    /**
     * Returns whether objects sitting idle in the pool will be validated by the
     * idle object evictor (if any - see
     * {@link #setTimeBetweenEvictionRunsMillis(long)}). Validation is performed
     * by the <code>validateObject()</code> method of the factory associated
     * with the pool. If the object fails to validate, it will be removed from
     * the pool and destroyed.
     *
     * @return <code>true</code> if objects will be validated by the evictor
     *
     * @see #setTestWhileIdle
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final boolean getTestWhileIdle() {
        return testWhileIdle;
    }

    /**
     * Returns whether objects sitting idle in the pool will be validated by the
     * idle object evictor (if any - see
     * {@link #setTimeBetweenEvictionRunsMillis(long)}). Validation is performed
     * by the <code>validateObject()</code> method of the factory associated
     * with the pool. If the object fails to validate, it will be removed from
     * the pool and destroyed.  Note that setting this property has no effect
     * unless the idle object evictor is enabled by setting
     * <code>timeBetweenEvictionRunsMillis</code> to a positive value.
     *
     * @param testWhileIdle
     *            <code>true</code> so objects will be validated by the evictor
     *
     * @see #getTestWhileIdle
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final void setTestWhileIdle(boolean testWhileIdle) {
        this.testWhileIdle = testWhileIdle;
    }

    /**
     * Returns the number of milliseconds to sleep between runs of the idle
     * object evictor thread. When non-positive, no idle object evictor thread
     * will be run.
     *
     * @return number of milliseconds to sleep between evictor runs
     *
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final long getTimeBetweenEvictionRunsMillis() {
        return timeBetweenEvictionRunsMillis;
    }

    /**
     * Sets the number of milliseconds to sleep between runs of the idle
     * object evictor thread. When non-positive, no idle object evictor thread
     * will be run.
     *
     * @param timeBetweenEvictionRunsMillis
     *            number of milliseconds to sleep between evictor runs
     *
     * @see #getTimeBetweenEvictionRunsMillis
     */
    public final void setTimeBetweenEvictionRunsMillis(
            long timeBetweenEvictionRunsMillis) {
        this.timeBetweenEvictionRunsMillis = timeBetweenEvictionRunsMillis;
        startEvictor(timeBetweenEvictionRunsMillis);
    }

    /**
     * Returns the maximum number of objects to examine during each run (if any)
     * of the idle object evictor thread. When positive, the number of tests
     * performed for a run will be the minimum of the configured value and the
     * number of idle instances in the pool. When negative, the number of tests
     * performed will be <code>ceil({@link #getNumIdle}/
     * abs({@link #getNumTestsPerEvictionRun}))</code> which means that when the
     * value is <code>-n</code> roughly one nth of the idle objects will be
     * tested per run.
     *
     * @return max number of objects to examine during each evictor run
     *
     * @see #setNumTestsPerEvictionRun
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final int getNumTestsPerEvictionRun() {
        return numTestsPerEvictionRun;
    }

    /**
     * Sets the maximum number of objects to examine during each run (if any)
     * of the idle object evictor thread. When positive, the number of tests
     * performed for a run will be the minimum of the configured value and the
     * number of idle instances in the pool. When negative, the number of tests
     * performed will be <code>ceil({@link #getNumIdle}/
     * abs({@link #getNumTestsPerEvictionRun}))</code> which means that when the
     * value is <code>-n</code> roughly one nth of the idle objects will be
     * tested per run.
     *
     * @param numTestsPerEvictionRun
     *            max number of objects to examine during each evictor run
     *
     * @see #getNumTestsPerEvictionRun
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final void setNumTestsPerEvictionRun(int numTestsPerEvictionRun) {
        this.numTestsPerEvictionRun = numTestsPerEvictionRun;
    }

    /**
     * Returns the minimum amount of time an object may sit idle in the pool
     * before it is eligible for eviction by the idle object evictor (if any -
     * see {@link #setTimeBetweenEvictionRunsMillis(long)}). When non-positive,
     * no objects will be evicted from the pool due to idle time alone.
     *
     * @return minimum amount of time an object may sit idle in the pool before
     *         it is eligible for eviction
     *
     * @see #setMinEvictableIdleTimeMillis
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final long getMinEvictableIdleTimeMillis() {
        return minEvictableIdleTimeMillis;
    }

    /**
     * Sets the minimum amount of time an object may sit idle in the pool
     * before it is eligible for eviction by the idle object evictor (if any -
     * see {@link #setTimeBetweenEvictionRunsMillis(long)}). When non-positive,
     * no objects will be evicted from the pool due to idle time alone.
     *
     * @param minEvictableIdleTimeMillis
     *            minimum amount of time an object may sit idle in the pool
     *            before it is eligible for eviction
     *
     * @see #getMinEvictableIdleTimeMillis
     * @see #setTimeBetweenEvictionRunsMillis
     */
    public final void setMinEvictableIdleTimeMillis(
            long minEvictableIdleTimeMillis) {
        this.minEvictableIdleTimeMillis = minEvictableIdleTimeMillis;
    }

    /**
     * Returns the minimum amount of time an object may sit idle in the pool
     * before it is eligible for eviction by the idle object evictor (if any -
     * see {@link #setTimeBetweenEvictionRunsMillis(long)}),
     * with the extra condition that at least <code>minIdle</code> object
     * instances remain in the pool. This setting is overridden by
     * {@link #getMinEvictableIdleTimeMillis} (that is, if
     * {@link #getMinEvictableIdleTimeMillis} is positive, then
     * {@link #getSoftMinEvictableIdleTimeMillis} is ignored).
     *
     * @return minimum amount of time an object may sit idle in the pool before
     *         it is eligible for eviction if minIdle instances are available
     *
     * @see #setSoftMinEvictableIdleTimeMillis
     */
    public final long getSoftMinEvictableIdleTimeMillis() {
        return softMinEvictableIdleTimeMillis;
    }

    /**
     * Sets the minimum amount of time an object may sit idle in the pool
     * before it is eligible for eviction by the idle object evictor (if any -
     * see {@link #setTimeBetweenEvictionRunsMillis(long)}),
     * with the extra condition that at least <code>minIdle</code> object
     * instances remain in the pool. This setting is overridden by
     * {@link #getMinEvictableIdleTimeMillis} (that is, if
     * {@link #getMinEvictableIdleTimeMillis} is positive, then
     * {@link #getSoftMinEvictableIdleTimeMillis} is ignored).
     *
     * @param softMinEvictableIdleTimeMillis
     *            minimum amount of time an object may sit idle in the pool
     *            before it is eligible for eviction if minIdle instances are
     *            available
     *
     * @see #getSoftMinEvictableIdleTimeMillis
     */
    public final void setSoftMinEvictableIdleTimeMillis(
            long softMinEvictableIdleTimeMillis) {
        this.softMinEvictableIdleTimeMillis = softMinEvictableIdleTimeMillis;
    }

    /**
     * Returns the name of the {@link EvictionPolicy} implementation that is
     * used by this pool.
     *
     * @return  The fully qualified class name of the {@link EvictionPolicy}
     *
     * @see #setEvictionPolicyClassName(String)
     */
    public final String getEvictionPolicyClassName() {
        return evictionPolicy.getClass().getName();
    }

    /**
     * Sets the name of the {@link EvictionPolicy} implementation that is
     * used by this pool.
     *
     * @param evictionPolicyClassName   the fully qualified class name of the
     *                                  new eviction policy
     *
     * @see #getEvictionPolicyClassName()
     */
    public final void setEvictionPolicyClassName(
            String evictionPolicyClassName) {
        try {
            Class<?> clazz = Class.forName(evictionPolicyClassName, true,
                    Thread.currentThread().getContextClassLoader());
            Object policy = clazz.newInstance();
            if (policy instanceof EvictionPolicy<?>) {
                @SuppressWarnings("unchecked") // safe, because we just checked the class
                EvictionPolicy<T> evicPolicy = (EvictionPolicy<T>) policy;
                this.evictionPolicy = evicPolicy;
            }
        } catch (ClassNotFoundException e) {
            throw new IllegalArgumentException(
                    "Unable to create EvictionPolicy instance of type " +
                    evictionPolicyClassName, e);
        } catch (InstantiationException e) {
            throw new IllegalArgumentException(
                    "Unable to create EvictionPolicy instance of type " +
                    evictionPolicyClassName, e);
        } catch (IllegalAccessException e) {
            throw new IllegalArgumentException(
                    "Unable to create EvictionPolicy instance of type " +
                    evictionPolicyClassName, e);
        }
    }


    /**
     * Closes the pool, destroys the remaining idle objects and, if registered
     * in JMX, deregisters it.
     */
    public abstract void close();

    /**
     * Has this pool instance been closed.
     * @return <code>true</code> when this pool has been closed.
     */
    public final boolean isClosed() {
        return closed;
    }

    /**
     * <p>Perform <code>numTests</code> idle object eviction tests, evicting
     * examined objects that meet the criteria for eviction. If
     * <code>testWhileIdle</code> is true, examined objects are validated
     * when visited (and removed if invalid); otherwise only objects that
     * have been idle for more than <code>minEvicableIdleTimeMillis</code>
     * are removed.</p>
     *
     * @throws Exception when there is a problem evicting idle objects.
     */
    public abstract void evict() throws Exception;

    /**
     * Returns the {@link EvictionPolicy} defined for this pool.
     * @return the eviction policy
     */
    final EvictionPolicy<T> getEvictionPolicy() {
        return evictionPolicy;
    }

    /**
     * Verifies that the pool is open.
     * @throws IllegalStateException if the pool is closed.
     */
    final void assertOpen() throws IllegalStateException {
        if (isClosed()) {
            throw new IllegalStateException("Pool not open");
        }
    }

    /**
     * <p>Starts the evictor with the given delay. If there is an evictor
     * running when this method is called, it is stopped and replaced with a
     * new evictor with the specified delay.</p>
     *
     * <p>This method needs to be final, since it is called from a constructor.
     * See POOL-195.</p>
     *
     * @param delay time in milliseconds before start and between eviction runs
     */
    final void startEvictor(long delay) {
        synchronized (evictionLock) {
            if (null != evictor) {
                EvictionTimer.cancel(evictor);
                evictor = null;
                evictionIterator = null;
            }
            if (delay > 0) {
                evictor = new Evictor();
                EvictionTimer.schedule(evictor, delay, delay);
            }
        }
    }

    /**
     * Tries to ensure that the configured minimum number of idle instances are
     * available in the pool.
     * @throws Exception if an error occurs creating idle instances
     */
    abstract void ensureMinIdle() throws Exception;


    // Monitoring (primarily JMX) related methods

    /**
     * Provides the name under which the pool has been registered with the
     * platform MBean server or <code>null</code> if the pool has not been
     * registered.
     * @return the JMX name
     */
    public final ObjectName getJmxName() {
        return oname;
    }

    /**
     * Provides the stack trace for the call that created this pool. JMX
     * registration may trigger a memory leak so it is important that pools are
     * deregistered when no longer used by calling the {@link #close()} method.
     * This method is provided to assist with identifying code that creates but
     * does not close it thereby creating a memory leak.
     * @return pool creation stack trace
     */
    public final String getCreationStackTrace() {
        return creationStackTrace;
    }

    /**
     * The total number of objects successfully borrowed from this pool over the
     * lifetime of the pool.
     * @return the borrowed object count
     */
    public final long getBorrowedCount() {
        return borrowedCount.get();
    }

    /**
     * The total number of objects returned to this pool over the lifetime of
     * the pool. This excludes attempts to return the same object multiple
     * times.
     * @return the returned object count
     */
    public final long getReturnedCount() {
        return returnedCount.get();
    }

    /**
     * The total number of objects created for this pool over the lifetime of
     * the pool.
     * @return the created object count
     */
    public final long getCreatedCount() {
        return createdCount.get();
    }

    /**
     * The total number of objects destroyed by this pool over the lifetime of
     * the pool.
     * @return the destroyed object count
     */
    public final long getDestroyedCount() {
        return destroyedCount.get();
    }

    /**
     * The total number of objects destroyed by the evictor associated with this
     * pool over the lifetime of the pool.
     * @return the evictor destroyed object count
     */
    public final long getDestroyedByEvictorCount() {
        return destroyedByEvictorCount.get();
    }

    /**
     * The total number of objects destroyed by this pool as a result of failing
     * validation during <code>borrowObject()</code> over the lifetime of the
     * pool.
     * @return validation destroyed object count
     */
    public final long getDestroyedByBorrowValidationCount() {
        return destroyedByBorrowValidationCount.get();
    }

    /**
     * The mean time objects are active for based on the last {@link
     * #MEAN_TIMING_STATS_CACHE_SIZE} objects returned to the pool.
     * @return mean time an object has been checked out from the pool among
     * recently returned objects
     */
    public final long getMeanActiveTimeMillis() {
        return activeTimes.getMean();
    }

    /**
     * The mean time objects are idle for based on the last {@link
     * #MEAN_TIMING_STATS_CACHE_SIZE} objects borrowed from the pool.
     * @return mean time an object has been idle in the pool among recently
     * borrowed objects
     */
    public final long getMeanIdleTimeMillis() {
        return idleTimes.getMean();
    }

    /**
     * The mean time threads wait to borrow an object based on the last {@link
     * #MEAN_TIMING_STATS_CACHE_SIZE} objects borrowed from the pool.
     * @return mean time in milliseconds that a recently served thread has had
     * to wait to borrow an object from the pool
     */
    public final long getMeanBorrowWaitTimeMillis() {
        return waitTimes.getMean();
    }

    /**
     * The maximum time a thread has waited to borrow objects from the pool.
     * @return maximum wait time in milliseconds since the pool was created
     */
    public final long getMaxBorrowWaitTimeMillis() {
        return maxBorrowWaitTimeMillis;
    }

    /**
     * The number of instances currently idle in this pool.
     * @return count of instances available for checkout from the pool
     */
    public abstract int getNumIdle();

    /**
     * The listener used (if any) to receive notifications of exceptions
     * unavoidably swallowed by the pool.
     *
     * @return The listener or <code>null</code> for no listener
     */
    public final SwallowedExceptionListener getSwallowedExceptionListener() {
        return swallowedExceptionListener;
    }

    /**
     * The listener used (if any) to receive notifications of exceptions
     * unavoidably swallowed by the pool.
     *
     * @param swallowedExceptionListener    The listener or <code>null</code>
     *                                      for no listener
     */
    public final void setSwallowedExceptionListener(
            SwallowedExceptionListener swallowedExceptionListener) {
        this.swallowedExceptionListener = swallowedExceptionListener;
    }

    /**
     * Swallows an exception and notifies the configured listener for swallowed
     * exceptions queue.
     *
     * @param e exception to be swallowed
     */
    final void swallowException(Exception e) {
        SwallowedExceptionListener listener = getSwallowedExceptionListener();

        if (listener == null) {
            return;
        }

        try {
            listener.onSwallowException(e);
        } catch (OutOfMemoryError oome) {
            throw oome;
        } catch (VirtualMachineError vme) {
            throw vme;
        } catch (Throwable t) {
            // Ignore. Enjoy the irony.
        }
    }

    /**
     * Updates statistics after an object is borrowed from the pool.
     * @param p object borrowed from the pool
     * @param waitTime time (in milliseconds) that the borrowing thread had to wait
     */
    final void updateStatsBorrow(PooledObject<T> p, long waitTime) {
        borrowedCount.incrementAndGet();
        idleTimes.add(p.getIdleTimeMillis());
        waitTimes.add(waitTime);
        synchronized (maxBorrowWaitTimeMillisLock) {
            if (waitTime > maxBorrowWaitTimeMillis) {
                maxBorrowWaitTimeMillis = waitTime;
            }
        }
    }

    /**
     * Updates statistics after an object is returned to the pool.
     * @param activeTime the amount of time (in milliseconds) that the returning
     * object was checked out
     */
    final void updateStatsReturn(long activeTime) {
        returnedCount.incrementAndGet();
        activeTimes.add(activeTime);
    }

    /**
     * Unregisters this pool's MBean.
     */
    final void jmxUnregister() {
        if (oname != null) {
            try {
                ManagementFactory.getPlatformMBeanServer().unregisterMBean(
                        oname);
            } catch (MBeanRegistrationException e) {
                swallowException(e);
            } catch (InstanceNotFoundException e) {
                swallowException(e);
            }
        }
    }

    /**
     * Registers the pool with the platform MBean server.
     * The registered name will be
     * <code>jmxNameBase + jmxNamePrefix + i</code> where i is the least
     * integer greater than or equal to 1 such that the name is not already
     * registered. Swallows MBeanRegistrationException, NotCompliantMBeanException
     * returning null.
     *
     * @param config Pool configuration
     * @param jmxNameBase default base JMX name for this pool
     * @param jmxNamePrefix name prefix
     * @return registered ObjectName, null if registration fails
     */
    private ObjectName jmxRegister(BaseObjectPoolConfig config,
            String jmxNameBase, String jmxNamePrefix) {
        ObjectName objectName = null;
        MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
        int i = 1;
        boolean registered = false;
        String base = config.getJmxNameBase();
        if (base == null) {
            base = jmxNameBase;
        }
        while (!registered) {
            try {
                ObjectName objName;
                // Skip the numeric suffix for the first pool in case there is
                // only one so the names are cleaner.
                if (i == 1) {
                    objName = new ObjectName(base + jmxNamePrefix);
                } else {
                    objName = new ObjectName(base + jmxNamePrefix + i);
                }
                mbs.registerMBean(this, objName);
                objectName = objName;
                registered = true;
            } catch (MalformedObjectNameException e) {
                if (BaseObjectPoolConfig.DEFAULT_JMX_NAME_PREFIX.equals(
                        jmxNamePrefix) && jmxNameBase.equals(base)) {
                    // Shouldn't happen. Skip registration if it does.
                    registered = true;
                } else {
                    // Must be an invalid name. Use the defaults instead.
                    jmxNamePrefix =
                            BaseObjectPoolConfig.DEFAULT_JMX_NAME_PREFIX;
                    base = jmxNameBase;
                }
            } catch (InstanceAlreadyExistsException e) {
                // Increment the index and try again
                i++;
            } catch (MBeanRegistrationException e) {
                // Shouldn't happen. Skip registration if it does.
                registered = true;
            } catch (NotCompliantMBeanException e) {
                // Shouldn't happen. Skip registration if it does.
                registered = true;
            }
        }
        return objectName;
    }

    /**
     * Gets the stack trace of an exception as a string.
     * @param e exception to trace
     * @return exception stack trace as a string
     */
    private String getStackTrace(Exception e) {
        // Need the exception in string form to prevent the retention of
        // references to classes in the stack trace that could trigger a memory
        // leak in a container environment.
        Writer w = new StringWriter();
        PrintWriter pw = new PrintWriter(w);
        e.printStackTrace(pw);
        return w.toString();
    }

    // Inner classes

    /**
     * The idle object evictor {@link TimerTask}.
     *
     * @see GenericKeyedObjectPool#setTimeBetweenEvictionRunsMillis
     */
    class Evictor extends TimerTask {
        /**
         * Run pool maintenance.  Evict objects qualifying for eviction and then
         * ensure that the minimum number of idle instances are available.
         * Since the Timer that invokes Evictors is shared for all Pools but
         * pools may exist in different class loaders, the Evictor ensures that
         * any actions taken are under the class loader of the factory
         * associated with the pool.
         */
        @Override
        public void run() {
            ClassLoader savedClassLoader =
                    Thread.currentThread().getContextClassLoader();
            try {
                if (factoryClassLoader != null) {
                    // Set the class loader for the factory
                    ClassLoader cl = factoryClassLoader.get();
                    if (cl == null) {
                        // The pool has been dereferenced and the class loader
                        // GC'd. Cancel this timer so the pool can be GC'd as
                        // well.
                        cancel();
                        return;
                    }
                    Thread.currentThread().setContextClassLoader(cl);
                }

                // Evict from the pool
                try {
                    evict();
                } catch(Exception e) {
                    swallowException(e);
                } catch(OutOfMemoryError oome) {
                    // Log problem but give evictor thread a chance to continue
                    // in case error is recoverable
                    oome.printStackTrace(System.err);
                }
                // Re-create idle instances.
                try {
                    ensureMinIdle();
                } catch (Exception e) {
                    swallowException(e);
                }
            } finally {
                // Restore the previous CCL
                Thread.currentThread().setContextClassLoader(savedClassLoader);
            }
        }
    }

    private class StatsStore {

        private final AtomicLong values[];
        private final int size;
        private int index;

        public StatsStore(int size) {
            this.size = size;
            values = new AtomicLong[size];
            for (int i = 0; i < size; i++) {
                values[i] = new AtomicLong(-1);
            }
        }

        public synchronized void add(long value) {
            values[index].set(value);
            index++;
            if (index == size) {
                index = 0;
            }
        }

        public long getMean() {
            double result = 0;
            int counter = 0;
            for (int i = 0; i < size; i++) {
                long value = values[i].get();
                if (value != -1) {
                    counter++;
                    result = result * ((counter - 1) / (double) counter) +
                            value/(double) counter;
                }
            }
            return (long) result;

        }
    }
}
