/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * The original version of this file carried the following notice:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
package org.apache.tomcat.util.threads;

import java.util.ArrayList;
import java.util.ConcurrentModificationException;
import java.util.HashSet;
import java.util.List;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

import org.apache.tomcat.util.res.StringManager;

/**
 * An {@link java.util.concurrent.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(Runnable)}, if fewer than corePoolSize threads are running, a new
 * thread is created to handle the request, even if other worker threads are idle. Else if fewer than maximumPoolSize
 * threads are running, a new thread will be created to handle the request 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(TimeUnit)}). 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(long, TimeUnit)}. 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 corePoolSize threads, 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>If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a
 * new thread.
 * <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.
 * </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 java.util.concurrent.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 faster on average than they can be
 * processed.
 * <li><em> Unbounded queues.</em> Using an unbounded queue (for example a
 * {@link java.util.concurrent.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 faster on average than they can be processed.
 * <li><em>Bounded queues.</em> A bounded queue (for example, an {@link java.util.concurrent.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.
 * </ol>
 * </dd>
 * <dt>Rejected tasks</dt>
 * <dd>New tasks submitted in method {@link #execute(Runnable)} 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(Runnable, ThreadPoolExecutor)} 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>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>In {@link ThreadPoolExecutor.DiscardPolicy}, a task that cannot be executed is simply dropped. This policy is
 * designed only for those rare cases in which task completion is never relied upon.
 * <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.) This policy
 * is rarely acceptable. In nearly all cases, you should also cancel the task to cause an exception in any component
 * waiting for its completion, and/or log the failure, as illustrated in {@link ThreadPoolExecutor.DiscardOldestPolicy}
 * documentation.
 * </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(Thread, Runnable)} and
 * {@link #afterExecute(Runnable, Throwable)} 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, callback, or BlockingQueue methods throw exceptions, internal worker threads may in turn fail, abruptly
 * terminate, and possibly be replaced.</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(Runnable)} and
 * {@link #purge} are available to assist in storage reclamation when large numbers of queued tasks become
 * cancelled.</dd>
 * <dt>Reclamation</dt>
 * <dd>A pool that is no longer referenced in a program <em>AND</em> has no remaining threads may be reclaimed (garbage
 * collected) without being explicitly shutdown. You can configure a pool to allow all unused threads to 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
 */
public class ThreadPoolExecutor extends AbstractExecutorService {

    /**
     * The string manager for this package.
     */
    protected static final StringManager sm = StringManager.getManager(ThreadPoolExecutor.class);

    /**
     * The main pool control state, ctl, is an atomic integer packing two conceptual fields:
     * <ul>
     * <li>workerCount, indicating the effective number of threads</li>
     * <li>runState, indicating whether running, shutting down etc</li>
     * </ul>
     * 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.
     * <p>
     * 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.
     * <p>
     * The runState provides the main lifecycle control, taking on values:
     * <ul>
     * <li>RUNNING: Accept new tasks and process queued tasks</li>
     * <li>SHUTDOWN: Don't accept new tasks, but process queued tasks</li>
     * <li>STOP: Don't accept new tasks, don't process queued tasks, and interrupt in-progress tasks</li>
     * <li>TIDYING: All tasks have terminated, workerCount is zero, the thread transitioning to state TIDYING will run
     * the terminated() hook method</li>
     * <li>TERMINATED: terminated() has completed</li>
     * </ul>
     * 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:
     * <ul>
     * <li>RUNNING -> SHUTDOWN On invocation of shutdown()</li>
     * <li>(RUNNING or SHUTDOWN) -> STOP On invocation of shutdownNow()</li>
     * <li>SHUTDOWN -> TIDYING When both queue and pool are empty</li>
     * <li>STOP -> TIDYING When pool is empty</li>
     * <li>TIDYING -> TERMINATED When the terminated() hook method has completed</li>
     * </ul>
     * Threads waiting in awaitTermination() will return when the state reaches TERMINATED.
     * <p>
     * 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 COUNT_MASK = (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;
    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 workerCountOf(int c) {
        return c & COUNT_MASK;
    }

    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;
    }

    /**
     * Attempts to CAS-increment the workerCount field of ctl.
     */
    private boolean compareAndIncrementWorkerCount(int expect) {
        return ctl.compareAndSet(expect, expect + 1);
    }

    /**
     * Attempts 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() {
        ctl.addAndGet(-1);
    }

    /**
     * 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<>();

    /**
     * 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;

    /**
     * The number of tasks submitted but not yet finished. This includes tasks in the queue and tasks that have been
     * handed to a worker thread but the latter did not start executing the task yet. This number is always greater or
     * equal to {@link #getActiveCount()}.
     */
    private final AtomicInteger submittedCount = new AtomicInteger(0);
    private final AtomicLong lastContextStoppedTime = new AtomicLong(0L);

    /**
     * Most recent time in ms when a thread decided to kill itself to avoid potential memory leaks. Useful to throttle
     * the rate of renewals of threads.
     */
    private final AtomicLong lastTimeThreadKilledItself = new AtomicLong(0L);

    /*
     * 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.
     */

    /**
     * Delay in ms between 2 threads being renewed. If negative, do not renew threads.
     */
    private volatile long threadRenewalDelay = Constants.DEFAULT_THREAD_RENEWAL_DELAY;

    /**
     * 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.
     * <p>
     * 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 RejectedExecutionHandler 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.
     * <p>
     * Since the worker count is actually stored in COUNT_BITS bits, the effective limit is
     * {@code corePoolSize & COUNT_MASK}.
     */
    private volatile int corePoolSize;

    /**
     * Maximum pool size.
     * <p>
     * Since the worker count is actually stored in COUNT_BITS bits, the effective limit is
     * {@code maximumPoolSize & COUNT_MASK}.
     */
    private volatile int maximumPoolSize;

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

    /**
     * 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;

        // TODO: switch to AbstractQueuedLongSynchronizer and move
        // completedTasks into the lock word.

        /**
         * 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) {
        // assert targetState == SHUTDOWN || targetState == STOP;
        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) || (runStateLessThan(c, STOP) && !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() {
        // assert mainLock.isHeldByCurrentThread();
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkPermission(shutdownPerm);
            for (Worker w : workers) {
                security.checkAccess(w.thread);
            }
        }
    }

    /**
     * Interrupts all threads, even if active. Ignores SecurityExceptions (in which case some threads may remain
     * uninterrupted).
     */
    private void interruptWorkers() {
        // assert mainLock.isHeldByCurrentThread();
        for (Worker w : workers) {
            w.interruptIfStarted();
        }
    }

    /**
     * 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() {
    }

    /**
     * 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;
        ArrayList<Runnable> taskList = new ArrayList<>();
        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();;) {
            // Check if queue empty only if necessary.
            if (runStateAtLeast(c, SHUTDOWN) &&
                    (runStateAtLeast(c, STOP) || firstTask != null || workQueue.isEmpty())) {
                return false;
            }

            for (;;) {
                if (workerCountOf(c) >= ((core ? corePoolSize : maximumPoolSize) & COUNT_MASK)) {
                    return false;
                }
                if (compareAndIncrementWorkerCount(c)) {
                    break retry;
                }
                c = ctl.get(); // Re-read ctl
                if (runStateAtLeast(c, SHUTDOWN)) {
                    continue retry;
                    // else CAS failed due to workerCount change; retry inner loop
                }
            }
        }

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

                    if (isRunning(c) || (runStateLessThan(c, STOP) && firstTask == null)) {
                        if (t.getState() != Thread.State.NEW) {
                            throw new IllegalThreadStateException();
                        }
                        workers.add(w);
                        workerAdded = true;
                        int s = workers.size();
                        if (s > largestPoolSize) {
                            largestPoolSize = s;
                        }
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (!workerStarted) {
                addWorkerFailed(w);
            }
        }
        return workerStarted;
    }

    /**
     * Rolls back the worker thread creation.
     * <ul>
     * <li>removes worker from workers, if present</li>
     * <li>decrements worker count</li>
     * <li>rechecks for termination, in case the existence of this worker was holding up termination</li>
     * </ul>
     */
    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) {
            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;
                }
                // https://bz.apache.org/bugzilla/show_bug.cgi?id=65454
                // If the work queue is not empty, it is likely that a task was
                // added to the work queue between this thread timing out and
                // the worker count being decremented a few lines above this
                // comment. In this case, create a replacement worker so that
                // the task isn't held in the queue waiting for one of the other
                // workers to finish.
                if (workerCountOf(c) >= min && workQueue.isEmpty()) {
                    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:
     * <ol>
     * <li>There are more than maximumPoolSize workers (due to a call to setMaximumPoolSize).</li>
     * <li>The pool is stopped.</li>
     * <li>The pool is shutdown and the queue is empty.</li>
     * <li>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, and if the
     * queue is non-empty, this worker is not the last thread in the pool.</li>
     * </ol>
     *
     * @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?

        for (;;) {
            int c = ctl.get();

            // Check if queue empty only if necessary.
            if (runStateAtLeast(c, SHUTDOWN) && (runStateAtLeast(c, STOP) || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c)) {
                    return null;
                }
                continue;
            }

            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:
     * <p>
     * 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.
     * <p>
     * 2. Before running any task, the lock is acquired to prevent other pool interrupts while the task is executing,
     * and then we ensure that unless pool is stopping, this thread does not have its interrupt set.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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);
                    try {
                        task.run();
                        afterExecute(task, null);
                    } catch (Throwable ex) {
                        afterExecute(task, ex);
                        throw ex;
                    }
                } 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, the
     * {@linkplain Executors#defaultThreadFactory default thread factory} and the
     * {@linkplain ThreadPoolExecutor.RejectPolicy default rejected execution handler}.
     * <p>
     * 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 ThreadPoolExecutor(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 the
     * {@linkplain ThreadPoolExecutor.RejectPolicy 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 ThreadPoolExecutor(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 the
     * {@linkplain Executors#defaultThreadFactory 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 ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> workQueue, RejectedExecutionHandler 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 ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
            BlockingQueue<Runnable> 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;

        prestartAllCoreThreads();
    }


    @Override
    public void execute(Runnable command) {
        submittedCount.incrementAndGet();
        try {
            executeInternal(command);
        } catch (RejectedExecutionException rx) {
            if (getQueue() instanceof RetryableQueue) {
                // If the Executor is close to maximum pool size, concurrent
                // calls to execute() may result (due to Tomcat's use of
                // TaskQueue) in some tasks being rejected rather than queued.
                // If this happens, add them to the queue.
                final RetryableQueue<Runnable> queue = (RetryableQueue<Runnable>) getQueue();
                if (!queue.force(command)) {
                    submittedCount.decrementAndGet();
                    throw new RejectedExecutionException(sm.getString("threadPoolExecutor.queueFull"));
                }
            } else {
                submittedCount.decrementAndGet();
                throw rx;
            }
        }
    }


    /**
     * 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 {@link 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
     */
    private void executeInternal(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 If a security manager exists and shutting down this
      *             pool requires the {@link RuntimePermission}{@code ("shutdownNow")} permission
      */
    @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 interrupts tasks via {@link Thread#interrupt}; any task that fails to respond to interrupts may
     * never terminate.
     *
    * @throws SecurityException If a security manager exists and shutting down this
      *             pool requires the {@link RuntimePermission}{@code ("shutdownNow")} permission
      */
    @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 runStateAtLeast(ctl.get(), SHUTDOWN);
    }

    /** Used by ScheduledThreadPoolExecutor. */
    boolean isStopped() {
        return runStateAtLeast(ctl.get(), STOP);
    }

    /**
     * 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 {@code true} if terminating but not yet terminated
     */
    public boolean isTerminating() {
        int c = ctl.get();
        return runStateAtLeast(c, SHUTDOWN) && 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 {
            while (runStateLessThan(ctl.get(), TERMINATED)) {
                if (nanos <= 0L) {
                    return false;
                }
                nanos = termination.awaitNanos(nanos);
            }
            return true;
        } finally {
            mainLock.unlock();
        }
    }

    /**
     * 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(ThreadFactory)
     */
    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(RejectedExecutionHandler)
     */
    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 {@code corePoolSize < 0} or {@code corePoolSize} is greater than the
     *                                      {@linkplain #getMaximumPoolSize() maximum pool size}
     *
     * @see #getCorePoolSize
     */
    public void setCorePoolSize(int corePoolSize) {
        if (corePoolSize < 0 || maximumPoolSize < corePoolSize) {
            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(sm.getString("threadPoolExecutor.invalidKeepAlive"));
        }
        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 thread keep-alive time, which is the amount of time that threads may remain idle before being
     * terminated. Threads that wait this amount of time without processing a task will be terminated if there are more
     * than the core number of threads currently in the pool, or if this pool {@linkplain #allowsCoreThreadTimeOut()
     * allows core thread timeout}. 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(TimeUnit)
     */
    public void setKeepAliveTime(long time, TimeUnit unit) {
        if (time < 0) {
            throw new IllegalArgumentException(sm.getString("threadPoolExecutor.invalidKeepAlive"));
        }
        if (time == 0 && allowsCoreThreadTimeOut()) {
            throw new IllegalArgumentException(sm.getString("threadPoolExecutor.invalidKeepAlive"));
        }
        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 may remain idle before being
     * terminated. Threads that wait this amount of time without processing a task will be terminated if there are more
     * than the core number of threads currently in the pool, or if this pool {@linkplain #allowsCoreThreadTimeOut()
     * allows core thread timeout}.
     *
     * @param unit the desired time unit of the result
     *
     * @return the time limit
     *
     * @see #setKeepAliveTime(long, TimeUnit)
     */
    public long getKeepAliveTime(TimeUnit unit) {
        return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
    }


    /**
     * Returns the delay in milliseconds after which threads should be renewed.
     *
     * @return the thread renewal delay
     */
    public long getThreadRenewalDelay() {
        return threadRenewalDelay;
    }


    /**
     * Sets the delay in milliseconds after which threads should be renewed.
     *
     * @param threadRenewalDelay the renewal delay
     */
    public void setThreadRenewalDelay(long threadRenewalDelay) {
        this.threadRenewalDelay = threadRenewalDelay;
    }


    /* 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 {@code 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 {
            q.removeIf(r -> r instanceof Future<?> && ((Future<?>) r).isCancelled());
        } 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
    }


    /**
     * Called when a context is stopping to trigger thread renewal for
     * threads that were created before the context was stopped.
     */
    public void contextStopping() {
        this.lastContextStoppedTime.set(System.currentTimeMillis());

        // save the current pool parameters to restore them later
        int savedCorePoolSize = this.getCorePoolSize();

        // setCorePoolSize(0) wakes idle threads
        this.setCorePoolSize(0);

        // TaskQueue.take() takes care of timing out, so that we are sure that
        // all threads of the pool are renewed in a limited time, something like
        // (threadKeepAlive + longest request time)

        this.setCorePoolSize(savedCorePoolSize);
    }


    /* 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 current number of threads in the pool. <br>
     * <b>NOTE</b>: this method only used in {@link TaskQueue#offer(Runnable)}, where operations are frequent, can
     * greatly reduce unnecessary performance overhead by a lock-free way.
     *
     * @return the number of threads
     */
    protected int getPoolSizeNoLock() {
        return runStateAtLeast(ctl.get(), TIDYING) ? 0 : workers.size();
    }

    /**
     * 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 the total number of tasks that have been submitted to this pool.
     *
     * @return the submitted task count
     */
    public int getSubmittedCount() {
        return submittedCount.get();
    }


    /**
     * 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 runState = isRunning(c) ? "Running" : runStateAtLeast(c, TERMINATED) ? "Terminated" : "Shutting down";
        return super.toString() + "[" + runState + ", 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 java.util.concurrent.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> {@code
     * class ExtendedExecutor extends ThreadPoolExecutor {
     *     // ...
     *     protected void afterExecute(Runnable r, Throwable t) {
     *         super.afterExecute(r, t);
     *         if (t == null && r instanceof Future<?> && ((Future<?>) r).isDone()) {
     *             try {
     *                 Object result = ((Future<?>) r).get();
     *             } catch (CancellationException ce) {
     *                 t = ce;
     *             } catch (ExecutionException ee) {
     *                 t = ee.getCause();
     *             } catch (InterruptedException ie) {
     *                 // ignore/reset
     *                 Thread.currentThread().interrupt();
     *             }
     *         }
     *         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) {
        // Throwing StopPooledThreadException is likely to cause this method to
        // be called more than once for a given task based on the typical
        // implementations of the parent class. This test ensures that
        // decrementAndGet() is only called once after each task execution.
        if (!(t instanceof StopPooledThreadException)) {
            submittedCount.decrementAndGet();
        }

        if (t == null) {
            stopCurrentThreadIfNeeded();
        }
    }


    /**
     * If the current thread was started before the last time when a context was stopped, an exception is thrown so that
     * the current thread is stopped.
     */
    protected void stopCurrentThreadIfNeeded() {
        if (currentThreadShouldBeStopped()) {
            long lastTime = lastTimeThreadKilledItself.longValue();
            if (lastTime + threadRenewalDelay < System.currentTimeMillis()) {
                if (lastTimeThreadKilledItself.compareAndSet(lastTime, System.currentTimeMillis() + 1)) {
                    // OK, it's really time to dispose of this thread

                    final String msg = sm.getString("threadPoolExecutor.threadStoppedToAvoidPotentialLeak",
                            Thread.currentThread().getName());

                    throw new StopPooledThreadException(msg);
                }
            }
        }
    }


    /**
     * Checks whether the current thread should be stopped based on the
     * thread renewal policy and the last context stop time.
     *
     * @return true if the current thread should be stopped
     */
    protected boolean currentThreadShouldBeStopped() {
        Thread currentThread = Thread.currentThread();
        if (threadRenewalDelay >= 0 && currentThread instanceof TaskThread) {
            TaskThread currentTaskThread = (TaskThread) currentThread;
            return currentTaskThread.getCreationTime() < this.lastContextStoppedTime.longValue();
        }
        return false;
    }


    /**
     * 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 RejectedExecutionHandler {
        /**
         * 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, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                r.run();
            }
        }
    }

    /**
     * A handler for rejected tasks that throws a {@link RejectedExecutionException}.
     */
    public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * 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, ThreadPoolExecutor e) {
            throw new RejectedExecutionException(
                    sm.getString("threadPoolExecutor.taskRejected", r.toString(), e.toString()));
        }
    }

    /**
     * A handler for rejected tasks that silently discards the rejected task.
     */
    public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * 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, ThreadPoolExecutor 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. This policy is rarely useful in cases where other
     * threads may be waiting for tasks to terminate, or failures must be recorded. Instead, consider using a handler of
     * the form:
     *
     * <pre> {@code
     * new RejectedExecutionHandler() {
     *     public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
     *         Runnable dropped = e.getQueue().poll();
     *         if (dropped instanceof Future<?>) {
     *             ((Future<?>) dropped).cancel(false);
     *             // also consider logging the failure
     *         }
     *         e.execute(r); // retry
     *     }
     * }
     * }</pre>
     */
    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
        /**
         * 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, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }

    private static class RejectPolicy implements RejectedExecutionHandler {
        @Override
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
            throw new RejectedExecutionException();
        }
    }


    /**
     * Handler for tasks that cannot be executed by the thread pool.
     */
    public interface RejectedExecutionHandler {

        /**
         * Method that may be invoked by a {@link ThreadPoolExecutor} when {@link ThreadPoolExecutor#execute execute}
         * cannot accept a task. This may occur when no more threads or queue slots are available because their bounds
         * would be exceeded, or upon shutdown of the Executor.
         * <p>
         * In the absence of other alternatives, the method may throw an unchecked {@link RejectedExecutionException},
         * which will be propagated to the caller of {@code execute}.
         *
         * @param r        the runnable task requested to be executed
         * @param executor the executor attempting to execute this task
         *
         * @throws RejectedExecutionException if there is no remedy
         */
        void rejectedExecution(Runnable r, ThreadPoolExecutor executor);
    }
}