uavstack-master/com.creditease.uav.helper/src/main/java/com/creditease/uav/helpers/thread/QueueWorkerThreadPoolExecutor.java
/*-
 * <<
 * UAVStack
 * ==
 * Copyright (C) 2016 - 2017 UAVStack
 * ==
 * Licensed 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 com.creditease.uav.helpers.thread;

import java.util.ArrayList;
import java.util.ConcurrentModificationException;
import java.util.Deque;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ConcurrentLinkedDeque;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An {@link ExecutorService} that executes each submitted task using one of possibly several pooled threads, normally
 * configured using {@link Executors} factory methods.
 *
 * <p>
 * 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 {@code ThreadPoolExecutor} also
 * maintains some basic statistics, such as the number of completed tasks.
 *
 * <p>
 * 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:
 *
 * <dl>
 *
 * <dt>Core and maximum pool sizes</dt>
 *
 * <dd>A {@code ThreadPoolExecutor} will automatically adjust the pool size (see {@link #getPoolSize}) according to the
 * bounds set by corePoolSize (see {@link #getCorePoolSize}) and maximumPoolSize (see {@link #getMaximumPoolSize}).
 *
 * When a new task is submitted in method {@link #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 {@code 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 #setCorePoolSize} and {@link #setMaximumPoolSize}.</dd>
 *
 * <dt>On-demand construction</dt>
 *
 * <dd>By default, even core threads are initially created and started only when new tasks arrive, but this can be
 * overridden dynamically using method {@link #prestartCoreThread} or {@link #prestartAllCoreThreads}. You probably want
 * to prestart threads if you construct the pool with a non-empty queue.</dd>
 *
 * <dt>Creating new threads</dt>
 *
 * <dd>New threads are created using a {@link 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 {@code 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 {@code ThreadFactory} fails to create a
 * thread when asked by returning null from {@code newThread}, the executor will continue, but might not be able to
 * execute any tasks. Threads should possess the "modifyThread" {@code RuntimePermission}. If worker threads or other
 * threads using the pool do not possess this permission, service may be degraded: configuration changes may not take
 * effect in a timely manner, and a shutdown pool may remain in a state in which termination is possible but not
 * completed.</dd>
 *
 * <dt>Keep-alive times</dt>
 *
 * <dd>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 #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 #setKeepAliveTime}. Using a value of
 * {@code 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 #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.</dd>
 *
 * <dt>Queuing</dt>
 *
 * <dd>Any {@link BlockingQueue} may be used to transfer and hold submitted tasks. The use of this queue interacts with
 * pool sizing:
 *
 * <ul>
 *
 * <li>If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than
 * queuing.</li>
 *
 * <li>If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a
 * new thread.</li>
 *
 * <li>If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case,
 * the task will be rejected.</li>
 *
 * </ul>
 *
 * There are three general strategies for queuing:
 * <ol>
 *
 * <li><em> Direct handoffs.</em> 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.</li>
 *
 * <li><em> Unbounded queues.</em> 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.</li>
 *
 * <li><em>Bounded queues.</em> 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.</li>
 *
 * </ol>
 *
 * </dd>
 *
 * <dt>Rejected tasks</dt>
 *
 * <dd>New tasks submitted in method {@link #execute} will be <em>rejected</em> 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 {@code
 * execute} method invokes the {@link RejectedExecutionHandler#rejectedExecution} method of its
 * {@link RejectedExecutionHandler}. Four predefined handler policies are provided:
 *
 * <ol>
 *
 * <li>In the default {@link ThreadPoolExecutor.AbortPolicy}, the handler throws a runtime
 * {@link RejectedExecutionException} upon rejection.</li>
 *
 * <li>In {@link ThreadPoolExecutor.CallerRunsPolicy}, the thread that invokes {@code execute} itself runs the task.
 * This provides a simple feedback control mechanism that will slow down the rate that new tasks are submitted.</li>
 *
 * <li>In {@link ThreadPoolExecutor.DiscardPolicy}, a task that cannot be executed is simply dropped.</li>
 *
 * <li>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.)</li>
 *
 * </ol>
 *
 * 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.</dd>
 *
 * <dt>Hook methods</dt>
 *
 * <dd>This class provides {@code protected} overridable {@link #beforeExecute} and {@link #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 #terminated} can be overridden to perform any special processing that needs to be done once the Executor has
 * fully terminated.
 *
 * <p>
 * If hook or callback methods throw exceptions, internal worker threads may in turn fail and abruptly terminate.</dd>
 *
 * <dt>Queue maintenance</dt>
 *
 * <dd>Method {@link #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 #remove} and {@link #purge} are
 * available to assist in storage reclamation when large numbers of queued tasks become cancelled.</dd>
 *
 * <dt>Finalization</dt>
 *
 * <dd>A pool that is no longer referenced in a program <em>AND</em> has no remaining threads will be {@code shutdown}
 * automatically. If you would like to ensure that unreferenced pools are reclaimed even if users forget to call
 * {@link #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 #allowCoreThreadTimeOut(boolean)}.</dd>
 *
 * </dl>
 *
 * <p>
 * <b>Extension example</b>. 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:
 *
 * <pre>
 *  {@code
 * 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();
 *     }
 *   }
 * }}
 * </pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public class QueueWorkerThreadPoolExecutor extends AbstractExecutorService {

    /**
     * The main pool control state, ctl, is an atomic integer packing two conceptual fields workerCount, indicating the
     * effective number of threads runState, indicating whether running, shutting down etc
     *
     * In order to pack them into one int, we limit workerCount to (2^29)-1 (about 500 million) threads rather than
     * (2^31)-1 (2 billion) otherwise representable. If this is ever an issue in the future, the variable can be changed
     * to be an AtomicLong, and the shift/mask constants below adjusted. But until the need arises, this code is a bit
     * faster and simpler using an int.
     *
     * The workerCount is the number of workers that have been permitted to start and not permitted to stop. The value
     * may be transiently different from the actual number of live threads, for example when a ThreadFactory fails to
     * create a thread when asked, and when exiting threads are still performing bookkeeping before terminating. The
     * user-visible pool size is reported as the current size of the workers set.
     *
     * The 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 TIDYING: All tasks have
     * terminated, workerCount is zero, the thread transitioning to state TIDYING will run the terminated() hook method
     * TERMINATED: terminated() has completed
     *
     * 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 -> TIDYING When both queue and pool are empty STOP -> TIDYING When pool
     * is empty TIDYING -> TERMINATED When the terminated() hook method has completed
     *
     * Threads waiting in awaitTermination() will return when the state reaches TERMINATED.
     *
     * Detecting the transition from SHUTDOWN to TIDYING is less straightforward than you'd like because the queue may
     * become empty after non-empty and vice versa during SHUTDOWN state, but we can only terminate if, after seeing
     * that it is empty, we see that workerCount is 0 (which sometimes entails a recheck -- see below).
     */
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING = -1 << COUNT_BITS;
    private static final int SHUTDOWN = 0 << COUNT_BITS;
    private static final int STOP = 1 << COUNT_BITS;
    private static final int TIDYING = 2 << COUNT_BITS;
    private static final int TERMINATED = 3 << COUNT_BITS;

    // Packing and unpacking ctl
    private static int runStateOf(int c) {

        return c & ~CAPACITY;
    }

    private static int workerCountOf(int c) {

        return c & CAPACITY;
    }

    private static int ctlOf(int rs, int wc) {

        return rs | wc;
    }

    /*
     * Bit field accessors that don't require unpacking ctl. These depend on the bit layout and on workerCount being
     * never negative.
     */

    private static boolean runStateLessThan(int c, int s) {

        return c < s;
    }

    private static boolean runStateAtLeast(int c, int s) {

        return c >= s;
    }

    private static boolean isRunning(int c) {

        return c < SHUTDOWN;
    }

    /**
     * Attempt to CAS-increment the workerCount field of ctl.
     */
    private boolean compareAndIncrementWorkerCount(int expect) {

        return ctl.compareAndSet(expect, expect + 1);
    }

    /**
     * Attempt to CAS-decrement the workerCount field of ctl.
     */
    private boolean compareAndDecrementWorkerCount(int expect) {

        return ctl.compareAndSet(expect, expect - 1);
    }

    /**
     * Decrements the workerCount field of ctl. This is called only on abrupt termination of a thread (see
     * processWorkerExit). Other decrements are performed within getTask.
     */
    private void decrementWorkerCount() {

        do {
        }
        while (!compareAndDecrementWorkerCount(ctl.get()));
    }

    /**
     * The queue used for holding tasks and handing off to worker threads. We do not require that workQueue.poll()
     * returning null necessarily means that workQueue.isEmpty(), so rely solely on isEmpty to see if the queue is empty
     * (which we must do for example when deciding whether to transition from SHUTDOWN to TIDYING). 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<Runnable> workQueue;

    /**
     * Lock held on access to workers set and related bookkeeping. While we could use a concurrent set of some sort, it
     * turns out to be generally preferable to use a lock. Among the reasons is that this serializes
     * interruptIdleWorkers, which avoids unnecessary interrupt storms, especially during shutdown. Otherwise exiting
     * threads would concurrently interrupt those that have not yet interrupted. It also simplifies some of the
     * associated statistics bookkeeping of largestPoolSize etc. We also hold mainLock on shutdown and shutdownNow, for
     * the sake of ensuring workers set is stable while separately checking permission to interrupt and actually
     * interrupting.
     */
    private final ReentrantLock mainLock = new ReentrantLock();

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

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

    /**
     * Tracks largest attained pool size. Accessed only under mainLock.
     */
    private int largestPoolSize;

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

    /*
     * All user control parameters are declared as volatiles so that ongoing actions are based on freshest values, but
     * without need for locking, since no internal invariants depend on them changing synchronously with respect to
     * other actions.
     */

    /**
     * Factory for new threads. All threads are created using this factory (via method addWorker). All callers must be
     * prepared for addWorker to fail, 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.
     *
     * We go further and preserve pool invariants even in the face of errors such as OutOfMemoryError, that might be
     * thrown while trying to create threads. Such errors are rather common due to the need to allocate a native stack
     * in Thread#start, and users will want to perform clean pool shutdown to clean up. There will likely be enough
     * memory available for the cleanup code to complete without encountering yet another OutOfMemoryError.
     */
    private volatile ThreadFactory threadFactory;

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

    /**
     * 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 is the minimum number of workers to keep alive (and not allow to time out etc) unless
     * allowCoreThreadTimeOut is set, in which case the minimum is zero.
     */
    private volatile int corePoolSize;

    /**
     * Maximum pool size. Note that the actual maximum is internally bounded by CAPACITY.
     */
    private volatile int maximumPoolSize;

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

    /**
     * Permission required for callers of shutdown and shutdownNow. We additionally require (see checkShutdownAccess)
     * that callers have permission to actually interrupt threads in the worker set (as governed by Thread.interrupt,
     * which relies on ThreadGroup.checkAccess, which in turn relies on SecurityManager.checkAccess). Shutdowns are
     * attempted only if these checks pass.
     *
     * All actual invocations of Thread.interrupt (see interruptIdleWorkers and interruptWorkers) ignore
     * SecurityExceptions, meaning that the attempted interrupts silently fail. In the case of shutdown, they should not
     * fail unless the SecurityManager has inconsistent policies, sometimes allowing access to a thread and sometimes
     * not. In such cases, failure to actually interrupt threads may disable or delay full termination. Other uses of
     * interruptIdleWorkers are advisory, and failure to actually interrupt will merely delay response to configuration
     * changes so is not handled exceptionally.
     */
    private static final RuntimePermission shutdownPerm = new RuntimePermission("modifyThread");

    /**
     * Class Worker mainly maintains interrupt control state for threads running tasks, along with other minor
     * bookkeeping. This class opportunistically extends AbstractQueuedSynchronizer to simplify acquiring and releasing
     * a lock surrounding each task execution. This protects against interrupts that are intended to wake up a worker
     * thread waiting for a task from instead interrupting a task being run. We implement a simple non-reentrant mutual
     * exclusion lock rather than use ReentrantLock because we do not want worker tasks to be able to reacquire the lock
     * when they invoke pool control methods like setCorePoolSize. Additionally, to suppress interrupts until the thread
     * actually starts running tasks, we initialize lock state to a negative value, and clear it upon start (in
     * runWorker).
     */
    private final class Worker extends AbstractQueuedSynchronizer implements Runnable {

        /**
         * This class will never be serialized, but we provide a serialVersionUID to suppress a javac warning.
         */
        private static final long serialVersionUID = 6138294804551838833L;

        /** Thread this worker is running in. Null if factory fails. */
        final Thread thread;
        /** Initial task to run. Possibly null. */
        Runnable firstTask;
        /** Per-thread task counter */
        volatile long completedTasks;

        /**
         * Creates with given first task and thread from ThreadFactory.
         * 
         * @param firstTask
         *            the first task (null if none)
         */
        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

        /** Delegates main run loop to outer runWorker */
        @Override
        public void run() {

            runWorker(this);
        }

        // Lock methods
        //
        // The value 0 represents the unlocked state.
        // The value 1 represents the locked state.

        @Override
        protected boolean isHeldExclusively() {

            return getState() != 0;
        }

        @Override
        protected boolean tryAcquire(int unused) {

            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        @Override
        protected boolean tryRelease(int unused) {

            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        public void lock() {

            acquire(1);
        }

        public boolean tryLock() {

            return tryAcquire(1);
        }

        public void unlock() {

            release(1);
        }

        public boolean isLocked() {

            return isHeldExclusively();
        }

        void interruptIfStarted() {

            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                }
                catch (SecurityException ignore) {
                }
            }
        }
    }

    /*
     * Methods for setting control state
     */

    /**
     * Transitions runState to given target, or leaves it alone if already at least the given target.
     *
     * @param targetState
     *            the desired state, either SHUTDOWN or STOP (but not TIDYING or TERMINATED -- use tryTerminate for
     *            that)
     */
    private void advanceRunState(int targetState) {

        for (;;) {
            int c = ctl.get();
            if (runStateAtLeast(c, targetState) || ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c))))
                break;
        }
    }

    /**
     * Transitions to TERMINATED state if either (SHUTDOWN and pool and queue empty) or (STOP and pool empty). If
     * otherwise eligible to terminate but workerCount is nonzero, interrupts an idle worker to ensure that shutdown
     * signals propagate. This method must be called following any action that might make termination possible --
     * reducing worker count or removing tasks from the queue during shutdown. The method is non-private to allow access
     * from ScheduledThreadPoolExecutor.
     */
    final void tryTerminate() {

        for (;;) {
            int c = ctl.get();
            if (isRunning(c) || runStateAtLeast(c, TIDYING) || (runStateOf(c) == SHUTDOWN && !workQueue.isEmpty()))
                return;
            if (workerCountOf(c) != 0) { // Eligible to terminate
                interruptIdleWorkers(ONLY_ONE);
                return;
            }

            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                    try {
                        terminated();
                    }
                    finally {
                        ctl.set(ctlOf(TERMINATED, 0));
                        termination.signalAll();
                    }
                    return;
                }
            }
            finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }

    /*
     * Methods for controlling interrupts to worker threads.
     */

    /**
     * If there is a security manager, makes sure caller has permission to shut down threads in general (see
     * shutdownPerm). If this passes, additionally makes sure the caller is allowed to interrupt each worker thread.
     * This might not be true even if first check passed, if the SecurityManager treats some threads specially.
     */
    private void checkShutdownAccess() {

        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkPermission(shutdownPerm);
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                for (Worker w : workers)
                    security.checkAccess(w.thread);
            }
            finally {
                mainLock.unlock();
            }
        }
    }

    /**
     * Interrupts all threads, even if active. Ignores SecurityExceptions (in which case some threads may remain
     * uninterrupted).
     */
    private void interruptWorkers() {

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers)
                w.interruptIfStarted();
        }
        finally {
            mainLock.unlock();
        }
    }

    /**
     * Interrupts threads that might be waiting for tasks (as indicated by not being locked) so they can check for
     * termination or configuration changes. Ignores SecurityExceptions (in which case some threads may remain
     * uninterrupted).
     *
     * @param onlyOne
     *            If true, interrupt at most one worker. This is called only from tryTerminate when termination is
     *            otherwise enabled but there are still other workers. In this case, at most one waiting worker is
     *            interrupted to propagate shutdown signals in case all threads are currently waiting. Interrupting any
     *            arbitrary thread ensures that newly arriving workers since shutdown began will also eventually exit.
     *            To guarantee eventual termination, it suffices to always interrupt only one idle worker, but
     *            shutdown() interrupts all idle workers so that redundant workers exit promptly, not waiting for a
     *            straggler task to finish.
     */
    private void interruptIdleWorkers(boolean onlyOne) {

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        t.interrupt();
                    }
                    catch (SecurityException ignore) {
                    }
                    finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        }
        finally {
            mainLock.unlock();
        }
    }

    /**
     * Common form of interruptIdleWorkers, to avoid having to remember what the boolean argument means.
     */
    private void interruptIdleWorkers() {

        interruptIdleWorkers(false);
    }

    private static final boolean ONLY_ONE = true;

    /*
     * Misc utilities, most of which are also exported to ScheduledThreadPoolExecutor
     */

    /**
     * Invokes the rejected execution handler for the given command. Package-protected for use by
     * ScheduledThreadPoolExecutor.
     */
    final void reject(Runnable command) {

        handler.rejectedExecution(command, this);
    }

    /**
     * Performs any further cleanup following run state transition on invocation of shutdown. A no-op here, but used by
     * ScheduledThreadPoolExecutor to cancel delayed tasks.
     */
    void onShutdown() {

    }

    /**
     * State check needed by ScheduledThreadPoolExecutor to enable running tasks during shutdown.
     *
     * @param shutdownOK
     *            true if should return true if SHUTDOWN
     */
    final boolean isRunningOrShutdown(boolean shutdownOK) {

        int rs = runStateOf(ctl.get());
        return rs == RUNNING || (rs == SHUTDOWN && shutdownOK);
    }

    /**
     * Drains the task queue into a new list, normally using drainTo. But if the queue is a DelayQueue or any other kind
     * of queue for which poll or drainTo may fail to remove some elements, it deletes them one by one.
     */
    private List<Runnable> drainQueue() {

        BlockingQueue<Runnable> q = workQueue;
        List<Runnable> taskList = new ArrayList<Runnable>();
        q.drainTo(taskList);
        if (!q.isEmpty()) {
            for (Runnable r : q.toArray(new Runnable[0])) {
                if (q.remove(r))
                    taskList.add(r);
            }
        }
        return taskList;
    }

    /*
     * Methods for creating, running and cleaning up after workers
     */

    /**
     * Checks if a new worker can be added with respect to current pool state and the given bound (either core or
     * maximum). If so, the worker count is adjusted accordingly, and, if possible, a new worker is created and started,
     * running firstTask as its first task. This method returns false if the pool is stopped or eligible to shut down.
     * It also returns false if the thread factory fails to create a thread when asked. If the thread creation fails,
     * either due to the thread factory returning null, or due to an exception (typically OutOfMemoryError in
     * Thread#start), we roll back cleanly.
     *
     * @param firstTask
     *            the task the new thread should run first (or null if none). Workers are created with an initial first
     *            task (in method execute()) 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 usually created via prestartCoreThread or to replace other dying workers.
     *
     * @param core
     *            if true use corePoolSize as bound, else maximumPoolSize. (A boolean indicator is used here rather than
     *            a value to ensure reads of fresh values after checking other pool state).
     * @return true if successful
     */
    private boolean addWorker(Runnable firstTask, boolean core) {

        retry: for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && !(rs == SHUTDOWN && firstTask == null && !workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get(); // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            final ReentrantLock mainLock = this.mainLock;
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int c = ctl.get();
                    int rs = runStateOf(c);

                    if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                }
                finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        }
        finally {
            if (!workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

    /**
     * Rolls back the worker thread creation. - removes worker from workers, if present - decrements worker count -
     * rechecks for termination, in case the existence of this worker was holding up termination
     */
    private void addWorkerFailed(Worker w) {

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (w != null)
                workers.remove(w);
            decrementWorkerCount();
            tryTerminate();
        }
        finally {
            mainLock.unlock();
        }
    }

    /**
     * Performs cleanup and bookkeeping for a dying worker. Called only from worker threads. Unless completedAbruptly is
     * set, assumes that workerCount has already been adjusted to account for exit. This method removes thread from
     * worker set, and possibly terminates the pool or replaces the worker if either it exited due to user task
     * exception or if fewer than corePoolSize workers are running or queue is non-empty but there are no workers.
     *
     * @param w
     *            the worker
     * @param completedAbruptly
     *            if the worker died due to user exception
     */
    private void processWorkerExit(Worker w, boolean completedAbruptly) {

        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);
        }
        finally {
            mainLock.unlock();
        }

        tryTerminate();

        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && !workQueue.isEmpty())
                    min = 1;
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            addWorker(null, false);
        }
    }

    /**
     * Performs blocking or timed wait for a task, depending on current configuration settings, or returns null if this
     * worker must exit because of any of: 1. There are more than maximumPoolSize workers (due to a call to
     * setMaximumPoolSize). 2. The pool is stopped. 3. The pool is shutdown and the queue is empty. 4. This worker timed
     * out waiting for a task, and timed-out workers are subject to termination (that is,
     * {@code allowCoreThreadTimeOut || workerCount > corePoolSize}) both before and after the timed wait.
     *
     * @return task, or null if the worker must exit, in which case workerCount is decremented
     */
    private Runnable getTask() {

        boolean timedOut = false; // Did the last poll() time out?

        retry: for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            boolean timed; // Are workers subject to culling?

            for (;;) {
                int wc = workerCountOf(c);
                timed = allowCoreThreadTimeOut || wc > corePoolSize;

                if (wc <= maximumPoolSize && !(timedOut && timed))
                    break;
                if (compareAndDecrementWorkerCount(c))
                    return null;
                c = ctl.get(); // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }

            /*** MODIFIED BY ALEX ***/
            if (DequeTaskPushPolicy.class.isAssignableFrom(handler.getClass())) {

                DequeTaskPushPolicy dhandler = (DequeTaskPushPolicy) handler;

                Runnable r = dhandler.getTaskInDequeMode();

                if (r != null) {
                    return r;
                }
            }
            /*** MODIFIED BY ALEX ***/

            try {
                Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            }
            catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

    /**
     * Main worker run loop. Repeatedly gets tasks from queue and executes them, while coping with a number of issues:
     *
     * 1. We may start out with an initial task, in which case we don't need to get the first one. Otherwise, as long as
     * pool is running, we get tasks from getTask. If it returns null then the worker exits due to changed pool state or
     * configuration parameters. Other exits result from exception throws in external code, in which case
     * completedAbruptly holds, which usually leads processWorkerExit to replace this thread.
     *
     * 2. Before running any task, the lock is acquired to prevent other pool interrupts while the task is executing,
     * and clearInterruptsForTaskRun called to ensure that unless pool is stopping, this thread does not have its
     * interrupt set.
     *
     * 3. Each task run is preceded by a call to beforeExecute, which might throw an exception, in which case we cause
     * thread to die (breaking loop with completedAbruptly true) without processing the task.
     *
     * 4. Assuming beforeExecute completes normally, we run the task, gathering any of its thrown exceptions to send to
     * afterExecute. We separately handle RuntimeException, Error (both of which the specs guarantee that we trap) and
     * arbitrary Throwables. Because we cannot rethrow Throwables within Runnable.run, we wrap them within Errors on the
     * way out (to the thread's UncaughtExceptionHandler). Any thrown exception also conservatively causes thread to
     * die.
     *
     * 5. After task.run completes, we call afterExecute, which may also throw an exception, which will also cause
     * thread to die. According to JLS Sec 14.20, this exception is the one that will be in effect even if task.run
     * throws.
     *
     * The net effect of the exception mechanics is that afterExecute and the thread's UncaughtExceptionHandler have as
     * accurate information as we can provide about any problems encountered by user code.
     *
     * @param w
     *            the worker
     */
    final void runWorker(Worker w) {

        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted. This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP)))
                        && !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();
                    }
                    catch (RuntimeException x) {
                        thrown = x;
                        throw x;
                    }
                    catch (Error x) {
                        thrown = x;
                        throw x;
                    }
                    catch (Throwable x) {
                        thrown = x;
                        throw new Error(x);
                    }
                    finally {
                        afterExecute(task, thrown);
                    }
                }
                finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        }
        finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

    // Public constructors and methods

    /**
     * Creates a new {@code 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, unless
     *            {@code allowCoreThreadTimeOut} is set
     * @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 {@code keepAliveTime} argument
     * @param workQueue
     *            the queue to use for holding tasks before they are executed. This queue will hold only the
     *            {@code Runnable} tasks submitted by the {@code execute} method.
     * @throws IllegalArgumentException
     *             if one of the following holds:<br>
     *             {@code corePoolSize < 0}<br>
     *             {@code keepAliveTime < 0}<br>
     *             {@code maximumPoolSize <= 0}<br>
     *             {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException
     *             if {@code workQueue} is null
     */
    public QueueWorkerThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(),
                defaultHandler);
    }

    /**
     * Creates a new {@code 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, unless
     *            {@code allowCoreThreadTimeOut} is set
     * @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 {@code keepAliveTime} argument
     * @param workQueue
     *            the queue to use for holding tasks before they are executed. This queue will hold only the
     *            {@code Runnable} tasks submitted by the {@code execute} method.
     * @param threadFactory
     *            the factory to use when the executor creates a new thread
     * @throws IllegalArgumentException
     *             if one of the following holds:<br>
     *             {@code corePoolSize < 0}<br>
     *             {@code keepAliveTime < 0}<br>
     *             {@code maximumPoolSize <= 0}<br>
     *             {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException
     *             if {@code workQueue} or {@code threadFactory} is null
     */
    public QueueWorkerThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, defaultHandler);
    }

    /**
     * Creates a new {@code 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, unless
     *            {@code allowCoreThreadTimeOut} is set
     * @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 {@code keepAliveTime} argument
     * @param workQueue
     *            the queue to use for holding tasks before they are executed. This queue will hold only the
     *            {@code Runnable} tasks submitted by the {@code execute} method.
     * @param handler
     *            the handler to use when execution is blocked because the thread bounds and queue capacities are
     *            reached
     * @throws IllegalArgumentException
     *             if one of the following holds:<br>
     *             {@code corePoolSize < 0}<br>
     *             {@code keepAliveTime < 0}<br>
     *             {@code maximumPoolSize <= 0}<br>
     *             {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException
     *             if {@code workQueue} or {@code handler} is null
     */
    public QueueWorkerThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> workQueue, QueueWorkerRejectedExecutionHandler handler) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), handler);
    }

    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial parameters.
     *
     * @param corePoolSize
     *            the number of threads to keep in the pool, even if they are idle, unless
     *            {@code allowCoreThreadTimeOut} is set
     * @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 {@code keepAliveTime} argument
     * @param workQueue
     *            the queue to use for holding tasks before they are executed. This queue will hold only the
     *            {@code Runnable} tasks submitted by the {@code 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 one of the following holds:<br>
     *             {@code corePoolSize < 0}<br>
     *             {@code keepAliveTime < 0}<br>
     *             {@code maximumPoolSize <= 0}<br>
     *             {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException
     *             if {@code workQueue} or {@code threadFactory} or {@code handler} is null
     */
    public QueueWorkerThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory,
            QueueWorkerRejectedExecutionHandler 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;
    }

    /**
     * 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 {@code RejectedExecutionHandler}.
     *
     * @param command
     *            the task to execute
     * @throws RejectedExecutionException
     *             at discretion of {@code RejectedExecutionHandler}, if the task cannot be accepted for execution
     * @throws NullPointerException
     *             if {@code command} is null
     */
    @Override
    public void execute(Runnable command) {

        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to start a new thread with the given command as its
         * first task. The call to addWorker atomically checks runState and workerCount, and so prevents false alarms
         * that would add threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need to double-check whether we should have added a
         * thread (because existing ones died since last checking) or that the pool shut down since entry into this
         * method. So we recheck state and if necessary roll back the enqueuing if stopped, or start a new thread if
         * there are none.
         *
         * 3. If we cannot queue task, then we try to add a new thread. If it fails, we know we are shut down or
         * saturated and so reject the task.
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (!isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

    /**
     * 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.
     *
     * <p>
     * This method does not wait for previously submitted tasks to complete execution. Use {@link #awaitTermination
     * awaitTermination} to do that.
     *
     * @throws SecurityException
     *             {@inheritDoc}
     */
    @Override
    public void shutdown() {

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        }
        finally {
            mainLock.unlock();
        }
        tryTerminate();
    }

    /**
     * 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.
     *
     * <p>
     * This method does not wait for actively executing tasks to terminate. Use {@link #awaitTermination
     * awaitTermination} to do that.
     *
     * <p>
     * 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.
     *
     * @throws SecurityException
     *             {@inheritDoc}
     */
    @Override
    public List<Runnable> shutdownNow() {

        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(STOP);
            interruptWorkers();
            tasks = drainQueue();
        }
        finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }

    @Override
    public boolean isShutdown() {

        return !isRunning(ctl.get());
    }

    /**
     * Returns true if this executor is in the process of terminating after {@link #shutdown} or {@link #shutdownNow}
     * but has not completely terminated. This method may be useful for debugging. A return of {@code 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 c = ctl.get();
        return !isRunning(c) && runStateLessThan(c, TERMINATED);
    }

    @Override
    public boolean isTerminated() {

        return runStateAtLeast(ctl.get(), TERMINATED);
    }

    @Override
    public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {

        long nanos = unit.toNanos(timeout);
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (;;) {
                if (runStateAtLeast(ctl.get(), TERMINATED))
                    return true;
                if (nanos <= 0)
                    return false;
                nanos = termination.awaitNanos(nanos);
            }
        }
        finally {
            mainLock.unlock();
        }
    }

    /**
     * Invokes {@code shutdown} when this executor is no longer referenced and it has no threads.
     */
    @Override
    protected void finalize() {

        shutdown();
    }

    /**
     * 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(QueueWorkerRejectedExecutionHandler handler) {

        if (handler == null)
            throw new NullPointerException();
        this.handler = handler;
    }

    /**
     * Returns the current handler for unexecutable tasks.
     *
     * @return the current handler
     * @see #setRejectedExecutionHandler
     */
    public QueueWorkerRejectedExecutionHandler 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 {@code corePoolSize < 0}
     * @see #getCorePoolSize
     */
    public void setCorePoolSize(int corePoolSize) {

        if (corePoolSize < 0)
            throw new IllegalArgumentException();
        int delta = corePoolSize - this.corePoolSize;
        this.corePoolSize = corePoolSize;
        if (workerCountOf(ctl.get()) > corePoolSize)
            interruptIdleWorkers();
        else if (delta > 0) {
            // We don't really know how many new threads are "needed".
            // As a heuristic, prestart enough new workers (up to new
            // core size) to handle the current number of tasks in
            // queue, but stop if queue becomes empty while doing so.
            int k = Math.min(delta, workQueue.size());
            while (k-- > 0 && addWorker(null, true)) {
                if (workQueue.isEmpty())
                    break;
            }
        }
    }

    /**
     * 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 {@code false} if all core threads have already
     * been started.
     *
     * @return {@code true} if a thread was started
     */
    public boolean prestartCoreThread() {

        return workerCountOf(ctl.get()) < corePoolSize && addWorker(null, true);
    }

    /**
     * Same as prestartCoreThread except arranges that at least one thread is started even if corePoolSize is 0.
     */
    void ensurePrestart() {

        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize)
            addWorker(null, true);
        else if (wc == 0)
            addWorker(null, false);
    }

    /**
     * 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 (addWorker(null, true))
            ++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 {@code true} if core threads are allowed to time out, else {@code 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
     * {@code true}. This method should in general be called before the pool is actively used.
     *
     * @param value
     *            {@code true} if should time out, else {@code false}
     * @throws IllegalArgumentException
     *             if value is {@code 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");
        if (value != allowCoreThreadTimeOut) {
            allowCoreThreadTimeOut = value;
            if (value)
                interruptIdleWorkers();
        }
    }

    /**
     * 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();
        this.maximumPoolSize = maximumPoolSize;
        if (workerCountOf(ctl.get()) > maximumPoolSize)
            interruptIdleWorkers();
    }

    /**
     * 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 {@code time} argument
     * @throws IllegalArgumentException
     *             if {@code time} less than zero or if {@code time} is zero and {@code 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");
        long keepAliveTime = unit.toNanos(time);
        long delta = keepAliveTime - this.keepAliveTime;
        this.keepAliveTime = keepAliveTime;
        if (delta < 0)
            interruptIdleWorkers();
    }

    /**
     * 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<Runnable> 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.
     *
     * <p>
     * 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
     * {@code submit} might be converted into a form that maintains {@code Future} status. However, in such cases,
     * method {@link #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) {

        boolean removed = workQueue.remove(task);
        tryTerminate(); // In case SHUTDOWN and now empty
        return removed;
    }

    /**
     * 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() {

        final BlockingQueue<Runnable> q = workQueue;
        try {
            Iterator<Runnable> it = q.iterator();
            while (it.hasNext()) {
                Runnable r = it.next();
                if (r instanceof Future<?> && ((Future<?>) r).isCancelled())
                    it.remove();
            }
        }
        catch (ConcurrentModificationException fallThrough) {
            // Take slow path if we encounter interference during traversal.
            // Make copy for traversal and call remove for cancelled entries.
            // The slow path is more likely to be O(N*N).
            for (Object r : q.toArray())
                if (r instanceof Future<?> && ((Future<?>) r).isCancelled())
                    q.remove(r);
        }

        tryTerminate(); // In case SHUTDOWN and now empty
    }

    /* Statistics */

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

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // Remove rare and surprising possibility of
            // isTerminated() && getPoolSize() > 0
            return runStateAtLeast(ctl.get(), TIDYING) ? 0 : workers.size();
        }
        finally {
            mainLock.unlock();
        }
    }

    /**
     * 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.isLocked())
                    ++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.isLocked())
                    ++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();
        }
    }

    /**
     * Returns a string identifying this pool, as well as its state, including indications of run state and estimated
     * worker and task counts.
     *
     * @return a string identifying this pool, as well as its state
     */
    @Override
    public String toString() {

        long ncompleted;
        int nworkers, nactive;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            ncompleted = completedTaskCount;
            nactive = 0;
            nworkers = workers.size();
            for (Worker w : workers) {
                ncompleted += w.completedTasks;
                if (w.isLocked())
                    ++nactive;
            }
        }
        finally {
            mainLock.unlock();
        }
        int c = ctl.get();
        String rs = (runStateLessThan(c, SHUTDOWN) ? "Running"
                : (runStateAtLeast(c, TERMINATED) ? "Terminated" : "Shutting down"));
        return super.toString() + "[" + rs + ", pool size = " + nworkers + ", active threads = " + nactive
                + ", queued tasks = " + workQueue.size() + ", completed tasks = " + ncompleted + "]";
    }

    /* Extension hooks */

    /**
     * Method invoked prior to executing the given Runnable in the given thread. This method is invoked by thread
     * {@code t} that will execute task {@code r}, and may be used to re-initialize ThreadLocals, or to perform logging.
     *
     * <p>
     * This implementation does nothing, but may be customized in subclasses. Note: To properly nest multiple
     * overridings, subclasses should generally invoke {@code super.beforeExecute} at the end of this method.
     *
     * @param t
     *            the thread that will run task {@code 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 {@code RuntimeException} or {@code Error} that
     * caused execution to terminate abruptly.
     *
     * <p>
     * This implementation does nothing, but may be customized in subclasses. Note: To properly nest multiple
     * overridings, subclasses should generally invoke {@code super.afterExecute} at the beginning of this method.
     *
     * <p>
     * <b>Note:</b> When actions are enclosed in tasks (such as {@link FutureTask}) either explicitly or via methods
     * such as {@code submit}, these task objects catch and maintain computational exceptions, and so they do not cause
     * abrupt termination, and the internal exceptions are <em>not</em> passed to this method. If you would like to trap
     * both kinds of failures in this method, you can further probe for such cases, as in this sample subclass that
     * prints either the direct cause or the underlying exception if a task has been aborted:
     *
     * <pre>
     * 
     * {
     *     &#64;code
     *     class ExtendedExecutor extends ThreadPoolExecutor {
     * 
     *         // ...
     *         protected void afterExecute(Runnable r, Throwable t) {
     * 
     *             super.afterExecute(r, t);
     *             if (t == null && r instanceof Future<?>) {
     *                 try {
     *                     Object result = ((Future<?>) r).get();
     *                 }
     *                 catch (CancellationException ce) {
     *                     t = ce;
     *                 }
     *                 catch (ExecutionException ee) {
     *                     t = ee.getCause();
     *                 }
     *                 catch (InterruptedException ie) {
     *                     Thread.currentThread().interrupt(); // ignore/reset
     *                 }
     *             }
     *             if (t != null)
     *                 System.out.println(t);
     *         }
     *     }
     * }
     * </pre>
     *
     * @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 {@code 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 {@code execute}
     * method, unless the executor has been shut down, in which case the task is discarded.
     */
    public static class CallerRunsPolicy implements QueueWorkerRejectedExecutionHandler {

        /**
         * Creates a {@code 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
         */
        @Override
        public void rejectedExecution(Runnable r, QueueWorkerThreadPoolExecutor e) {

            if (!e.isShutdown()) {
                r.run();
            }
        }
    }

    /**
     * A handler for rejected tasks that throws a {@code RejectedExecutionException}.
     */
    public static class AbortPolicy implements QueueWorkerRejectedExecutionHandler {

        /**
         * Creates an {@code 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.
         */
        @Override
        public void rejectedExecution(Runnable r, QueueWorkerThreadPoolExecutor e) {

            throw new RejectedExecutionException("Task " + r.toString() + " rejected from " + e.toString());
        }
    }

    /**
     * A handler for rejected tasks that silently discards the rejected task.
     */
    public static class DiscardPolicy implements QueueWorkerRejectedExecutionHandler {

        /**
         * Creates a {@code 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
         */
        @Override
        public void rejectedExecution(Runnable r, QueueWorkerThreadPoolExecutor e) {

        }
    }

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

        /**
         * Creates a {@code 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
         */
        @Override
        public void rejectedExecution(Runnable r, QueueWorkerThreadPoolExecutor e) {

            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }

    /*** MODIFIED BY ALEX ***/
    public static class DequeTaskPushPolicy implements QueueWorkerRejectedExecutionHandler {

        private AtomicInteger globalThreadRoundRobinNumber = new AtomicInteger(0);

        public DequeTaskPushPolicy() {
        }

        public Runnable getTaskInDequeMode() {

            Runnable r = null;

            // poll task from current thread's Deque
            QueueWorkerThread qwt = (QueueWorkerThread) Thread.currentThread();

            // NOT in DequeMode
            if (qwt.isInDequeMode() == false) {
                return r;
            }

            r = qwt.pollQueueTask();

            // if there is no any task from Deque
            // we should get out of Deque Mode
            if (r == null) {
                qwt.disableDequeMode();
            }

            return r;
        }

        @Override
        public void rejectedExecution(Runnable r, QueueWorkerThreadPoolExecutor executor) {

            // step 1: get the ThreadGroup to enum all running threads in executor
            QueueWorkerThreadFactory qtf = (QueueWorkerThreadFactory) executor.getThreadFactory();

            ThreadGroup globalThreadGroup = qtf.getThreadGroup();

            int activeCount = executor.getPoolSize();

            Thread[] thd = new Thread[activeCount];

            globalThreadGroup.enumerate(thd);

            QueueWorkerThread qwt = null;

            boolean isDequeThreadFound = false;

            // step 2: should check every enum thread which is alive & not interrupted
            // then drain all tasks on BQueue & put current task to its Deque
            // then the thread should run in Deque Mode
            for (int i = 0; i < activeCount; i++) {

                int n = doRoundRobin(activeCount);

                qwt = ((QueueWorkerThread) thd[n]);

                if (qwt.isAlive() && !qwt.isInterrupted()) {
                    // when rejection shows to us, that means the threads reach MAX & BQueue is FULL
                    // then let's turn on Deque mode
                    qwt.enableDequeMode(executor.getQueue());
                    qwt.putQueueTask(r);
                    isDequeThreadFound = true;
                    break;
                }
            }

            // if there is no any DequeThread Found, then back CallerRunPolicy
            if (isDequeThreadFound == false) {
                if (!executor.isShutdown()) {
                    r.run();
                }
            }
        }

        /**
         * doRoundRobin
         * 
         * @param activeCount
         * @return
         */
        private int doRoundRobin(int activeCount) {

            int seed = globalThreadRoundRobinNumber.getAndIncrement();

            int n = seed % activeCount;

            if (seed == Integer.MAX_VALUE) {

                synchronized (globalThreadRoundRobinNumber) {

                    if (globalThreadRoundRobinNumber.get() == Integer.MAX_VALUE) {
                        globalThreadRoundRobinNumber.set(0);
                    }
                }
            }

            return n;
        }
    }

    public static class QueueWorkerThread extends Thread {

        private Deque<Runnable> workDeque = new ConcurrentLinkedDeque<Runnable>();

        private AtomicBoolean dequeEnableFlag = new AtomicBoolean(false);

        private final String threadCacheName;

        public QueueWorkerThread(ThreadGroup tg, Runnable r, String name) {
            super(tg, r, THD_PREFIX + name);
            threadCacheName = THD_PREFIX + name;
        }

        /**
         * enableDequeMode
         * 
         * @param consumeBQueueSizeBeforeDeque
         */
        protected void enableDequeMode(BlockingQueue<Runnable> workQueue) {

            if (dequeEnableFlag.get() == false) {

                synchronized (this) {

                    if (dequeEnableFlag.get() == false) {
                        // change deque enable flag=true
                        dequeEnableFlag.compareAndSet(false, true);

                        this.setName(threadCacheName + "-InDequeMode");

                        // we need drain out all tasks in BQueue
                        if (null != workQueue) {
                            List<Runnable> tasks = new ArrayList<Runnable>();
                            workQueue.drainTo(tasks);
                            workDeque.addAll(tasks);
                        }
                    }
                }
            }
        }

        /**
         * disable deque mode
         */
        protected void disableDequeMode() {

            if (dequeEnableFlag.get() == true) {

                synchronized (this) {

                    if (dequeEnableFlag.get() == true) {
                        dequeEnableFlag.compareAndSet(true, false);

                        this.setName(threadCacheName);
                    }
                }
            }
        }

        public boolean isInDequeMode() {

            return dequeEnableFlag.get();
        }

        public void putQueueTask(Runnable qt) {

            if (null == qt) {
                return;
            }

            workDeque.addLast(qt);
        }

        public Runnable pollQueueTask() {

            Runnable r = workDeque.pollFirst();

            return r;
        }

        public int getWorkQueueSize() {

            return workDeque.size();
        }
    }

    public static class QueueWorkerThreadFactory implements ThreadFactory {

        private AtomicInteger id = new AtomicInteger(0);

        private final String name;

        // thread group
        private final ThreadGroup globalThreadGroup;

        public QueueWorkerThreadFactory(String name) {
            this.name = name + "-";
            globalThreadGroup = new ThreadGroup(TG_THDGROUP + "-" + name);
        }

        @Override
        public Thread newThread(Runnable r) {

            Thread thd = new QueueWorkerThread(globalThreadGroup, r, this.name + String.valueOf(id.incrementAndGet()));
            // set thread as daemon for better release when reload or jvm out
            thd.setDaemon(true);

            return thd;
        }

        public ThreadGroup getThreadGroup() {

            return this.globalThreadGroup;
        }
    }

    // const
    private static final String THD_PREFIX = "CE-QueueWorker-";
    private static final String TG_THDGROUP = "CE-QWTPE-ThreadGroup";

    /*** MODIFIED BY ALEX ***/
}
