content: /*
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * 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 java.util.concurrent;
  
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  import static java.util.concurrent.TimeUnit.NANOSECONDS;
  
  import java.util.AbstractQueue;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.List;
  import java.util.NoSuchElementException;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.ReentrantLock;
  
  // BEGIN android-note
  // omit class-level docs on setRemoveOnCancelPolicy()
  // END android-note
  
  /**
   * 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.
   *
   * <p>Successive executions of a periodic task scheduled via
   * {@link #scheduleAtFixedRate scheduleAtFixedRate} or
   * {@link #scheduleWithFixedDelay 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>
   *
   * @author Doug Lea
   * @since 1.5
   */
  public class ScheduledThreadPoolExecutor
          extends ThreadPoolExecutor
          implements ScheduledExecutorService {
  
      /*
       * This class specializes ThreadPoolExecutor implementation by
       *
       * 1. Using a custom task type ScheduledFutureTask, even for tasks
       *    that don't require scheduling because they are submitted
       *    using ExecutorService rather than ScheduledExecutorService
       *    methods, which are treated as 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.
       */
      volatile boolean removeOnCancel;
  
      /**
       * Sequence number to break scheduling ties, and in turn to
       * guarantee FIFO order among tied entries.
       */
      private static final AtomicLong sequencer = new AtomicLong();
  
      private class ScheduledFutureTask<V>
              extends FutureTask<V> implements RunnableScheduledFuture<V> {
  
          /**
           * Sequence number to break ties FIFO
           */
          private final long sequenceNumber;
  
          /**
           * The nanoTime-based time when the task is enabled to execute.
           */
          private volatile long time;
  
          /**
           * Period for repeating tasks, in nanoseconds.
           * A positive value indicates fixed-rate execution.
           * A negative value indicates fixed-delay execution.
           * A value of 0 indicates a non-repeating (one-shot) 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 triggerTime,
                              long sequenceNumber) {
              super(r, result);
              this.time = triggerTime;
              this.period = 0;
              this.sequenceNumber = sequenceNumber;
          }
  
          /**
           * Creates a periodic action with given nanoTime-based initial
           * trigger time and period.
           */
          ScheduledFutureTask(Runnable r, V result, long triggerTime,
                              long period, long sequenceNumber) {
              super(r, result);
              this.time = triggerTime;
              this.period = period;
              this.sequenceNumber = sequenceNumber;
          }
  
          /**
           * Creates a one-shot action with given nanoTime-based trigger time.
           */
          ScheduledFutureTask(Callable<V> callable, long triggerTime,
                              long sequenceNumber) {
              super(callable);
              this.time = triggerTime;
              this.period = 0;
              this.sequenceNumber = sequenceNumber;
          }
  
          public long getDelay(TimeUnit unit) {
              return unit.convert(time - System.nanoTime(), 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) {
              // The racy read of heapIndex below is benign:
              // if heapIndex < 0, then OOTA guarantees that we have surely
              // been removed; else we recheck under lock in remove()
              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)
                  super.run();
              else if (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;
      }
  
      /**
       * The default keep-alive time for pool threads.
       * <p>
       * Normally, this value is unused because all pool threads will be
       * core threads, but if a user creates a pool with a corePoolSize
       * of zero (against our advice), we keep a thread alive as long as
       * there are queued tasks.  If the keep alive time is zero (the
       * historic value), we end up hot-spinning in getTask, wasting a
       * CPU.  But on the other hand, if we set the value too high, and
       * users create a one-shot pool which they don't cleanly shutdown,
       * the pool's non-daemon threads will prevent JVM termination.  A
       * small but non-zero value (relative to a JVM's lifetime) seems
       * best.
       */
      private static final long DEFAULT_KEEPALIVE_MILLIS = 10L;
  
      /**
       * 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) {
          //核心线程自定义，总线程数无限大，自定义强
          // workQueue是DelayedWorkQueue，可以实现延时/定时获取任务
  
          //1。当运行的线程数小于核心线程数就会创建核心线程数来执行任务
          //2。当运行的线程数大于等于核心线程数时，就会将任务加入到DelayedWorkQueue中，
          //3。DelayedWorkQueue的size为16，所以当加入到队列中的数据大于16的时候，
          // 会创建新的非核心线程来执行任务，
          //4。当非核心线程满的话，就会拒绝任务
          super(corePoolSize, Integer.MAX_VALUE,
                  DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
                  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,
                  DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
                  new DelayedWorkQueue(), threadFactory);
      }
  
      /**
       * 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 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,
                  DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
                  new DelayedWorkQueue(), handler);
      }
  
      /**
       * 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
       * @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,
                  DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
                  new DelayedWorkQueue(), threadFactory, handler);
      }
  
      /**
       * Returns the nanoTime-based trigger time of a delayed action.
       */
      private long triggerTime(long delay, TimeUnit unit) {
          return triggerTime(unit.toNanos((delay < 0) ? 0 : delay));
      }
  
      /**
       * Returns the nanoTime-based trigger time of a delayed action.
       */
      long triggerTime(long delay) {
          return System.nanoTime() +
                  ((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<Void> t = decorateTask(command,
                  new ScheduledFutureTask<Void>(command, null,
                          triggerTime(delay, unit),
                          sequencer.getAndIncrement()));
          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),
                          sequencer.getAndIncrement()));
          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 <= 0L)
              throw new IllegalArgumentException();
          ScheduledFutureTask<Void> sft =
                  new ScheduledFutureTask<Void>(command,
                          null,
                          triggerTime(initialDelay, unit),
                          unit.toNanos(period),
                          sequencer.getAndIncrement());
          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 <= 0L)
              throw new IllegalArgumentException();
          ScheduledFutureTask<Void> sft =
                  new ScheduledFutureTask<Void>(command,
                          null,
                          triggerTime(initialDelay, unit),
                          -unit.toNanos(delay),
                          sequencer.getAndIncrement());
          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.
       */
      // android-note: Removed "throws SecurityException" doc.
      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. 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.
       *
       * @return list of tasks that never commenced execution.
       * Each element of this list is a {@link ScheduledFuture}.
       * For tasks submitted via one of the {@code schedule}
       * methods, the element will be identical to the returned
       * {@code ScheduledFuture}.  For tasks submitted using
       * {@link #execute execute}, the element will be a
       * zero-delay {@code ScheduledFuture}.
       */
      // android-note: Removed "throws SecurityException" doc.
      public List<Runnable> shutdownNow() {
          return super.shutdownNow();
      }
  
      /**
       * 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.
       *
       * <p>Each element of this queue is a {@link ScheduledFuture}.
       * For tasks submitted via one of the {@code schedule} methods, the
       * element will be identical to the returned {@code ScheduledFuture}.
       * For tasks submitted using {@link #execute execute}, the element
       * will be a zero-delay {@code ScheduledFuture}.
       *
       * <p>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;
  
          /**
           * 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;
  
          /**
           * 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];
                  return (first == null || first.getDelay(NANOSECONDS) > 0)
                          ? null
                          : 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 <= 0L)
                              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 <= 0L)
                              return null;
                          else
                              nanos = available.awaitNanos(nanos);
                      } else {
                          long delay = first.getDelay(NANOSECONDS);
                          if (delay <= 0L)
                              return finishPoll(first);
                          if (nanos <= 0L)
                              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;        // index of next element to return; initially 0
              int lastRet = -1;  // index of last element returned; -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;
              }
          }
      }
  }
  