Module java.base

Class ScheduledThreadPoolExecutor

All Implemented Interfaces:
AutoCloseable, Executor, ExecutorService, ScheduledExecutorService

public class ScheduledThreadPoolExecutor
extends ThreadPoolExecutor
implements ScheduledExecutorService
A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically. This class is preferable to 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, use setRemoveOnCancelPolicy(boolean) to cause tasks to be immediately removed from the work queue at time of cancellation.

Successive executions of a periodic task scheduled via scheduleAtFixedRate or 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 corePoolSize threads and an unbounded queue, adjustments to maximumPoolSize have no useful effect. Additionally, it is almost never a good idea to set corePoolSize to zero or use allowCoreThreadTimeOut because this may leave the pool without threads to handle tasks once they become eligible to run.

As with ThreadPoolExecutor, if not otherwise specified, this class uses Executors.defaultThreadFactory() as the default thread factory, and ThreadPoolExecutor.AbortPolicy as the default rejected execution handler.

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 decorateTask (one version each for Runnable and Callable) that can be used to customize the concrete task types used to execute commands entered via execute, submit, schedule, scheduleAtFixedRate, and scheduleWithFixedDelay. By default, a ScheduledThreadPoolExecutor uses a task type extending FutureTask. However, this may be modified or replaced using subclasses of the form:

 
 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.
 }
Since:
1.5
  • Constructor Details

    • ScheduledThreadPoolExecutor

      public ScheduledThreadPoolExecutor(int corePoolSize)
      Creates a new ScheduledThreadPoolExecutor with the given core pool size.
      Parameters:
      corePoolSize - the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
      Throws:
      IllegalArgumentException - if corePoolSize < 0
    • ScheduledThreadPoolExecutor

      public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory)
      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 allowCoreThreadTimeOut is set
      threadFactory - the factory to use when the executor creates a new thread
      Throws:
      IllegalArgumentException - if corePoolSize < 0
      NullPointerException - if threadFactory is null
    • ScheduledThreadPoolExecutor

      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 allowCoreThreadTimeOut is set
      handler - the handler to use when execution is blocked because the thread bounds and queue capacities are reached
      Throws:
      IllegalArgumentException - if corePoolSize < 0
      NullPointerException - if handler is null
    • ScheduledThreadPoolExecutor

      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 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 corePoolSize < 0
      NullPointerException - if threadFactory or handler is null
  • Method Details

    • decorateTask

      protected <V> 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.
      Type Parameters:
      V - the type of the task's result
      Parameters:
      runnable - the submitted Runnable
      task - the task created to execute the runnable
      Returns:
      a task that can execute the runnable
      Since:
      1.6
    • decorateTask

      protected <V> 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.
      Type Parameters:
      V - the type of the task's result
      Parameters:
      callable - the submitted Callable
      task - the task created to execute the callable
      Returns:
      a task that can execute the callable
      Since:
      1.6
    • schedule

      public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
      Description copied from interface: ScheduledExecutorService
      Submits a one-shot task that becomes enabled after the given delay.
      Specified by:
      schedule in interface ScheduledExecutorService
      Parameters:
      command - the task to execute
      delay - the time from now to delay execution
      unit - the time unit of the delay parameter
      Returns:
      a ScheduledFuture representing pending completion of the task and whose get() method will return null upon completion
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if command or unit is null
    • schedule

      public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit)
      Description copied from interface: ScheduledExecutorService
      Submits a value-returning one-shot task that becomes enabled after the given delay.
      Specified by:
      schedule in interface ScheduledExecutorService
      Type Parameters:
      V - the type of the callable's result
      Parameters:
      callable - the function to execute
      delay - the time from now to delay execution
      unit - the time unit of the delay parameter
      Returns:
      a ScheduledFuture that can be used to extract result or cancel
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if callable or unit is null
    • scheduleAtFixedRate

      public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit)
      Submits a periodic action that becomes enabled first after the given initial delay, and subsequently with the given period; that is, executions will commence after initialDelay, then initialDelay + period, then initialDelay + 2 * period, and so on.

      The sequence of task executions continues indefinitely until one of the following exceptional completions occur:

      Subsequent executions are suppressed. Subsequent calls to isDone() on the returned future will return true.

      If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.

      Specified by:
      scheduleAtFixedRate in interface ScheduledExecutorService
      Parameters:
      command - the task to execute
      initialDelay - the time to delay first execution
      period - the period between successive executions
      unit - the time unit of the initialDelay and period parameters
      Returns:
      a ScheduledFuture representing pending completion of the series of repeated tasks. The future's get() method will never return normally, and will throw an exception upon task cancellation or abnormal termination of a task execution.
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if command or unit is null
      IllegalArgumentException - if period less than or equal to zero
    • scheduleWithFixedDelay

      public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit)
      Submits a periodic action that becomes enabled first after the given initial delay, and subsequently with the given delay between the termination of one execution and the commencement of the next.

      The sequence of task executions continues indefinitely until one of the following exceptional completions occur:

      Subsequent executions are suppressed. Subsequent calls to isDone() on the returned future will return true.
      Specified by:
      scheduleWithFixedDelay in interface ScheduledExecutorService
      Parameters:
      command - the task to execute
      initialDelay - the time to delay first execution
      delay - the delay between the termination of one execution and the commencement of the next
      unit - the time unit of the initialDelay and delay parameters
      Returns:
      a ScheduledFuture representing pending completion of the series of repeated tasks. The future's get() method will never return normally, and will throw an exception upon task cancellation or abnormal termination of a task execution.
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if command or unit is null
      IllegalArgumentException - if delay less than or equal to zero
    • execute

      public void execute(Runnable command)
      Executes 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 shutdownNow will access the zero-delayed ScheduledFuture, not the command itself.

      A consequence of the use of ScheduledFuture objects is that afterExecute is always called with a null second Throwable argument, even if the command terminated abruptly. Instead, the Throwable thrown by such a task can be obtained via Future.get().

      Specified by:
      execute in interface Executor
      Overrides:
      execute in class ThreadPoolExecutor
      Parameters:
      command - the task to execute
      Throws:
      RejectedExecutionException - at discretion of RejectedExecutionHandler, if the task cannot be accepted for execution because the executor has been shut down
      NullPointerException - if command is null
    • submit

      public Future<?> submit(Runnable task)
      Description copied from interface: ExecutorService
      Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Parameters:
      task - the task to submit
      Returns:
      a Future representing pending completion of the task
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if the task is null
    • submit

      public <T> Future<T> submit(Runnable task, T result)
      Description copied from interface: ExecutorService
      Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return the given result upon successful completion.
      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Type Parameters:
      T - the type of the result
      Parameters:
      task - the task to submit
      result - the result to return
      Returns:
      a Future representing pending completion of the task
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if the task is null
    • submit

      public <T> Future<T> submit(Callable<T> task)
      Description copied from interface: ExecutorService
      Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future's get method will return the task's result upon successful completion.

      If you would like to immediately block waiting for a task, you can use constructions of the form result = exec.submit(aCallable).get();

      Note: The Executors class includes a set of methods that can convert some other common closure-like objects, for example, PrivilegedAction to Callable form so they can be submitted.

      Specified by:
      submit in interface ExecutorService
      Overrides:
      submit in class AbstractExecutorService
      Type Parameters:
      T - the type of the task's result
      Parameters:
      task - the task to submit
      Returns:
      a Future representing pending completion of the task
      Throws:
      RejectedExecutionException - if the task cannot be scheduled for execution
      NullPointerException - if the task is null
    • setContinueExistingPeriodicTasksAfterShutdownPolicy

      public void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value)
      Sets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown. In this case, executions will continue until shutdownNow or the policy is set to false when already shutdown. This value is by default false.
      Parameters:
      value - if true, continue after shutdown, else don't
      See Also:
      getContinueExistingPeriodicTasksAfterShutdownPolicy()
    • getContinueExistingPeriodicTasksAfterShutdownPolicy

      public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy()
      Gets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown. In this case, executions will continue until shutdownNow or the policy is set to false when already shutdown. This value is by default false.
      Returns:
      true if will continue after shutdown
      See Also:
      setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean)
    • setExecuteExistingDelayedTasksAfterShutdownPolicy

      public void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value)
      Sets the policy on whether to execute existing delayed tasks even when this executor has been shutdown. In this case, these tasks will only terminate upon shutdownNow, or after setting the policy to false when already shutdown. This value is by default true.
      Parameters:
      value - if true, execute after shutdown, else don't
      See Also:
      getExecuteExistingDelayedTasksAfterShutdownPolicy()
    • getExecuteExistingDelayedTasksAfterShutdownPolicy

      public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy()
      Gets the policy on whether to execute existing delayed tasks even when this executor has been shutdown. In this case, these tasks will only terminate upon shutdownNow, or after setting the policy to false when already shutdown. This value is by default true.
      Returns:
      true if will execute after shutdown
      See Also:
      setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean)
    • setRemoveOnCancelPolicy

      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 false.
      Parameters:
      value - if true, remove on cancellation, else don't
      Since:
      1.7
      See Also:
      getRemoveOnCancelPolicy()
    • getRemoveOnCancelPolicy

      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 false.
      Returns:
      true if cancelled tasks are immediately removed from the queue
      Since:
      1.7
      See Also:
      setRemoveOnCancelPolicy(boolean)
    • shutdown

      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 ExecuteExistingDelayedTasksAfterShutdownPolicy has been set false, existing delayed tasks whose delays have not yet elapsed are cancelled. And unless the ContinueExistingPeriodicTasksAfterShutdownPolicy has been set true, future executions of existing periodic tasks will be cancelled.

      Specified by:
      shutdown in interface ExecutorService
      Overrides:
      shutdown in class ThreadPoolExecutor
      Throws:
      SecurityException - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.
    • shutdownNow

      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. These tasks are drained (removed) from the task queue upon return from this method.

      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 interrupts tasks via Thread.interrupt(); any task that fails to respond to interrupts may never terminate.

      Specified by:
      shutdownNow in interface ExecutorService
      Overrides:
      shutdownNow in class ThreadPoolExecutor
      Returns:
      list of tasks that never commenced execution. Each element of this list is a ScheduledFuture. For tasks submitted via one of the schedule methods, the element will be identical to the returned ScheduledFuture. For tasks submitted using execute, the element will be a zero-delay ScheduledFuture.
      Throws:
      SecurityException - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.
    • getQueue

      public BlockingQueue<Runnable> getQueue()
      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.

      Each element of this queue is a ScheduledFuture. For tasks submitted via one of the schedule methods, the element will be identical to the returned ScheduledFuture. For tasks submitted using execute, the element will be a zero-delay ScheduledFuture.

      Iteration over this queue is not guaranteed to traverse tasks in the order in which they will execute.

      Overrides:
      getQueue in class ThreadPoolExecutor
      Returns:
      the task queue