【Java原理系列】 ScheduledThreadPoolExecutor原理用法示例源码详解_scheduledthreadpoolexecutor中一个任务执行,其余都在队列里等待

ScheduledThreadPoolExecutor原理用法示例源码详解

原理

下面是ScheduledThreadPoolExecutor的原理：

1. 创建线程池：ScheduledThreadPoolExecutor在初始化时会创建一个线程池，这个线程池用于执行任务。
2. 排队任务：当调用ScheduledThreadPoolExecutor的schedule方法来提交一个任务时，任务会被封装成一个ScheduledFutureTask，并加入到任务队列中等待执行。
3. 延时执行：ScheduledFutureTask包含了任务的执行时间信息。ScheduledThreadPoolExecutor会根据任务的执行时间，将任务放置在延时队列中等待执行。
4. 选择任务：线程池中的线程会不断地从延时队列中选择需要执行的任务。
5. 执行任务：如果选择到了需要执行的任务，线程池会创建一个线程来执行该任务。
6. 完成任务：任务执行完成后，线程池会将任务标记为已完成，并从延时队列中移除。
7. 关闭线程池：当调用ScheduledThreadPoolExecutor的shutdown方法时，线程池会停止接收新的任务，并等待已提交的任务执行完成。执行完成后，线程池会关闭。 ScheduledThreadPoolExecutor的原理可以帮助我们理解如何使用它来实现定时任务的调度。通过调整线程池的大小和任务的执行时间，我们可以实现不同的调度策略。

用法

这个类是ThreadPoolExecutor的一个扩展，它允许提交在给定延迟后运行的任务，或者按固定间隔执行的任务。与java.util.Timer相比，当需要多个工作线程时，或者当需要ThreadPoolExecutor提供的额外灵活性和功能时，这个类更有优势。

此类实现了ScheduledExecutorService接口，提供了以下功能：

1. 延迟任务：任务可以在给定的延迟后执行，或者按固定间隔执行。
2. FIFO顺序：任务按提交的顺序执行（First-In-First-Out）。
3. 任务取消：可以取消正在提交的任务。取消的任务在执行前会被抑制。默认情况下，取消的任务会在其延迟过期后自动从队列中移除。
4. 任务定制：子类可以定制用于执行命令的任务类型。
5. 关闭行为：此类支持在关闭执行器时处理任务的不同行为。可以允许在关闭后继续运行任务，或者取消它们。

还提供了一些有用的方法，如now()、getDelay()和compareTo()，用于处理任务调度和延迟

这个类在处理需要执行定时任务或者需要按固定间隔执行任务时非常有用。

示例

下面是一些使用 ScheduledThreadPoolExecutor 的示例：

创建一个 ScheduledThreadPoolExecutor:

ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(5);
1

这里我们创建了一个 ScheduledThreadPoolExecutor 实例，并设置了核心线程数为5。

提交一个延迟任务:

executor.schedule(() -> System.out.println("Task 1 executed"), 5, TimeUnit.SECONDS);
1

这里我们提交了一个任务，该任务在5秒后执行，并打印 “Task 1 executed”。

提交一个按固定间隔执行的任务:

executor.scheduleAtFixedRate(() -> System.out.println("Task 2 executed"), 0, 2, TimeUnit.SECONDS);
1

这里我们提交了一个任务，该任务每隔2秒执行一次，并打印 “Task 2 executed”。

关闭执行器并等待所有任务完成:

executor.shutdown();
try {
    if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
        executor.shutdownNow();
    }
} catch (InterruptedException e) {
    executor.shutdownNow();
}
1
2
3
4
5
6
7
8

这里我们首先调用 shutdown() 方法通知执行器关闭，然后等待所有任务完成。如果在60秒内所有任务都没有完成，我们将调用 shutdownNow() 方法强制关闭执行器。

源码

/**
 * A {@link ThreadPoolExecutor} that can additionally schedule
 * commands to run after a given delay, or to execute
 * periodically. This class is preferable to {@link java.util.Timer}
 * when multiple worker threads are needed, or when the additional
 * flexibility or capabilities of {@link ThreadPoolExecutor} (which
 * this class extends) are required.
 *
 * <p>Delayed tasks execute no sooner than they are enabled, but
 * without any real-time guarantees about when, after they are
 * enabled, they will commence. Tasks scheduled for exactly the same
 * execution time are enabled in first-in-first-out (FIFO) order of
 * submission.
 *
 * <p>When a submitted task is cancelled before it is run, execution
 * is suppressed. By default, such a cancelled task is not
 * automatically removed from the work queue until its delay
 * elapses. While this enables further inspection and monitoring, it
 * may also cause unbounded retention of cancelled tasks. To avoid
 * this, set {@link #setRemoveOnCancelPolicy} to {@code true}, which
 * causes tasks to be immediately removed from the work queue at
 * time of cancellation.
 *
 * <p>Successive executions of a task scheduled via
 * {@code scheduleAtFixedRate} or
 * {@code scheduleWithFixedDelay} do not overlap. While different
 * executions may be performed by different threads, the effects of
 * prior executions <a
 * href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * those of subsequent ones.
 *
 * <p>While this class inherits from {@link ThreadPoolExecutor}, a few
 * of the inherited tuning methods are not useful for it. In
 * particular, because it acts as a fixed-sized pool using
 * {@code corePoolSize} threads and an unbounded queue, adjustments
 * to {@code maximumPoolSize} have no useful effect. Additionally, it
 * is almost never a good idea to set {@code corePoolSize} to zero or
 * use {@code allowCoreThreadTimeOut} because this may leave the pool
 * without threads to handle tasks once they become eligible to run.
 *
 * <p><b>Extension notes:</b> This class overrides the
 * {@link ThreadPoolExecutor#execute(Runnable) execute} and
 * {@link AbstractExecutorService#submit(Runnable) submit}
 * methods to generate internal {@link ScheduledFuture} objects to
 * control per-task delays and scheduling.  To preserve
 * functionality, any further overrides of these methods in
 * subclasses must invoke superclass versions, which effectively
 * disables additional task customization.  However, this class
 * provides alternative protected extension method
 * {@code decorateTask} (one version each for {@code Runnable} and
 * {@code Callable}) that can be used to customize the concrete task
 * types used to execute commands entered via {@code execute},
 * {@code submit}, {@code schedule}, {@code scheduleAtFixedRate},
 * and {@code scheduleWithFixedDelay}.  By default, a
 * {@code ScheduledThreadPoolExecutor} uses a task type extending
 * {@link FutureTask}. However, this may be modified or replaced using
 * subclasses of the form:
 *
 *  <pre> {@code
 * public class CustomScheduledExecutor extends ScheduledThreadPoolExecutor {
 *
 *   static class CustomTask<V> implements RunnableScheduledFuture<V> { ... }
 *
 *   protected <V> RunnableScheduledFuture<V> decorateTask(
 *                Runnable r, RunnableScheduledFuture<V> task) {
 *       return new CustomTask<V>(r, task);
 *   }
 *
 *   protected <V> RunnableScheduledFuture<V> decorateTask(
 *                Callable<V> c, RunnableScheduledFuture<V> task) {
 *       return new CustomTask<V>(c, task);
 *   }
 *   // ... add constructors, etc.
 * }}</pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public class ScheduledThreadPoolExecutor
        extends ThreadPoolExecutor
        implements ScheduledExecutorService {

    /*
     * This class specializes ThreadPoolExecutor implementation by
     *
     * 1. Using a custom task type, ScheduledFutureTask for
     *    tasks, even those that don't require scheduling (i.e.,
     *    those submitted using ExecutorService execute, not
     *    ScheduledExecutorService methods) which are treated as
     *    delayed tasks with a delay of zero.
     *
     * 2. Using a custom queue (DelayedWorkQueue), a variant of
     *    unbounded DelayQueue. The lack of capacity constraint and
     *    the fact that corePoolSize and maximumPoolSize are
     *    effectively identical simplifies some execution mechanics
     *    (see delayedExecute) compared to ThreadPoolExecutor.
     *
     * 3. Supporting optional run-after-shutdown parameters, which
     *    leads to overrides of shutdown methods to remove and cancel
     *    tasks that should NOT be run after shutdown, as well as
     *    different recheck logic when task (re)submission overlaps
     *    with a shutdown.
     *
     * 4. Task decoration methods to allow interception and
     *    instrumentation, which are needed because subclasses cannot
     *    otherwise override submit methods to get this effect. These
     *    don't have any impact on pool control logic though.
     */

    /**
     * False if should cancel/suppress periodic tasks on shutdown.
     */
    private volatile boolean continueExistingPeriodicTasksAfterShutdown;

    /**
     * False if should cancel non-periodic tasks on shutdown.
     */
    private volatile boolean executeExistingDelayedTasksAfterShutdown = true;

    /**
     * True if ScheduledFutureTask.cancel should remove from queue
     */
    private volatile boolean removeOnCancel = false;

    /**
     * Sequence number to break scheduling ties, and in turn to
     * guarantee FIFO order among tied entries.
     */
    private static final AtomicLong sequencer = new AtomicLong();

    /**
     * Returns current nanosecond time.
     */
    final long now() {
        return System.nanoTime();
    }

    private class ScheduledFutureTask<V>
            extends FutureTask<V> implements RunnableScheduledFuture<V> {

        /** Sequence number to break ties FIFO */
        private final long sequenceNumber;

        /** The time the task is enabled to execute in nanoTime units */
        private long time;

        /**
         * Period in nanoseconds for repeating tasks.  A positive
         * value indicates fixed-rate execution.  A negative value
         * indicates fixed-delay execution.  A value of 0 indicates a
         * non-repeating task.
         */
        private final long period;

        /** The actual task to be re-enqueued by reExecutePeriodic */
        RunnableScheduledFuture<V> outerTask = this;

        /**
         * Index into delay queue, to support faster cancellation.
         */
        int heapIndex;

        /**
         * Creates a one-shot action with given nanoTime-based trigger time.
         */
        ScheduledFutureTask(Runnable r, V result, long ns) {
            super(r, result);
            this.time = ns;
            this.period = 0;
            this.sequenceNumber = sequencer.getAndIncrement();
        }

        /**
         * Creates a periodic action with given nano time and period.
         */
        ScheduledFutureTask(Runnable r, V result, long ns, long period) {
            super(r, result);
            this.time = ns;
            this.period = period;
            this.sequenceNumber = sequencer.getAndIncrement();
        }

        /**
         * Creates a one-shot action with given nanoTime-based trigger time.
         */
        ScheduledFutureTask(Callable<V> callable, long ns) {
            super(callable);
            this.time = ns;
            this.period = 0;
            this.sequenceNumber = sequencer.getAndIncrement();
        }

        public long getDelay(TimeUnit unit) {
            return unit.convert(time - now(), NANOSECONDS);
        }

        public int compareTo(Delayed other) {
            if (other == this) // compare zero if same object
                return 0;
            if (other instanceof ScheduledFutureTask) {
                ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other;
                long diff = time - x.time;
                if (diff < 0)
                    return -1;
                else if (diff > 0)
                    return 1;
                else if (sequenceNumber < x.sequenceNumber)
                    return -1;
                else
                    return 1;
            }
            long diff = getDelay(NANOSECONDS) - other.getDelay(NANOSECONDS);
            return (diff < 0) ? -1 : (diff > 0) ? 1 : 0;
        }

        /**
         * Returns {@code true} if this is a periodic (not a one-shot) action.
         *
         * @return {@code true} if periodic
         */
        public boolean isPeriodic() {
            return period != 0;
        }

        /**
         * Sets the next time to run for a periodic task.
         */
        private void setNextRunTime() {
            long p = period;
            if (p > 0)
                time += p;
            else
                time = triggerTime(-p);
        }

        public boolean cancel(boolean mayInterruptIfRunning) {
            boolean cancelled = super.cancel(mayInterruptIfRunning);
            if (cancelled && removeOnCancel && heapIndex >= 0)
                remove(this);
            return cancelled;
        }

        /**
         * Overrides FutureTask version so as to reset/requeue if periodic.
         */
        public void run() {
            boolean periodic = isPeriodic();
            if (!canRunInCurrentRunState(periodic))
                cancel(false);
            else if (!periodic)
                ScheduledFutureTask.super.run();
            else if (ScheduledFutureTask.super.runAndReset()) {
                setNextRunTime();
                reExecutePeriodic(outerTask);
            }
        }
    }

    /**
     * Returns true if can run a task given current run state
     * and run-after-shutdown parameters.
     *
     * @param periodic true if this task periodic, false if delayed
     */
    boolean canRunInCurrentRunState(boolean periodic) {
        return isRunningOrShutdown(periodic ?
                                   continueExistingPeriodicTasksAfterShutdown :
                                   executeExistingDelayedTasksAfterShutdown);
    }

    /**
     * Main execution method for delayed or periodic tasks.  If pool
     * is shut down, rejects the task. Otherwise adds task to queue
     * and starts a thread, if necessary, to run it.  (We cannot
     * prestart the thread to run the task because the task (probably)
     * shouldn't be run yet.)  If the pool is shut down while the task
     * is being added, cancel and remove it if required by state and
     * run-after-shutdown parameters.
     *
     * @param task the task
     */
    private void delayedExecute(RunnableScheduledFuture<?> task) {
        if (isShutdown())
            reject(task);
        else {
            super.getQueue().add(task);
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }

    /**
     * Requeues a periodic task unless current run state precludes it.
     * Same idea as delayedExecute except drops task rather than rejecting.
     *
     * @param task the task
     */
    void reExecutePeriodic(RunnableScheduledFuture<?> task) {
        if (canRunInCurrentRunState(true)) {
            super.getQueue().add(task);
            if (!canRunInCurrentRunState(true) && remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }

    /**
     * Cancels and clears the queue of all tasks that should not be run
     * due to shutdown policy.  Invoked within super.shutdown.
     */
    @Override void onShutdown() {
        BlockingQueue<Runnable> q = super.getQueue();
        boolean keepDelayed =
            getExecuteExistingDelayedTasksAfterShutdownPolicy();
        boolean keepPeriodic =
            getContinueExistingPeriodicTasksAfterShutdownPolicy();
        if (!keepDelayed && !keepPeriodic) {
            for (Object e : q.toArray())
                if (e instanceof RunnableScheduledFuture<?>)
                    ((RunnableScheduledFuture<?>) e).cancel(false);
            q.clear();
        }
        else {
            // Traverse snapshot to avoid iterator exceptions
            for (Object e : q.toArray()) {
                if (e instanceof RunnableScheduledFuture) {
                    RunnableScheduledFuture<?> t =
                        (RunnableScheduledFuture<?>)e;
                    if ((t.isPeriodic() ? !keepPeriodic : !keepDelayed) ||
                        t.isCancelled()) { // also remove if already cancelled
                        if (q.remove(t))
                            t.cancel(false);
                    }
                }
            }
        }
        tryTerminate();
    }

    /**
     * Modifies or replaces the task used to execute a runnable.
     * This method can be used to override the concrete
     * class used for managing internal tasks.
     * The default implementation simply returns the given task.
     *
     * @param runnable the submitted Runnable
     * @param task the task created to execute the runnable
     * @param <V> the type of the task's result
     * @return a task that can execute the runnable
     * @since 1.6
     */
    protected <V> RunnableScheduledFuture<V> decorateTask(
        Runnable runnable, RunnableScheduledFuture<V> task) {
        return task;
    }

    /**
     * Modifies or replaces the task used to execute a callable.
     * This method can be used to override the concrete
     * class used for managing internal tasks.
     * The default implementation simply returns the given task.
     *
     * @param callable the submitted Callable
     * @param task the task created to execute the callable
     * @param <V> the type of the task's result
     * @return a task that can execute the callable
     * @since 1.6
     */
    protected <V> RunnableScheduledFuture<V> decorateTask(
        Callable<V> callable, RunnableScheduledFuture<V> task) {
        return task;
    }

    /**
     * Creates a new {@code ScheduledThreadPoolExecutor} with the
     * given core pool size.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     */
    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

    /**
     * Creates a new {@code ScheduledThreadPoolExecutor} 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 threadFactory the factory to use when the executor
     *        creates a new thread
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     * @throws NullPointerException if {@code threadFactory} is null
     */
    public ScheduledThreadPoolExecutor(int corePoolSize,
                                       ThreadFactory threadFactory) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue(), threadFactory);
    }

    /**
     * Creates a new ScheduledThreadPoolExecutor 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 handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     * @throws NullPointerException if {@code handler} is null
     */
    public ScheduledThreadPoolExecutor(int corePoolSize,
                                       RejectedExecutionHandler handler) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue(), handler);
    }

    /**
     * Creates a new ScheduledThreadPoolExecutor 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 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 {@code corePoolSize < 0}
     * @throws NullPointerException if {@code threadFactory} or
     *         {@code handler} is null
     */
    public ScheduledThreadPoolExecutor(int corePoolSize,
                                       ThreadFactory threadFactory,
                                       RejectedExecutionHandler handler) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue(), threadFactory, handler);
    }

    /**
     * Returns the trigger time of a delayed action.
     */
    private long triggerTime(long delay, TimeUnit unit) {
        return triggerTime(unit.toNanos((delay < 0) ? 0 : delay));
    }

    /**
     * Returns the trigger time of a delayed action.
     */
    long triggerTime(long delay) {
        return now() +
            ((delay < (Long.MAX_VALUE >> 1)) ? delay : overflowFree(delay));
    }

    /**
     * Constrains the values of all delays in the queue to be within
     * Long.MAX_VALUE of each other, to avoid overflow in compareTo.
     * This may occur if a task is eligible to be dequeued, but has
     * not yet been, while some other task is added with a delay of
     * Long.MAX_VALUE.
     */
    private long overflowFree(long delay) {
        Delayed head = (Delayed) super.getQueue().peek();
        if (head != null) {
            long headDelay = head.getDelay(NANOSECONDS);
            if (headDelay < 0 && (delay - headDelay < 0))
                delay = Long.MAX_VALUE + headDelay;
        }
        return delay;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public ScheduledFuture<?> schedule(Runnable command,
                                       long delay,
                                       TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        RunnableScheduledFuture<?> t = decorateTask(command,
            new ScheduledFutureTask<Void>(command, null,
                                          triggerTime(delay, unit)));
        delayedExecute(t);
        return t;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                           long delay,
                                           TimeUnit unit) {
        if (callable == null || unit == null)
            throw new NullPointerException();
        RunnableScheduledFuture<V> t = decorateTask(callable,
            new ScheduledFutureTask<V>(callable,
                                       triggerTime(delay, unit)));
        delayedExecute(t);
        return t;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     * @throws IllegalArgumentException   {@inheritDoc}
     */
    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (period <= 0)
            throw new IllegalArgumentException();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(period));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     * @throws IllegalArgumentException   {@inheritDoc}
     */
    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                     long initialDelay,
                                                     long delay,
                                                     TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (delay <= 0)
            throw new IllegalArgumentException();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(-delay));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
    }

    /**
     * Executes {@code command} with zero required delay.
     * This has effect equivalent to
     * {@link #schedule(Runnable,long,TimeUnit) schedule(command, 0, anyUnit)}.
     * Note that inspections of the queue and of the list returned by
     * {@code shutdownNow} will access the zero-delayed
     * {@link ScheduledFuture}, not the {@code command} itself.
     *
     * <p>A consequence of the use of {@code ScheduledFuture} objects is
     * that {@link ThreadPoolExecutor#afterExecute afterExecute} is always
     * called with a null second {@code Throwable} argument, even if the
     * {@code command} terminated abruptly.  Instead, the {@code Throwable}
     * thrown by such a task can be obtained via {@link Future#get}.
     *
     * @throws RejectedExecutionException at discretion of
     *         {@code RejectedExecutionHandler}, if the task
     *         cannot be accepted for execution because the
     *         executor has been shut down
     * @throws NullPointerException {@inheritDoc}
     */
    public void execute(Runnable command) {
        schedule(command, 0, NANOSECONDS);
    }

    // Override AbstractExecutorService methods

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<?> submit(Runnable task) {
        return schedule(task, 0, NANOSECONDS);
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        return schedule(Executors.callable(task, result), 0, NANOSECONDS);
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        return schedule(task, 0, NANOSECONDS);
    }

    /**
     * Sets the policy on whether to continue executing existing
     * periodic tasks even when this executor has been {@code shutdown}.
     * In this case, these tasks will only terminate upon
     * {@code shutdownNow} or after setting the policy to
     * {@code false} when already shutdown.
     * This value is by default {@code false}.
     *
     * @param value if {@code true}, continue after shutdown, else don't
     * @see #getContinueExistingPeriodicTasksAfterShutdownPolicy
     */
    public void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value) {
        continueExistingPeriodicTasksAfterShutdown = value;
        if (!value && isShutdown())
            onShutdown();
    }

    /**
     * Gets the policy on whether to continue executing existing
     * periodic tasks even when this executor has been {@code shutdown}.
     * In this case, these tasks will only terminate upon
     * {@code shutdownNow} or after setting the policy to
     * {@code false} when already shutdown.
     * This value is by default {@code false}.
     *
     * @return {@code true} if will continue after shutdown
     * @see #setContinueExistingPeriodicTasksAfterShutdownPolicy
     */
    public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy() {
        return continueExistingPeriodicTasksAfterShutdown;
    }

    /**
     * Sets the policy on whether to execute existing delayed
     * tasks even when this executor has been {@code shutdown}.
     * In this case, these tasks will only terminate upon
     * {@code shutdownNow}, or after setting the policy to
     * {@code false} when already shutdown.
     * This value is by default {@code true}.
     *
     * @param value if {@code true}, execute after shutdown, else don't
     * @see #getExecuteExistingDelayedTasksAfterShutdownPolicy
     */
    public void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value) {
        executeExistingDelayedTasksAfterShutdown = value;
        if (!value && isShutdown())
            onShutdown();
    }

    /**
     * Gets the policy on whether to execute existing delayed
     * tasks even when this executor has been {@code shutdown}.
     * In this case, these tasks will only terminate upon
     * {@code shutdownNow}, or after setting the policy to
     * {@code false} when already shutdown.
     * This value is by default {@code true}.
     *
     * @return {@code true} if will execute after shutdown
     * @see #setExecuteExistingDelayedTasksAfterShutdownPolicy
     */
    public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy() {
        return executeExistingDelayedTasksAfterShutdown;
    }

    /**
     * Sets the policy on whether cancelled tasks should be immediately
     * removed from the work queue at time of cancellation.  This value is
     * by default {@code false}.
     *
     * @param value if {@code true}, remove on cancellation, else don't
     * @see #getRemoveOnCancelPolicy
     * @since 1.7
     */
    public void setRemoveOnCancelPolicy(boolean value) {
        removeOnCancel = value;
    }

    /**
     * Gets the policy on whether cancelled tasks should be immediately
     * removed from the work queue at time of cancellation.  This value is
     * by default {@code false}.
     *
     * @return {@code true} if cancelled tasks are immediately removed
     *         from the queue
     * @see #setRemoveOnCancelPolicy
     * @since 1.7
     */
    public boolean getRemoveOnCancelPolicy() {
        return removeOnCancel;
    }

    /**
     * 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.
     *
     * <p>If the {@code ExecuteExistingDelayedTasksAfterShutdownPolicy}
     * has been set {@code false}, existing delayed tasks whose delays
     * have not yet elapsed are cancelled.  And unless the {@code
     * ContinueExistingPeriodicTasksAfterShutdownPolicy} has been set
     * {@code true}, future executions of existing periodic tasks will
     * be cancelled.
     *
     * @throws SecurityException {@inheritDoc}
     */
    public void shutdown() {
        super.shutdown();
    }

    /**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution.
     *
     * <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.
     *
     * @return list of tasks that never commenced execution.
     *         Each element of this list is a {@link ScheduledFuture},
     *         including those tasks submitted using {@code execute},
     *         which are for scheduling purposes used as the basis of a
     *         zero-delay {@code ScheduledFuture}.
     * @throws SecurityException {@inheritDoc}
     */
    public List<Runnable> shutdownNow() {
        return super.shutdownNow();
    }

    /**
     * Returns the task queue used by this executor.  Each element of
     * this queue is a {@link ScheduledFuture}, including those
     * tasks submitted using {@code execute} which are for scheduling
     * purposes used as the basis of a zero-delay
     * {@code ScheduledFuture}.  Iteration over this queue is
     * <em>not</em> guaranteed to traverse tasks in the order in
     * which they will execute.
     *
     * @return the task queue
     */
    public BlockingQueue<Runnable> getQueue() {
        return super.getQueue();
    }

    /**
     * Specialized delay queue. To mesh with TPE declarations, this
     * class must be declared as a BlockingQueue<Runnable> even though
     * it can only hold RunnableScheduledFutures.
     */
    static class DelayedWorkQueue extends AbstractQueue<Runnable>
        implements BlockingQueue<Runnable> {

        /*
         * A DelayedWorkQueue is based on a heap-based data structure
         * like those in DelayQueue and PriorityQueue, except that
         * every ScheduledFutureTask also records its index into the
         * heap array. This eliminates the need to find a task upon
         * cancellation, greatly speeding up removal (down from O(n)
         * to O(log n)), and reducing garbage retention that would
         * otherwise occur by waiting for the element to rise to top
         * before clearing. But because the queue may also hold
         * RunnableScheduledFutures that are not ScheduledFutureTasks,
         * we are not guaranteed to have such indices available, in
         * which case we fall back to linear search. (We expect that
         * most tasks will not be decorated, and that the faster cases
         * will be much more common.)
         *
         * All heap operations must record index changes -- mainly
         * within siftUp and siftDown. Upon removal, a task's
         * heapIndex is set to -1. Note that ScheduledFutureTasks can
         * appear at most once in the queue (this need not be true for
         * other kinds of tasks or work queues), so are uniquely
         * identified by heapIndex.
         */

        private static final int INITIAL_CAPACITY = 16;
        private RunnableScheduledFuture<?>[] queue =
            new RunnableScheduledFuture<?>[INITIAL_CAPACITY];
        private final ReentrantLock lock = new ReentrantLock();
        private int size = 0;

        /**
         * Thread designated to wait for the task at the head of the
         * queue.  This variant of the Leader-Follower pattern
         * (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
         * minimize unnecessary timed waiting.  When a thread becomes
         * the leader, it waits only for the next delay to elapse, but
         * other threads await indefinitely.  The leader thread must
         * signal some other thread before returning from take() or
         * poll(...), unless some other thread becomes leader in the
         * interim.  Whenever the head of the queue is replaced with a
         * task with an earlier expiration time, the leader field is
         * invalidated by being reset to null, and some waiting
         * thread, but not necessarily the current leader, is
         * signalled.  So waiting threads must be prepared to acquire
         * and lose leadership while waiting.
         */
        private Thread leader = null;

        /**
         * Condition signalled when a newer task becomes available at the
         * head of the queue or a new thread may need to become leader.
         */
        private final Condition available = lock.newCondition();

        /**
         * Sets f's heapIndex if it is a ScheduledFutureTask.
         */
        private void setIndex(RunnableScheduledFuture<?> f, int idx) {
            if (f instanceof ScheduledFutureTask)
                ((ScheduledFutureTask)f).heapIndex = idx;
        }

        /**
         * Sifts element added at bottom up to its heap-ordered spot.
         * Call only when holding lock.
         */
        private void siftUp(int k, RunnableScheduledFuture<?> key) {
            while (k > 0) {
                int parent = (k - 1) >>> 1;
                RunnableScheduledFuture<?> e = queue[parent];
                if (key.compareTo(e) >= 0)
                    break;
                queue[k] = e;
                setIndex(e, k);
                k = parent;
            }
            queue[k] = key;
            setIndex(key, k);
        }

        /**
         * Sifts element added at top down to its heap-ordered spot.
         * Call only when holding lock.
         */
        private void siftDown(int k, RunnableScheduledFuture<?> key) {
            int half = size >>> 1;
            while (k < half) {
                int child = (k << 1) + 1;
                RunnableScheduledFuture<?> c = queue[child];
                int right = child + 1;
                if (right < size && c.compareTo(queue[right]) > 0)
                    c = queue[child = right];
                if (key.compareTo(c) <= 0)
                    break;
                queue[k] = c;
                setIndex(c, k);
                k = child;
            }
            queue[k] = key;
            setIndex(key, k);
        }

        /**
         * Resizes the heap array.  Call only when holding lock.
         */
        private void grow() {
            int oldCapacity = queue.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1); // grow 50%
            if (newCapacity < 0) // overflow
                newCapacity = Integer.MAX_VALUE;
            queue = Arrays.copyOf(queue, newCapacity);
        }

        /**
         * Finds index of given object, or -1 if absent.
         */
        private int indexOf(Object x) {
            if (x != null) {
                if (x instanceof ScheduledFutureTask) {
                    int i = ((ScheduledFutureTask) x).heapIndex;
                    // Sanity check; x could conceivably be a
                    // ScheduledFutureTask from some other pool.
                    if (i >= 0 && i < size && queue[i] == x)
                        return i;
                } else {
                    for (int i = 0; i < size; i++)
                        if (x.equals(queue[i]))
                            return i;
                }
            }
            return -1;
        }

        public boolean contains(Object x) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                return indexOf(x) != -1;
            } finally {
                lock.unlock();
            }
        }

        public boolean remove(Object x) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                int i = indexOf(x);
                if (i < 0)
                    return false;

                setIndex(queue[i], -1);
                int s = --size;
                RunnableScheduledFuture<?> replacement = queue[s];
                queue[s] = null;
                if (s != i) {
                    siftDown(i, replacement);
                    if (queue[i] == replacement)
                        siftUp(i, replacement);
                }
                return true;
            } finally {
                lock.unlock();
            }
        }

        public int size() {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                return size;
            } finally {
                lock.unlock();
            }
        }

        public boolean isEmpty() {
            return size() == 0;
        }

        public int remainingCapacity() {
            return Integer.MAX_VALUE;
        }

        public RunnableScheduledFuture<?> peek() {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                return queue[0];
            } finally {
                lock.unlock();
            }
        }

        public boolean offer(Runnable x) {
            if (x == null)
                throw new NullPointerException();
            RunnableScheduledFuture<?> e = (RunnableScheduledFuture<?>)x;
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                int i = size;
                if (i >= queue.length)
                    grow();
                size = i + 1;
                if (i == 0) {
                    queue[0] = e;
                    setIndex(e, 0);
                } else {
                    siftUp(i, e);
                }
                if (queue[0] == e) {
                    leader = null;
                    available.signal();
                }
            } finally {
                lock.unlock();
            }
            return true;
        }

        public void put(Runnable e) {
            offer(e);
        }

        public boolean add(Runnable e) {
            return offer(e);
        }

        public boolean offer(Runnable e, long timeout, TimeUnit unit) {
            return offer(e);
        }

        /**
         * Performs common bookkeeping for poll and take: Replaces
         * first element with last and sifts it down.  Call only when
         * holding lock.
         * @param f the task to remove and return
         */
        private RunnableScheduledFuture<?> finishPoll(RunnableScheduledFuture<?> f) {
            int s = --size;
            RunnableScheduledFuture<?> x = queue[s];
            queue[s] = null;
            if (s != 0)
                siftDown(0, x);
            setIndex(f, -1);
            return f;
        }

        public RunnableScheduledFuture<?> poll() {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                RunnableScheduledFuture<?> first = queue[0];
                if (first == null || first.getDelay(NANOSECONDS) > 0)
                    return null;
                else
                    return finishPoll(first);
            } finally {
                lock.unlock();
            }
        }

        public RunnableScheduledFuture<?> take() throws InterruptedException {
            final ReentrantLock lock = this.lock;
            lock.lockInterruptibly();
            try {
                for (;;) {
                    RunnableScheduledFuture<?> first = queue[0];
                    if (first == null)
                        available.await();
                    else {
                        long delay = first.getDelay(NANOSECONDS);
                        if (delay <= 0)
                            return finishPoll(first);
                        first = null; // don't retain ref while waiting
                        if (leader != null)
                            available.await();
                        else {
                            Thread thisThread = Thread.currentThread();
                            leader = thisThread;
                            try {
                                available.awaitNanos(delay);
                            } finally {
                                if (leader == thisThread)
                                    leader = null;
                            }
                        }
                    }
                }
            } finally {
                if (leader == null && queue[0] != null)
                    available.signal();
                lock.unlock();
            }
        }

        public RunnableScheduledFuture<?> poll(long timeout, TimeUnit unit)
            throws InterruptedException {
            long nanos = unit.toNanos(timeout);
            final ReentrantLock lock = this.lock;
            lock.lockInterruptibly();
            try {
                for (;;) {
                    RunnableScheduledFuture<?> first = queue[0];
                    if (first == null) {
                        if (nanos <= 0)
                            return null;
                        else
                            nanos = available.awaitNanos(nanos);
                    } else {
                        long delay = first.getDelay(NANOSECONDS);
                        if (delay <= 0)
                            return finishPoll(first);
                        if (nanos <= 0)
                            return null;
                        first = null; // don't retain ref while waiting
                        if (nanos < delay || leader != null)
                            nanos = available.awaitNanos(nanos);
                        else {
                            Thread thisThread = Thread.currentThread();
                            leader = thisThread;
                            try {
                                long timeLeft = available.awaitNanos(delay);
                                nanos -= delay - timeLeft;
                            } finally {
                                if (leader == thisThread)
                                    leader = null;
                            }
                        }
                    }
                }
            } finally {
                if (leader == null && queue[0] != null)
                    available.signal();
                lock.unlock();
            }
        }

        public void clear() {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                for (int i = 0; i < size; i++) {
                    RunnableScheduledFuture<?> t = queue[i];
                    if (t != null) {
                        queue[i] = null;
                        setIndex(t, -1);
                    }
                }
                size = 0;
            } finally {
                lock.unlock();
            }
        }

        /**
         * Returns first element only if it is expired.
         * Used only by drainTo.  Call only when holding lock.
         */
        private RunnableScheduledFuture<?> peekExpired() {
            // assert lock.isHeldByCurrentThread();
            RunnableScheduledFuture<?> first = queue[0];
            return (first == null || first.getDelay(NANOSECONDS) > 0) ?
                null : first;
        }

        public int drainTo(Collection<? super Runnable> c) {
            if (c == null)
                throw new NullPointerException();
            if (c == this)
                throw new IllegalArgumentException();
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                RunnableScheduledFuture<?> first;
                int n = 0;
                while ((first = peekExpired()) != null) {
                    c.add(first);   // In this order, in case add() throws.
                    finishPoll(first);
                    ++n;
                }
                return n;
            } finally {
                lock.unlock();
            }
        }

        public int drainTo(Collection<? super Runnable> c, int maxElements) {
            if (c == null)
                throw new NullPointerException();
            if (c == this)
                throw new IllegalArgumentException();
            if (maxElements <= 0)
                return 0;
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                RunnableScheduledFuture<?> first;
                int n = 0;
                while (n < maxElements && (first = peekExpired()) != null) {
                    c.add(first);   // In this order, in case add() throws.
                    finishPoll(first);
                    ++n;
                }
                return n;
            } finally {
                lock.unlock();
            }
        }

        public Object[] toArray() {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                return Arrays.copyOf(queue, size, Object[].class);
            } finally {
                lock.unlock();
            }
        }

        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                if (a.length < size)
                    return (T[]) Arrays.copyOf(queue, size, a.getClass());
                System.arraycopy(queue, 0, a, 0, size);
                if (a.length > size)
                    a[size] = null;
                return a;
            } finally {
                lock.unlock();
            }
        }

        public Iterator<Runnable> iterator() {
            return new Itr(Arrays.copyOf(queue, size));
        }

        /**
         * Snapshot iterator that works off copy of underlying q array.
         */
        private class Itr implements Iterator<Runnable> {
            final RunnableScheduledFuture<?>[] array;
            int cursor = 0;     // index of next element to return
            int lastRet = -1;   // index of last element, or -1 if no such

            Itr(RunnableScheduledFuture<?>[] array) {
                this.array = array;
            }

            public boolean hasNext() {
                return cursor < array.length;
            }

            public Runnable next() {
                if (cursor >= array.length)
                    throw new NoSuchElementException();
                lastRet = cursor;
                return array[cursor++];
            }

            public void remove() {
                if (lastRet < 0)
                    throw new IllegalStateException();
                DelayedWorkQueue.this.remove(array[lastRet]);
                lastRet = -1;
            }
        }
    }
}
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 a[size] = null;
            return a;
        } finally {
            lock.unlock();
        }
    }

    public Iterator<Runnable> iterator() {
        return new Itr(Arrays.copyOf(queue, size));
    }

    /**
     * Snapshot iterator that works off copy of underlying q array.
     */
    private class Itr implements Iterator<Runnable> {
        final RunnableScheduledFuture<?>[] array;
        int cursor = 0;     // index of next element to return
        int lastRet = -1;   // index of last element, or -1 if no such

        Itr(RunnableScheduledFuture<?>[] array) {
            this.array = array;
        }

        public boolean hasNext() {
            return cursor < array.length;
        }

        public Runnable next() {
            if (cursor >= array.length)
                throw new NoSuchElementException();
            lastRet = cursor;
            return array[cursor++];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            DelayedWorkQueue.this.remove(array[lastRet]);
            lastRet = -1;
        }
    }
}
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}

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