theadpoolexecutor

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/*
 * %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.*;

/**
 * An {@link ExecutorService} that executes each submitted task using
 * one of possibly several pooled threads, normally configured
 * using {@link Executors} factory methods.
 *
 * 
Thread pools address two different problems: they usually
 * provide improved performance when executing large numbers of
 * asynchronous tasks, due to reduced per-task invocation overhead,
 * and they provide a means of bounding and managing the resources,
 * including threads, consumed when executing a collection of tasks.
 * Each ThreadPoolExecutor also maintains some basic
 * statistics, such as the number of completed tasks.
 *
 * 
To be useful across a wide range of contexts, this class
 * provides many adjustable parameters and extensibility
 * hooks. However, programmers are urged to use the more convenient
 * {@link Executors} factory methods {@link
 * Executors#newCachedThreadPool} (unbounded thread pool, with
 * automatic thread reclamation), {@link Executors#newFixedThreadPool}
 * (fixed size thread pool) and {@link
 * Executors#newSingleThreadExecutor} (single background thread), that
 * preconfigure settings for the most common usage
 * scenarios. Otherwise, use the following guide when manually
 * configuring and tuning this class:
 *
 * 

 *
 * 
Core and maximum pool sizes
 *
 * 
A ThreadPoolExecutor will automatically adjust the
 * pool size
 * (see {@link ThreadPoolExecutor#getPoolSize})
 * according to the bounds set by corePoolSize
 * (see {@link ThreadPoolExecutor#getCorePoolSize})
 * and
 * maximumPoolSize
 * (see {@link ThreadPoolExecutor#getMaximumPoolSize}).
 * When a new task is submitted in method {@link
 * ThreadPoolExecutor#execute}, and fewer than corePoolSize threads
 * are running, a new thread is created to handle the request, even if
 * other worker threads are idle.  If there are more than
 * corePoolSize but less than maximumPoolSize threads running, a new
 * thread will be created only if the queue is full.  By setting
 * corePoolSize and maximumPoolSize the same, you create a fixed-size
 * thread pool. By setting maximumPoolSize to an essentially unbounded
 * value such as Integer.MAX_VALUE, you allow the pool to
 * accommodate an arbitrary number of concurrent tasks. Most typically,
 * core and maximum pool sizes are set only upon construction, but they
 * may also be changed dynamically using {@link
 * ThreadPoolExecutor#setCorePoolSize} and {@link
 * ThreadPoolExecutor#setMaximumPoolSize}. 
 *
 * 
On-demand construction
 *
 * 
 By default, even core threads are initially created and
 * started only when new tasks arrive, but this can be overridden
 * dynamically using method {@link
 * ThreadPoolExecutor#prestartCoreThread} or
 * {@link ThreadPoolExecutor#prestartAllCoreThreads}.
 * You probably want to prestart threads if you construct the
 * pool with a non-empty queue. 
 *
 * 
Creating new threads
 *
 * 
New threads are created using a {@link
 * java.util.concurrent.ThreadFactory}.  If not otherwise specified, a
 * {@link Executors#defaultThreadFactory} is used, that creates threads to all
 * be in the same {@link ThreadGroup} and with the same
 * NORM_PRIORITY priority and non-daemon status. By supplying
 * a different ThreadFactory, you can alter the thread's name, thread
 * group, priority, daemon status, etc. If a ThreadFactory fails to create
 * a thread when asked by returning null from newThread,
 * the executor will continue, but might
 * not be able to execute any tasks. 
 *
 * 
Keep-alive times
 *
 * 
If the pool currently has more than corePoolSize threads,
 * excess threads will be terminated if they have been idle for more
 * than the keepAliveTime (see {@link
 * ThreadPoolExecutor#getKeepAliveTime}). This provides a means of
 * reducing resource consumption when the pool is not being actively
 * used. If the pool becomes more active later, new threads will be
 * constructed. This parameter can also be changed dynamically using
 * method {@link ThreadPoolExecutor#setKeepAliveTime}. Using a value
 * of Long.MAX_VALUE {@link TimeUnit#NANOSECONDS} effectively
 * disables idle threads from ever terminating prior to shut down. By
 * default, the keep-alive policy applies only when there are more
 * than corePoolSizeThreads. But method {@link
 * ThreadPoolExecutor#allowCoreThreadTimeOut(boolean)} can be used to apply
 * this time-out policy to core threads as well, so long as
 * the keepAliveTime value is non-zero. 
 *
 * 
Queuing
 *
 * 
Any {@link BlockingQueue} may be used to transfer and hold
 * submitted tasks.  The use of this queue interacts with pool sizing:
 *
 * 

 *
 * 
 If fewer than corePoolSize threads are running, the Executor
 * always prefers adding a new thread
 * rather than queuing.
 *
 * 
 If corePoolSize or more threads are running, the Executor
 * always prefers queuing a request rather than adding a new
 * thread.
 *
 * 
 If a request cannot be queued, a new thread is created unless
 * this would exceed maximumPoolSize, in which case, the task will be
 * rejected.
 *
 * 
 *
 * There are three general strategies for queuing:
 * 

 *
 * 
  Direct handoffs. A good default choice for a work
 * queue is a {@link SynchronousQueue} that hands off tasks to threads
 * without otherwise holding them. Here, an attempt to queue a task
 * will fail if no threads are immediately available to run it, so a
 * new thread will be constructed. This policy avoids lockups when
 * handling sets of requests that might have internal dependencies.
 * Direct handoffs generally require unbounded maximumPoolSizes to
 * avoid rejection of new submitted tasks. This in turn admits the
 * possibility of unbounded thread growth when commands continue to
 * arrive on average faster than they can be processed.  
 *
 * 
 Unbounded queues. Using an unbounded queue (for
 * example a {@link LinkedBlockingQueue} without a predefined
 * capacity) will cause new tasks to wait in the queue when all
 * corePoolSize threads are busy. Thus, no more than corePoolSize
 * threads will ever be created. (And the value of the maximumPoolSize
 * therefore doesn't have any effect.)  This may be appropriate when
 * each task is completely independent of others, so tasks cannot
 * affect each others execution; for example, in a web page server.
 * While this style of queuing can be useful in smoothing out
 * transient bursts of requests, it admits the possibility of
 * unbounded work queue growth when commands continue to arrive on
 * average faster than they can be processed.  
 *
 * 
Bounded queues. A bounded queue (for example, an
 * {@link ArrayBlockingQueue}) helps prevent resource exhaustion when
 * used with finite maximumPoolSizes, but can be more difficult to
 * tune and control.  Queue sizes and maximum pool sizes may be traded
 * off for each other: Using large queues and small pools minimizes
 * CPU usage, OS resources, and context-switching overhead, but can
 * lead to artificially low throughput.  If tasks frequently block (for
 * example if they are I/O bound), a system may be able to schedule
 * time for more threads than you otherwise allow. Use of small queues
 * generally requires larger pool sizes, which keeps CPUs busier but
 * may encounter unacceptable scheduling overhead, which also
 * decreases throughput.  
 *
 * 
 *
 * 
 *
 * 
Rejected tasks
 *
 * 
 New tasks submitted in method {@link
 * ThreadPoolExecutor#execute} will be rejected when the
 * Executor has been shut down, and also when the Executor uses finite
 * bounds for both maximum threads and work queue capacity, and is
 * saturated.  In either case, the execute method invokes the
 * {@link RejectedExecutionHandler#rejectedExecution} method of its
 * {@link RejectedExecutionHandler}.  Four predefined handler policies
 * are provided:
 *
 * 

 *
 * 
 In the
 * default {@link ThreadPoolExecutor.AbortPolicy}, the handler throws a
 * runtime {@link RejectedExecutionException} upon rejection. 
 *
 * 
 In {@link
 * ThreadPoolExecutor.CallerRunsPolicy}, the thread that invokes
 * execute itself runs the task. This provides a simple
 * feedback control mechanism that will slow down the rate that new
 * tasks are submitted. 
 *
 * 
 In {@link ThreadPoolExecutor.DiscardPolicy},
 * a task that cannot be executed is simply dropped.  
 *
 * 
In {@link
 * ThreadPoolExecutor.DiscardOldestPolicy}, if the executor is not
 * shut down, the task at the head of the work queue is dropped, and
 * then execution is retried (which can fail again, causing this to be
 * repeated.) 
 *
 * 
 *
 * It is possible to define and use other kinds of {@link
 * RejectedExecutionHandler} classes. Doing so requires some care
 * especially when policies are designed to work only under particular
 * capacity or queuing policies. 
 *
 * 
Hook methods
 *
 * 
This class provides protected overridable {@link
 * ThreadPoolExecutor#beforeExecute} and {@link
 * ThreadPoolExecutor#afterExecute} methods that are called before and
 * after execution of each task.  These can be used to manipulate the
 * execution environment; for example, reinitializing ThreadLocals,
 * gathering statistics, or adding log entries. Additionally, method
 * {@link ThreadPoolExecutor#terminated} can be overridden to perform
 * any special processing that needs to be done once the Executor has
 * fully terminated.
 *
 * 
If hook or callback methods throw
 * exceptions, internal worker threads may in turn fail and
 * abruptly terminate.
 *
 * 
Queue maintenance
 *
 * 
 Method {@link ThreadPoolExecutor#getQueue} allows access to
 * the work queue for purposes of monitoring and debugging.  Use of
 * this method for any other purpose is strongly discouraged.  Two
 * supplied methods, {@link ThreadPoolExecutor#remove} and {@link
 * ThreadPoolExecutor#purge} are available to assist in storage
 * reclamation when large numbers of queued tasks become
 * cancelled.
 *
 * 
Finalization
 *
 * 
 A pool that is no longer referenced in a program AND
 * has no remaining threads will be shutdown
 * automatically. If you would like to ensure that unreferenced pools
 * are reclaimed even if users forget to call {@link
 * ThreadPoolExecutor#shutdown}, then you must arrange that unused
 * threads eventually die, by setting appropriate keep-alive times,
 * using a lower bound of zero core threads and/or setting {@link
 * ThreadPoolExecutor#allowCoreThreadTimeOut(boolean)}.  
 
 *
 * 
 Extension example. Most extensions of this class
 * override one or more of the protected hook methods. For example,
 * here is a subclass that adds a simple pause/resume feature:
 *
 * 
 * class PausableThreadPoolExecutor extends ThreadPoolExecutor {
 *   private boolean isPaused;
 *   private ReentrantLock pauseLock = new ReentrantLock();
 *   private Condition unpaused = pauseLock.newCondition();
 *
 *   public PausableThreadPoolExecutor(...) { super(...); }
 *
 *   protected void beforeExecute(Thread t, Runnable r) {
 *     super.beforeExecute(t, r);
 *     pauseLock.lock();
 *     try {
 *       while (isPaused) unpaused.await();
 *     } catch (InterruptedException ie) {
 *       t.interrupt();
 *     } finally {
 *       pauseLock.unlock();
 *     }
 *   }
 *
 *   public void pause() {
 *     pauseLock.lock();
 *     try {
 *       isPaused = true;
 *     } finally {
 *       pauseLock.unlock();
 *     }
 *   }
 *
 *   public void resume() {
 *     pauseLock.lock();
 *     try {
 *       isPaused = false;
 *       unpaused.signalAll();
 *     } finally {
 *       pauseLock.unlock();
 *     }
 *   }
 * }
 * 
 * @since 1.5
 * @author Doug Lea
 */
public class ThreadPoolExecutor extends AbstractExecutorService {

    /**
     * Permission for checking shutdown
     */
    private static final RuntimePermission shutdownPerm =
        new RuntimePermission("modifyThread");

    /*
     * A ThreadPoolExecutor manages a largish set of control fields.
     * State changes in fields that affect execution control
     * guarantees only occur within mainLock regions. These include
     * fields runState, poolSize, corePoolSize, and maximumPoolSize
     * However, these fields are also declared volatile, so can be
     * read outside of locked regions. (Also, the workers Set is
     * accessed only under lock).
     *
     * The other fields representing user control parameters do not
     * affect execution invariants, so are declared volatile and
     * allowed to change (via user methods) asynchronously with
     * execution. These fields: allowCoreThreadTimeOut, keepAliveTime,
     * the rejected execution handler, and threadFactory, are not
     * updated within locks.
     *
     * The extensive use of volatiles here enables the most
     * performance-critical actions, such as enqueuing and dequeuing
     * tasks in the workQueue, to normally proceed without holding the
     * mainLock when they see that the state allows actions, although,
     * as described below, sometimes at the expense of re-checks
     * following these actions.
     */

    /**
     * runState provides the main lifecyle control, taking on values:
     *
     *   RUNNING:  Accept new tasks and process queued tasks
     *   SHUTDOWN: Don't accept new tasks, but process queued tasks
     *   STOP:     Don't accept new tasks, don't process queued tasks,
     *             and interrupt in-progress tasks
     *   TERMINATED: Same as STOP, plus all threads have terminated
     *
     * The numerical order among these values matters, to allow
     * ordered comparisons. The runState monotonically increases over
     * time, but need not hit each state. The transitions are:
     *
     * RUNNING -> SHUTDOWN
     *    On invocation of shutdown(), perhaps implicitly in finalize()
     * (RUNNING or SHUTDOWN) -> STOP
     *    On invocation of shutdownNow()
     * SHUTDOWN -> TERMINATED
     *    When both queue and pool are empty
     * STOP -> TERMINATED
     *    When pool is empty
     */
    volatile int runState;
    static final int RUNNING    = 0;
    static final int SHUTDOWN   = 1;
    static final int STOP       = 2;
    static final int TERMINATED = 3;

    /**
     * The queue used for holding tasks and handing off to worker
     * threads.  Note that when using this queue, we do not require
     * that workQueue.poll() returning null necessarily means that
     * workQueue.isEmpty(), so must sometimes check both. This
     * accommodates special-purpose queues such as DelayQueues for
     * which poll() is allowed to return null even if it may later
     * return non-null when delays expire.
     */
    private final BlockingQueue workQueue;

    /**
     * Lock held on updates to poolSize, corePoolSize,
     * maximumPoolSize, runState, and workers set.
     */
    private final ReentrantLock mainLock = new ReentrantLock();

    /**
     * Wait condition to support awaitTermination
     */
    private final Condition termination = mainLock.newCondition();

    /**
     * Set containing all worker threads in pool. Accessed only when
     * holding mainLock.
     */
    private final HashSet workers = new HashSet();

    /**
     * Timeout in nanoseconds for idle threads waiting for work.
     * Threads use this timeout when there are more than corePoolSize
     * present or if allowCoreThreadTimeOut. Otherwise they wait
     * forever for new work.
     */
    private volatile long  keepAliveTime;

    /**
     * If false (default) core threads stay alive even when idle.  If
     * true, core threads use keepAliveTime to time out waiting for
     * work.
     */
    private volatile boolean allowCoreThreadTimeOut;

    /**
     * Core pool size, updated only while holding mainLock, but
     * volatile to allow concurrent readability even during updates.
     */
    private volatile int   corePoolSize;

    /**
     * Maximum pool size, updated only while holding mainLock but
     * volatile to allow concurrent readability even during updates.
     */
    private volatile int   maximumPoolSize;

    /**
     * Current pool size, updated only while holding mainLock but
     * volatile to allow concurrent readability even during updates.
     */
    private volatile int   poolSize;

    /**
     * Handler called when saturated or shutdown in execute.
     */
    private volatile RejectedExecutionHandler handler;

    /**
     * Factory for new threads. All threads are created using this
     * factory (via method addThread).  All callers must be prepared
     * for addThread to fail by returning null, which may reflect a
     * system or user's policy limiting the number of threads.  Even
     * though it is not treated as an error, failure to create threads
     * may result in new tasks being rejected or existing ones
     * remaining stuck in the queue. On the other hand, no special
     * precautions exist to handle OutOfMemoryErrors that might be
     * thrown while trying to create threads, since there is generally
     * no recourse from within this class.
     */
    private volatile ThreadFactory threadFactory;

    /**
     * Tracks largest attained pool size.
     */
    private int largestPoolSize;

    /**
     * Counter for completed tasks. Updated only on termination of
     * worker threads.
     */
    private long completedTaskCount;

    /**
     * The default rejected execution handler
     */
    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();

    // Constructors

    /**
     * Creates a new ThreadPoolExecutor with the given initial
     * parameters and default thread factory and rejected execution handler.
     * It may be more convenient to use one of the {@link Executors} factory
     * methods instead of this general purpose constructor.
     *
     * @param corePoolSize the number of threads to keep in the
     * pool, even if they are idle.
     * @param maximumPoolSize the maximum number of threads to allow in the
     * pool.
     * @param keepAliveTime when the number of threads is greater than
     * the core, this is the maximum time that excess idle threads
     * will wait for new tasks before terminating.
     * @param unit the time unit for the keepAliveTime
     * argument.
     * @param workQueue the queue to use for holding tasks before they
     * are executed. This queue will hold only the Runnable
     * tasks submitted by the execute method.
     * @throws IllegalArgumentException if corePoolSize or
     * keepAliveTime less than zero, or if maximumPoolSize less than or
     * equal to zero, or if corePoolSize greater than maximumPoolSize.
     * @throws NullPointerException if workQueue is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

    /**
     * Creates a new ThreadPoolExecutor with the given initial
     * parameters and default rejected execution handler.
     *
     * @param corePoolSize the number of threads to keep in the
     * pool, even if they are idle.
     * @param maximumPoolSize the maximum number of threads to allow in the
     * pool.
     * @param keepAliveTime when the number of threads is greater than
     * the core, this is the maximum time that excess idle threads
     * will wait for new tasks before terminating.
     * @param unit the time unit for the keepAliveTime
     * argument.
     * @param workQueue the queue to use for holding tasks before they
     * are executed. This queue will hold only the Runnable
     * tasks submitted by the execute method.
     * @param threadFactory the factory to use when the executor
     * creates a new thread.
     * @throws IllegalArgumentException if corePoolSize or
     * keepAliveTime less than zero, or if maximumPoolSize less than or
     * equal to zero, or if corePoolSize greater than maximumPoolSize.
     * @throws NullPointerException if workQueue
     * or threadFactory are null.
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

    /**
     * Creates a new ThreadPoolExecutor with the given initial
     * parameters and default thread factory.
     *
     * @param corePoolSize the number of threads to keep in the
     * pool, even if they are idle.
     * @param maximumPoolSize the maximum number of threads to allow in the
     * pool.
     * @param keepAliveTime when the number of threads is greater than
     * the core, this is the maximum time that excess idle threads
     * will wait for new tasks before terminating.
     * @param unit the time unit for the keepAliveTime
     * argument.
     * @param workQueue the queue to use for holding tasks before they
     * are executed. This queue will hold only the Runnable
     * tasks submitted by the execute method.
     * @param handler the handler to use when execution is blocked
     * because the thread bounds and queue capacities are reached.
     * @throws IllegalArgumentException if corePoolSize or
     * keepAliveTime less than zero, or if maximumPoolSize less than or
     * equal to zero, or if corePoolSize greater than maximumPoolSize.
     * @throws NullPointerException if workQueue
     * or handler are null.
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              RejectedExecutionHandler handler) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), handler);
    }

    /**
     * Creates a new ThreadPoolExecutor with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the
     * pool, even if they are idle.
     * @param maximumPoolSize the maximum number of threads to allow in the
     * pool.
     * @param keepAliveTime when the number of threads is greater than
     * the core, this is the maximum time that excess idle threads
     * will wait for new tasks before terminating.
     * @param unit the time unit for the keepAliveTime
     * argument.
     * @param workQueue the queue to use for holding tasks before they
     * are executed. This queue will hold only the Runnable
     * tasks submitted by the execute method.
     * @param threadFactory the factory to use when the executor
     * creates a new thread.
     * @param handler the handler to use when execution is blocked
     * because the thread bounds and queue capacities are reached.
     * @throws IllegalArgumentException if corePoolSize or
     * keepAliveTime less than zero, or if maximumPoolSize less than or
     * equal to zero, or if corePoolSize greater than maximumPoolSize.
     * @throws NullPointerException if workQueue
     * or threadFactory or handler are null.
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

    /*
     * Support for execute().
     *
     * Method execute() and its helper methods handle the various
     * cases encountered when new tasks are submitted.  The main
     * execute() method proceeds in 3 steps:
     *
     * 1. If it appears that fewer than corePoolSize threads are
     * running, try to start a new thread with the given command as
     * its first task.  The check here errs on the side of caution.
     * The call to addIfUnderCorePoolSize rechecks runState and pool
     * size under lock (they change only under lock) so prevents false
     * alarms that would add threads when it shouldn't, but may also
     * fail to add them when they should. This is compensated within
     * the following steps.
     *
     * 2. If a task can be successfully queued, then we are done, but
     * still need to compensate for missing the fact that we should
     * have added a thread (because existing ones died) or that
     * shutdown occurred since entry into this method. So we recheck
     * state and if necessary (in ensureQueuedTaskHandled) roll back
     * the enqueuing if shut down, or start a new thread if there are
     * none.
     *
     * 3. If we cannot queue task, then we try to add a new
     * thread. There's no guesswork here (addIfUnderMaximumPoolSize)
     * since it is performed under lock.  If it fails, we know we are
     * shut down or saturated.
     *
     * The reason for taking this overall approach is to normally
     * avoid holding mainLock during this method, which would be a
     * serious scalability bottleneck.  After warmup, almost all calls
     * take step 2 in a way that entails no locking.
     */

    /**
     * Executes the given task sometime in the future.  The task
     * may execute in a new thread or in an existing pooled thread.
     *
     * If the task cannot be submitted for execution, either because this
     * executor has been shutdown or because its capacity has been reached,
     * the task is handled by the current RejectedExecutionHandler.
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     * RejectedExecutionHandler, if task cannot be accepted
     * for execution
     * @throws NullPointerException if command is null
     */
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
            if (runState == RUNNING && workQueue.offer(command)) {
                if (runState != RUNNING || poolSize == 0)
                    ensureQueuedTaskHandled(command);
            }
            else if (!addIfUnderMaximumPoolSize(command))
                reject(command); // is shutdown or saturated
        }
    }

    /**
     * Creates and returns a new thread running firstTask as its first
     * task. Call only while holding mainLock.
     *
     * @param firstTask the task the new thread should run first (or
     * null if none)
     * @return the new thread, or null if threadFactory fails to create thread
     */
    private Thread addThread(Runnable firstTask) {
        Worker w = new Worker(firstTask);
        Thread t = threadFactory.newThread(w);
        if (t != null) {
            w.thread = t;
            workers.add(w);
            int nt = ++poolSize;
            if (nt > largestPoolSize)
                largestPoolSize = nt;
        }
        return t;
    }

    /**
     * Creates and starts a new thread running firstTask as its first
     * task, only if fewer than corePoolSize threads are running
     * and the pool is not shut down.
     * @param firstTask the task the new thread should run first (or
     * null if none)
     * @return true if successful
     */
    private boolean addIfUnderCorePoolSize(Runnable firstTask) {
        Thread t = null;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (poolSize < corePoolSize && runState == RUNNING)
                t = addThread(firstTask);
        } finally {
            mainLock.unlock();
        }
        if (t == null)
            return false;
        t.start();
        return true;
    }

    /**
     * Creates and starts a new thread running firstTask as its first
     * task, only if fewer than maximumPoolSize threads are running
     * and pool is not shut down.
     * @param firstTask the task the new thread should run first (or
     * null if none)
     * @return true if successful
     */
    private boolean addIfUnderMaximumPoolSize(Runnable firstTask) {
        Thread t = null;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (poolSize < maximumPoolSize && runState == RUNNING)
                t = addThread(firstTask);
        } finally {
            mainLock.unlock();
        }
        if (t == null)
            return false;
        t.start();
        return true;
    }

    /**
     * Rechecks state after queuing a task. Called from execute when
     * pool state has been observed to change after queuing a task. If
     * the task was queued concurrently with a call to shutdownNow,
     * and is still present in the queue, this task must be removed
     * and rejected to preserve shutdownNow guarantees.  Otherwise,
     * this method ensures (unless addThread fails) that there is at
     * least one live thread to handle this task
     * @param command the task
     */
    private void ensureQueuedTaskHandled(Runnable command) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        boolean reject = false;
        Thread t = null;
        try {
            int state = runState;
            if (state != RUNNING && workQueue.remove(command))
                reject = true;
            else if (state < STOP &&
                     poolSize < Math.max(corePoolSize, 1) &&
                     !workQueue.isEmpty())
                t = addThread(null);
        } finally {
            mainLock.unlock();
        }
        if (reject)
            reject(command);
        else if (t != null)
            t.start();
    }

    /**
     * Invokes the rejected execution handler for the given command.
     */
    void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }


    /**
     * Worker threads.
     *
     * Worker threads can start out life either with an initial first
     * task, or without one. Normally, they are started with a first
     * task. This enables execute(), etc to bypass queuing when there
     * are fewer than corePoolSize threads (in which case we always
     * start one), or when the queue is full.(in which case we must
     * bypass queue.) Initially idle threads are created either by
     * users (prestartCoreThread and setCorePoolSize) or when methods
     * ensureQueuedTaskHandled and tryTerminate notice that the queue
     * is not empty but there are no active threads to handle them.
     *
     * After completing a task, workers try to get another one,
     * via method getTask. If they cannot (i.e., getTask returns
     * null), they exit, calling workerDone to update pool state.
     *
     * When starting to run a task, unless the pool is stopped, each
     * worker thread ensures that it is not interrupted, and uses
     * runLock to prevent the pool from interrupting it in the midst
     * of execution. This shields user tasks from any interrupts that
     * may otherwise be needed during shutdown (see method
     * interruptIdleWorkers), unless the pool is stopping (via
     * shutdownNow) in which case interrupts are let through to affect
     * both tasks and workers. However, this shielding does not
     * necessarily protect the workers from lagging interrupts from
     * other user threads directed towards tasks that have already
     * been completed. Thus, a worker thread may be interrupted
     * needlessly (for example in getTask), in which case it rechecks
     * pool state to see if it should exit.
     */
    private final class Worker implements Runnable {
        /**
         * The runLock is acquired and released surrounding each task
         * execution. It mainly protects against interrupts that are
         * intended to cancel the worker thread from instead
         * interrupting the task being run.
         */
        private final ReentrantLock runLock = new ReentrantLock();

        /**
         * Initial task to run before entering run loop. Possibly null.
         */
        private Runnable firstTask;

        /**
         * Per thread completed task counter; accumulated
         * into completedTaskCount upon termination.
         */
        volatile long completedTasks;

        /**
         * Thread this worker is running in.  Acts as a final field,
         * but cannot be set until thread is created.
         */
        Thread thread;

        Worker(Runnable firstTask) {
            this.firstTask = firstTask;
        }

        boolean isActive() {
            return runLock.isLocked();
        }

        /**
         * Interrupts thread if not running a task.
         */
        void interruptIfIdle() {
            final ReentrantLock runLock = this.runLock;
            if (runLock.tryLock()) {
                try {
		    if (thread != Thread.currentThread())
			thread.interrupt();
                } finally {
                    runLock.unlock();
                }
            }
        }

        /**
         * Interrupts thread even if running a task.
         */
        void interruptNow() {
            thread.interrupt();
        }

        /**
         * Runs a single task between before/after methods.
         */
        private void runTask(Runnable task) {
            final ReentrantLock runLock = this.runLock;
            runLock.lock();
            try {
                /*
                 * Ensure that unless pool is stopping, this thread
                 * does not have its interrupt set. This requires a
                 * double-check of state in case the interrupt was
                 * cleared concurrently with a shutdownNow -- if so,
                 * the interrupt is re-enabled.
                 */
                if (runState < STOP &&
                    Thread.interrupted() &&
                    runState >= STOP)
                    thread.interrupt();
                /*
                 * Track execution state to ensure that afterExecute
                 * is called only if task completed or threw
                 * exception. Otherwise, the caught runtime exception
                 * will have been thrown by afterExecute itself, in
                 * which case we don't want to call it again.
                 */
                boolean ran = false;
                beforeExecute(thread, task);
                try {
                    task.run();
                    ran = true;
                    afterExecute(task, null);
                    ++completedTasks;
                } catch (RuntimeException ex) {
                    if (!ran)
                        afterExecute(task, ex);
                    throw ex;
                }
            } finally {
                runLock.unlock();
            }
        }

        /**
         * Main run loop
         */
        public void run() {
            try {
                Runnable task = firstTask;
                firstTask = null;
                while (task != null || (task = getTask()) != null) {
                    runTask(task);
                    task = null;
                }
            } finally {
                workerDone(this);
            }
        }
    }

    /* Utilities for worker thread control */

    /**
     * Gets the next task for a worker thread to run.  The general
     * approach is similar to execute() in that worker threads trying
     * to get a task to run do so on the basis of prevailing state
     * accessed outside of locks.  This may cause them to choose the
     * "wrong" action, such as trying to exit because no tasks
     * appear to be available, or entering a take when the pool is in
     * the process of being shut down.  These potential problems are
     * countered by (1) rechecking pool state (in workerCanExit)
     * before giving up, and (2) interrupting other workers upon
     * shutdown, so they can recheck state. All other user-based state
     * changes (to allowCoreThreadTimeOut etc) are OK even when
     * performed asynchronously wrt getTask.
     *
     * @return the task
     */
    Runnable getTask() {
        for (;;) {
            try {
                int state = runState;
                if (state > SHUTDOWN)
                    return null;
                Runnable r;
                if (state == SHUTDOWN)  // Help drain queue
                    r = workQueue.poll();
                else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
                    r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
                else
                    r = workQueue.take();
                if (r != null)
                    return r;
                if (workerCanExit()) {
                    if (runState >= SHUTDOWN) // Wake up others
                        interruptIdleWorkers();
                    return null;
                }
                // Else retry
            } catch (InterruptedException ie) {
                // On interruption, re-check runState
            }
        }
    }

    /**
     * Check whether a worker thread that fails to get a task can
     * exit.  We allow a worker thread to die if the pool is stopping,
     * or the queue is empty, or there is at least one thread to
     * handle possibly non-empty queue, even if core timeouts are
     * allowed.
     */
    private boolean workerCanExit() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        boolean canExit;
        try {
            canExit = runState >= STOP ||
                workQueue.isEmpty() ||
                (allowCoreThreadTimeOut &&
                 poolSize > Math.max(1, corePoolSize));
        } finally {
            mainLock.unlock();
        }
        return canExit;
    }

    /**
     * Wakes up all threads that might be waiting for tasks so they
     * can check for termination. Note: this method is also called by
     * ScheduledThreadPoolExecutor.
     */
    void interruptIdleWorkers() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers)
                w.interruptIfIdle();
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Performs bookkeeping for an exiting worker thread.
     * @param w the worker
     */
    void workerDone(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);
            if (--poolSize == 0)
                tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }

    /* Termination support. */

    /**
     * Transitions to TERMINATED state if either (SHUTDOWN and pool
     * and queue empty) or (STOP and pool empty), otherwise unless
     * stopped, ensuring that there is at least one live thread to
     * handle queued tasks.
     *
     * This method is called from the three places in which
     * termination can occur: in workerDone on exit of the last thread
     * after pool has been shut down, or directly within calls to
     * shutdown or shutdownNow, if there are no live threads.
     */
    private void tryTerminate() {
        if (poolSize == 0) {
            int state = runState;
            if (state < STOP && !workQueue.isEmpty()) {
                state = RUNNING; // disable termination check below
                Thread t = addThread(null);
                if (t != null)
                    t.start();
            }
            if (state == STOP || state == SHUTDOWN) {
                runState = TERMINATED;
                termination.signalAll();
                terminated();
            }
        }
    }

    /**
     * Initiates an orderly shutdown in which previously submitted
     * tasks are executed, but no new tasks will be
     * accepted. Invocation has no additional effect if already shut
     * down.
     * @throws SecurityException if a security manager exists and
     * shutting down this ExecutorService may manipulate threads that
     * the caller is not permitted to modify because it does not hold
     * {@link java.lang.RuntimePermission}("modifyThread"),
     * or the security manager's checkAccess method denies access.
     */
    public void shutdown() {
        /*
         * Conceptually, shutdown is just a matter of changing the
         * runState to SHUTDOWN, and then interrupting any worker
         * threads that might be blocked in getTask() to wake them up
         * so they can exit. Then, if there happen not to be any
         * threads or tasks, we can directly terminate pool via
         * tryTerminate.  Else, the last worker to leave the building
         * turns off the lights (in workerDone).
         *
         * But this is made more delicate because we must cooperate
         * with the security manager (if present), which may implement
         * policies that make more sense for operations on Threads
         * than they do for ThreadPools. This requires 3 steps:
         *
         * 1. Making sure caller has permission to shut down threads
         * in general (see shutdownPerm).
         *
         * 2. If (1) passes, making sure the caller is allowed to
         * modify each of our threads. This might not be true even if
         * first check passed, if the SecurityManager treats some
         * threads specially. If this check passes, then we can try
         * to set runState.
         *
         * 3. If both (1) and (2) pass, dealing with inconsistent
         * security managers that allow checkAccess but then throw a
         * SecurityException when interrupt() is invoked.  In this
         * third case, because we have already set runState, we can
         * only try to back out from the shutdown as cleanly as
         * possible. Some workers may have been killed but we remain
         * in non-shutdown state (which may entail tryTerminate from
         * workerDone starting a new worker to maintain liveness.)
         */

	SecurityManager security = System.getSecurityManager();
	if (security != null)
            security.checkPermission(shutdownPerm);

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (security != null) { // Check if caller can modify our threads
                for (Worker w : workers)
                    security.checkAccess(w.thread);
            }

            int state = runState;
            if (state < SHUTDOWN)
                runState = SHUTDOWN;

            try {
                for (Worker w : workers) {
                    w.interruptIfIdle();
                }
            } catch (SecurityException se) { // Try to back out
                runState = state;
                // tryTerminate() here would be a no-op
                throw se;
            }

            tryTerminate(); // Terminate now if pool and queue empty
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution. These tasks are drained (removed)
     * from the task queue upon return from this method.
     *
     * 
There are no guarantees beyond best-effort attempts to stop
     * processing actively executing tasks.  This implementation
     * cancels tasks via {@link Thread#interrupt}, so any task that
     * fails to respond to interrupts may never terminate.
     *
     * @return list of tasks that never commenced execution
     * @throws SecurityException if a security manager exists and
     * shutting down this ExecutorService may manipulate threads that
     * the caller is not permitted to modify because it does not hold
     * {@link java.lang.RuntimePermission}("modifyThread"),
     * or the security manager's checkAccess method denies access.
     */
    public List shutdownNow() {
        /*
         * shutdownNow differs from shutdown only in that
         * 1. runState is set to STOP,
         * 2. all worker threads are interrupted, not just the idle ones, and
         * 3. the queue is drained and returned.
         */
	SecurityManager security = System.getSecurityManager();
	if (security != null)
            security.checkPermission(shutdownPerm);

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (security != null) { // Check if caller can modify our threads
                for (Worker w : workers)
                    security.checkAccess(w.thread);
            }

            int state = runState;
            if (state < STOP)
                runState = STOP;

            try {
                for (Worker w : workers) {
                    w.interruptNow();
                }
            } catch (SecurityException se) { // Try to back out
                runState = state;
                // tryTerminate() here would be a no-op
                throw se;
            }

            List tasks = drainQueue();
            tryTerminate(); // Terminate now if pool and queue empty
            return tasks;
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Drains the task queue into a new list. Used by shutdownNow.
     * Call only while holding main lock.
     */
    private List drainQueue() {
        List taskList = new ArrayList();
        workQueue.drainTo(taskList);
        /*
         * If the queue is a DelayQueue or any other kind of queue
         * for which poll or drainTo may fail to remove some elements,
         * we need to manually traverse and remove remaining tasks.
         * To guarantee atomicity wrt other threads using this queue,
         * we need to create a new iterator for each element removed.
         */
        while (!workQueue.isEmpty()) {
            Iterator it = workQueue.iterator();
            try {
                if (it.hasNext()) {
                    Runnable r = it.next();
                    if (workQueue.remove(r))
                        taskList.add(r);
                }
            } catch (ConcurrentModificationException ignore) {
            }
        }
        return taskList;
    }

    public boolean isShutdown() {
        return runState != RUNNING;
    }

    /**
     * Returns true if shutdownNow has been invoked but this executor
     * has not completely terminated.
     */
    boolean isStopped() {
        return runState == STOP;
    }

    /**
     * Returns true if this executor is in the process of terminating
     * after shutdown or shutdownNow but has not
     * completely terminated.  This method may be useful for
     * debugging. A return of true reported a sufficient
     * period after shutdown may indicate that submitted tasks have
     * ignored or suppressed interruption, causing this executor not
     * to properly terminate.
     * @return true if terminating but not yet terminated
     */
    public boolean isTerminating() {
        int state = runState;
        return state == SHUTDOWN || state == STOP;
    }

    public boolean isTerminated() {
        return runState == TERMINATED;
    }

    public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (;;) {
                if (runState == TERMINATED)
                    return true;
                if (nanos <= 0)
                    return false;
                nanos = termination.awaitNanos(nanos);
            }
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Invokes shutdown when this executor is no longer
     * referenced.
     */
    protected void finalize()  {
        shutdown();
    }

    /* Getting and setting tunable parameters */

    /**
     * Sets the thread factory used to create new threads.
     *
     * @param threadFactory the new thread factory
     * @throws NullPointerException if threadFactory is null
     * @see #getThreadFactory
     */
    public void setThreadFactory(ThreadFactory threadFactory) {
        if (threadFactory == null)
            throw new NullPointerException();
        this.threadFactory = threadFactory;
    }

    /**
     * Returns the thread factory used to create new threads.
     *
     * @return the current thread factory
     * @see #setThreadFactory
     */
    public ThreadFactory getThreadFactory() {
        return threadFactory;
    }

    /**
     * Sets a new handler for unexecutable tasks.
     *
     * @param handler the new handler
     * @throws NullPointerException if handler is null
     * @see #getRejectedExecutionHandler
     */
    public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
        if (handler == null)
            throw new NullPointerException();
        this.handler = handler;
    }

    /**
     * Returns the current handler for unexecutable tasks.
     *
     * @return the current handler
     * @see #setRejectedExecutionHandler
     */
    public RejectedExecutionHandler getRejectedExecutionHandler() {
        return handler;
    }

    /**
     * Sets the core number of threads.  This overrides any value set
     * in the constructor.  If the new value is smaller than the
     * current value, excess existing threads will be terminated when
     * they next become idle. If larger, new threads will, if needed,
     * be started to execute any queued tasks.
     *
     * @param corePoolSize the new core size
     * @throws IllegalArgumentException if corePoolSize
     * less than zero
     * @see #getCorePoolSize
     */
    public void setCorePoolSize(int corePoolSize) {
        if (corePoolSize < 0)
            throw new IllegalArgumentException();
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int extra = this.corePoolSize - corePoolSize;
            this.corePoolSize = corePoolSize;
            if (extra < 0) {
                int n = workQueue.size(); // don't add more threads than tasks
                while (extra++ < 0 && n-- > 0 && poolSize < corePoolSize) {
                    Thread t = addThread(null);
                    if (t != null)
                        t.start();
                    else
                        break;
                }
            }
            else if (extra > 0 && poolSize > corePoolSize) {
                try {
                    Iterator it = workers.iterator();
                    while (it.hasNext() &&
                           extra-- > 0 &&
                           poolSize > corePoolSize &&
                           workQueue.remainingCapacity() == 0)
                        it.next().interruptIfIdle();
                } catch (SecurityException ignore) {
                    // Not an error; it is OK if the threads stay live
                }
            }
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Returns the core number of threads.
     *
     * @return the core number of threads
     * @see #setCorePoolSize
     */
    public int getCorePoolSize() {
        return corePoolSize;
    }

    /**
     * Starts a core thread, causing it to idly wait for work. This
     * overrides the default policy of starting core threads only when
     * new tasks are executed. This method will return false
     * if all core threads have already been started.
     * @return true if a thread was started
     */
    public boolean prestartCoreThread() {
        return addIfUnderCorePoolSize(null);
    }

    /**
     * Starts all core threads, causing them to idly wait for work. This
     * overrides the default policy of starting core threads only when
     * new tasks are executed.
     * @return the number of threads started
     */
    public int prestartAllCoreThreads() {
        int n = 0;
        while (addIfUnderCorePoolSize(null))
            ++n;
        return n;
    }

    /**
     * Returns true if this pool allows core threads to time out and
     * terminate if no tasks arrive within the keepAlive time, being
     * replaced if needed when new tasks arrive. When true, the same
     * keep-alive policy applying to non-core threads applies also to
     * core threads. When false (the default), core threads are never
     * terminated due to lack of incoming tasks.
     * @return true if core threads are allowed to time out,
     * else false
     *
     * @since 1.6
     */
    public boolean allowsCoreThreadTimeOut() {
        return allowCoreThreadTimeOut;
    }

    /**
     * Sets the policy governing whether core threads may time out and
     * terminate if no tasks arrive within the keep-alive time, being
     * replaced if needed when new tasks arrive. When false, core
     * threads are never terminated due to lack of incoming
     * tasks. When true, the same keep-alive policy applying to
     * non-core threads applies also to core threads. To avoid
     * continual thread replacement, the keep-alive time must be
     * greater than zero when setting true. This method
     * should in general be called before the pool is actively used.
     * @param value true if should time out, else false
     * @throws IllegalArgumentException if value is true
     * and the current keep-alive time is not greater than zero.
     *
     * @since 1.6
     */
    public void allowCoreThreadTimeOut(boolean value) {
        if (value && keepAliveTime <= 0)
            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");

        allowCoreThreadTimeOut = value;
    }

    /**
     * Sets the maximum allowed number of threads. This overrides any
     * value set in the constructor. If the new value is smaller than
     * the current value, excess existing threads will be
     * terminated when they next become idle.
     *
     * @param maximumPoolSize the new maximum
     * @throws IllegalArgumentException if the new maximum is
     *         less than or equal to zero, or
     *         less than the {@linkplain #getCorePoolSize core pool size}
     * @see #getMaximumPoolSize
     */
    public void setMaximumPoolSize(int maximumPoolSize) {
        if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize)
            throw new IllegalArgumentException();
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int extra = this.maximumPoolSize - maximumPoolSize;
            this.maximumPoolSize = maximumPoolSize;
            if (extra > 0 && poolSize > maximumPoolSize) {
                try {
                    Iterator it = workers.iterator();
                    while (it.hasNext() &&
                           extra > 0 &&
                           poolSize > maximumPoolSize) {
                        it.next().interruptIfIdle();
                        --extra;
                    }
                } catch (SecurityException ignore) {
                    // Not an error; it is OK if the threads stay live
                }
            }
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Returns the maximum allowed number of threads.
     *
     * @return the maximum allowed number of threads
     * @see #setMaximumPoolSize
     */
    public int getMaximumPoolSize() {
        return maximumPoolSize;
    }

    /**
     * Sets the time limit for which threads may remain idle before
     * being terminated.  If there are more than the core number of
     * threads currently in the pool, after waiting this amount of
     * time without processing a task, excess threads will be
     * terminated.  This overrides any value set in the constructor.
     * @param time the time to wait.  A time value of zero will cause
     * excess threads to terminate immediately after executing tasks.
     * @param unit the time unit of the time argument
     * @throws IllegalArgumentException if time less than zero or
     * if time is zero and allowsCoreThreadTimeOut
     * @see #getKeepAliveTime
     */
    public void setKeepAliveTime(long time, TimeUnit unit) {
        if (time < 0)
            throw new IllegalArgumentException();
        if (time == 0 && allowsCoreThreadTimeOut())
            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
        this.keepAliveTime = unit.toNanos(time);
    }

    /**
     * Returns the thread keep-alive time, which is the amount of time
     * that threads in excess of the core pool size may remain
     * idle before being terminated.
     *
     * @param unit the desired time unit of the result
     * @return the time limit
     * @see #setKeepAliveTime
     */
    public long getKeepAliveTime(TimeUnit unit) {
        return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
    }

    /* User-level queue utilities */

    /**
     * Returns the task queue used by this executor. Access to the
     * task queue is intended primarily for debugging and monitoring.
     * This queue may be in active use.  Retrieving the task queue
     * does not prevent queued tasks from executing.
     *
     * @return the task queue
     */
    public BlockingQueue getQueue() {
        return workQueue;
    }

    /**
     * Removes this task from the executor's internal queue if it is
     * present, thus causing it not to be run if it has not already
     * started.
     *
     * 
 This method may be useful as one part of a cancellation
     * scheme.  It may fail to remove tasks that have been converted
     * into other forms before being placed on the internal queue. For
     * example, a task entered using submit might be
     * converted into a form that maintains Future status.
     * However, in such cases, method {@link ThreadPoolExecutor#purge}
     * may be used to remove those Futures that have been cancelled.
     *
     * @param task the task to remove
     * @return true if the task was removed
     */
    public boolean remove(Runnable task) {
        return getQueue().remove(task);
    }

    /**
     * Tries to remove from the work queue all {@link Future}
     * tasks that have been cancelled. This method can be useful as a
     * storage reclamation operation, that has no other impact on
     * functionality. Cancelled tasks are never executed, but may
     * accumulate in work queues until worker threads can actively
     * remove them. Invoking this method instead tries to remove them now.
     * However, this method may fail to remove tasks in
     * the presence of interference by other threads.
     */
    public void purge() {
        // Fail if we encounter interference during traversal
        try {
            Iterator it = getQueue().iterator();
            while (it.hasNext()) {
                Runnable r = it.next();
                if (r instanceof Future) {
                    Future c = (Future)r;
                    if (c.isCancelled())
                        it.remove();
                }
            }
        }
        catch (ConcurrentModificationException ex) {
            return;
        }
    }

    /* Statistics */

    /**
     * Returns the current number of threads in the pool.
     *
     * @return the number of threads
     */
    public int getPoolSize() {
        return poolSize;
    }

    /**
     * Returns the approximate number of threads that are actively
     * executing tasks.
     *
     * @return the number of threads
     */
    public int getActiveCount() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int n = 0;
            for (Worker w : workers) {
                if (w.isActive())
                    ++n;
            }
            return n;
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Returns the largest number of threads that have ever
     * simultaneously been in the pool.
     *
     * @return the number of threads
     */
    public int getLargestPoolSize() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            return largestPoolSize;
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Returns the approximate total number of tasks that have ever been
     * scheduled for execution.  Because the states of tasks and
     * threads may change dynamically during computation, the returned
     * value is only an approximation.
     *
     * @return the number of tasks
     */
    public long getTaskCount() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            long n = completedTaskCount;
            for (Worker w : workers) {
                n += w.completedTasks;
                if (w.isActive())
                    ++n;
            }
            return n + workQueue.size();
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * Returns the approximate total number of tasks that have
     * completed execution. Because the states of tasks and threads
     * may change dynamically during computation, the returned value
     * is only an approximation, but one that does not ever decrease
     * across successive calls.
     *
     * @return the number of tasks
     */
    public long getCompletedTaskCount() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            long n = completedTaskCount;
            for (Worker w : workers)
                n += w.completedTasks;
            return n;
        } finally {
            mainLock.unlock();
        }
    }

    /* Extension hooks */

    /**
     * Method invoked prior to executing the given Runnable in the
     * given thread.  This method is invoked by thread t that
     * will execute task r, and may be used to re-initialize
     * ThreadLocals, or to perform logging.
     *
     * 
This implementation does nothing, but may be customized in
     * subclasses. Note: To properly nest multiple overridings, subclasses
     * should generally invoke super.beforeExecute at the end of
     * this method.
     *
     * @param t the thread that will run task r.
     * @param r the task that will be executed.
     */
    protected void beforeExecute(Thread t, Runnable r) { }

    /**
     * Method invoked upon completion of execution of the given Runnable.
     * This method is invoked by the thread that executed the task. If
     * non-null, the Throwable is the uncaught RuntimeException
     * or Error that caused execution to terminate abruptly.
     *
     * 
Note: When actions are enclosed in tasks (such as
     * {@link FutureTask}) either explicitly or via methods such as
     * submit, these task objects catch and maintain
     * computational exceptions, and so they do not cause abrupt
     * termination, and the internal exceptions are not
     * passed to this method.
     *
     * 
This implementation does nothing, but may be customized in
     * subclasses. Note: To properly nest multiple overridings, subclasses
     * should generally invoke super.afterExecute at the
     * beginning of this method.
     *
     * @param r the runnable that has completed.
     * @param t the exception that caused termination, or null if
     * execution completed normally.
     */
    protected void afterExecute(Runnable r, Throwable t) { }

    /**
     * Method invoked when the Executor has terminated.  Default
     * implementation does nothing. Note: To properly nest multiple
     * overridings, subclasses should generally invoke
     * super.terminated within this method.
     */
    protected void terminated() { }

    /* Predefined RejectedExecutionHandlers */

    /**
     * A handler for rejected tasks that runs the rejected task
     * directly in the calling thread of the execute method,
     * unless the executor has been shut down, in which case the task
     * is discarded.
     */
    public static class CallerRunsPolicy implements RejectedExecutionHandler {
        /**
         * Creates a CallerRunsPolicy.
         */
        public CallerRunsPolicy() { }

        /**
         * Executes task r in the caller's thread, unless the executor
         * has been shut down, in which case the task is discarded.
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                r.run();
            }
        }
    }

    /**
     * A handler for rejected tasks that throws a
     * RejectedExecutionException.
     */
    public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * Creates an AbortPolicy.
         */
        public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always.
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException();
        }
    }

    /**
     * A handler for rejected tasks that silently discards the
     * rejected task.
     */
    public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * Creates a DiscardPolicy.
         */
        public DiscardPolicy() { }

        /**
         * Does nothing, which has the effect of discarding task r.
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }

    /**
     * A handler for rejected tasks that discards the oldest unhandled
     * request and then retries execute, unless the executor
     * is shut down, in which case the task is discarded.
     */
    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
        /**
         * Creates a DiscardOldestPolicy for the given executor.
         */
        public DiscardOldestPolicy() { }

        /**
         * Obtains and ignores the next task that the executor
         * would otherwise execute, if one is immediately available,
         * and then retries execution of task r, unless the executor
         * is shut down, in which case task r is instead discarded.
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }
}