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java.util.concurrent
public class: ScheduledThreadPoolExecutor [javadoc | source]
java.lang.Object
   java.util.concurrent.AbstractExecutorService
      java.util.concurrent.ThreadPoolExecutor
         java.util.concurrent.ScheduledThreadPoolExecutor

All Implemented Interfaces:
    ScheduledExecutorService, ExecutorService

A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically. This class is preferable to java.util.Timer when multiple worker threads are needed, or when the additional flexibility or capabilities of ThreadPoolExecutor (which this class extends) are required.

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.

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 #setRemoveOnCancelPolicy to {@code true}, which causes tasks to be immediately removed from the work queue at time of cancellation.

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 happen-before those of subsequent ones.

While this class inherits from 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.

Extension notes: This class overrides the execute and submit methods to generate internal 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 FutureTask . However, this may be modified or replaced using subclasses of the form:

 {@code
public class CustomScheduledExecutor extends ScheduledThreadPoolExecutor {

  static class CustomTask implements RunnableScheduledFuture { ... }

  protected  RunnableScheduledFuture decorateTask(
               Runnable r, RunnableScheduledFuture task) {
      return new CustomTask(r, task);
  }

  protected  RunnableScheduledFuture decorateTask(
               Callable c, RunnableScheduledFuture task) {
      return new CustomTask(c, task);
  }
  // ... add constructors, etc.
}}
Nested Class Summary:
static class  ScheduledThreadPoolExecutor.DelayedWorkQueue  Specialized delay queue. To mesh with TPE declarations, this class must be declared as a BlockingQueue even though it can only hold RunnableScheduledFutures. 
Constructor:
 public ScheduledThreadPoolExecutor(int corePoolSize) 
 public ScheduledThreadPoolExecutor(int corePoolSize,
    ThreadFactory threadFactory) 
    Creates a new {@code ScheduledThreadPoolExecutor} with the given initial parameters.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set
    threadFactory - the factory to use when the executor creates a new thread
    Throws:
    IllegalArgumentException - if {@code corePoolSize < 0}
    NullPointerException - if {@code threadFactory} is null
 public ScheduledThreadPoolExecutor(int corePoolSize,
    RejectedExecutionHandler handler) 
    Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set
    handler - the handler to use when execution is blocked because the thread bounds and queue capacities are reached
    Throws:
    IllegalArgumentException - if {@code corePoolSize < 0}
    NullPointerException - if {@code handler} is null
 public ScheduledThreadPoolExecutor(int corePoolSize,
    ThreadFactory threadFactory,
    RejectedExecutionHandler handler) 
    Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set
    threadFactory - the factory to use when the executor creates a new thread
    handler - the handler to use when execution is blocked because the thread bounds and queue capacities are reached
    Throws:
    IllegalArgumentException - if {@code corePoolSize < 0}
    NullPointerException - if {@code threadFactory} or {@code handler} is null
Method from java.util.concurrent.ScheduledThreadPoolExecutor Summary:
canRunInCurrentRunState,   decorateTask,   decorateTask,   execute,   getContinueExistingPeriodicTasksAfterShutdownPolicy,   getExecuteExistingDelayedTasksAfterShutdownPolicy,   getQueue,   getRemoveOnCancelPolicy,   now,   onShutdown,   reExecutePeriodic,   schedule,   schedule,   scheduleAtFixedRate,   scheduleWithFixedDelay,   setContinueExistingPeriodicTasksAfterShutdownPolicy,   setExecuteExistingDelayedTasksAfterShutdownPolicy,   setRemoveOnCancelPolicy,   shutdown,   shutdownNow,   submit,   submit,   submit,   triggerTime
Methods from java.util.concurrent.ThreadPoolExecutor:
afterExecute,   allowCoreThreadTimeOut,   allowsCoreThreadTimeOut,   awaitTermination,   beforeExecute,   execute,   finalize,   getActiveCount,   getCompletedTaskCount,   getCorePoolSize,   getKeepAliveTime,   getLargestPoolSize,   getMaximumPoolSize,   getPoolSize,   getQueue,   getRejectedExecutionHandler,   getTaskCount,   getThreadFactory,   isRunningOrShutdown,   isShutdown,   isTerminated,   isTerminating,   onShutdown,   prestartAllCoreThreads,   prestartCoreThread,   purge,   reject,   remove,   runWorker,   setCorePoolSize,   setKeepAliveTime,   setMaximumPoolSize,   setRejectedExecutionHandler,   setThreadFactory,   shutdown,   shutdownNow,   terminated,   toString,   tryTerminate
Methods from java.util.concurrent.AbstractExecutorService:
invokeAll,   invokeAll,   invokeAny,   invokeAny,   newTaskFor,   newTaskFor,   submit,   submit,   submit
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.util.concurrent.ScheduledThreadPoolExecutor Detail:
 boolean canRunInCurrentRunState(boolean periodic) 
    Returns true if can run a task given current run state and run-after-shutdown parameters.
 protected RunnableScheduledFuture<V> decorateTask(Runnable runnable,
    RunnableScheduledFuture<V> task) 
    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.
 protected RunnableScheduledFuture<V> decorateTask(Callable<V> callable,
    RunnableScheduledFuture<V> 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.
 public  void execute(Runnable command) 
    Executes {@code command} with zero required delay. This has effect equivalent to schedule(command, 0, anyUnit) . Note that inspections of the queue and of the list returned by {@code shutdownNow} will access the zero-delayed ScheduledFuture , not the {@code command} itself.

    A consequence of the use of {@code ScheduledFuture} objects is that 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 Future#get .

 public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy() 
    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}.
 public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy() 
    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}.
 public BlockingQueue<Runnable> getQueue() 
    Returns the task queue used by this executor. Each element of this queue is a 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 not guaranteed to traverse tasks in the order in which they will execute.
 public boolean getRemoveOnCancelPolicy() 
    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}.
 final long now() 
    Returns current nanosecond time.
  void onShutdown() 
    Cancels and clears the queue of all tasks that should not be run due to shutdown policy. Invoked within super.shutdown.
  void reExecutePeriodic(RunnableScheduledFuture<?> task) 
    Requeues a periodic task unless current run state precludes it. Same idea as delayedExecute except drops task rather than rejecting.
 public ScheduledFuture<?> schedule(Runnable command,
    long delay,
    TimeUnit unit) 
 public ScheduledFuture<V> schedule(Callable<V> callable,
    long delay,
    TimeUnit unit) 
 public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
    long initialDelay,
    long period,
    TimeUnit unit) 
 public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
    long initialDelay,
    long delay,
    TimeUnit unit) 
 public  void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value) 
    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}.
 public  void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value) 
    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}.
 public  void setRemoveOnCancelPolicy(boolean value) 
    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}.
 public  void shutdown() 
    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.

    This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

    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.

 public List<Runnable> shutdownNow() 
    Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

    This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that.

    There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. This implementation cancels tasks via Thread#interrupt , so any task that fails to respond to interrupts may never terminate.

 public Future<?> submit(Runnable task) 
 public Future<T> submit(Callable<T> task) 
 public Future<T> submit(Runnable task,
    T result) 
 long triggerTime(long delay) 
    Returns the trigger time of a delayed action.