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ScheduledThreadPoolExecutor


java.util.concurrent

Class ScheduledThreadPoolExecutor

  • All Implemented Interfaces:
    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. 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.

    Successive executions of a 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.

    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 Detail

      • 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 Detail

      • 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.
        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.
        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
        Creates and executes a one-shot action 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 is null
      • scheduleAtFixedRate

        public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                             long initialDelay,
                                             long period,
                                             TimeUnit unit)
        Description copied from interface: ScheduledExecutorService
        Creates and executes 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. If any execution of the task encounters an exception, subsequent executions are suppressed. Otherwise, the task will only terminate via cancellation or termination of the executor. 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 task, and whose get() method will throw an exception upon cancellation
        Throws:
        RejectedExecutionException - if the task cannot be scheduled for execution
        NullPointerException - if command is null
        IllegalArgumentException - if period less than or equal to zero
      • scheduleWithFixedDelay

        public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                long initialDelay,
                                                long delay,
                                                TimeUnit unit)
        Description copied from interface: ScheduledExecutorService
        Creates and executes 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. If any execution of the task encounters an exception, subsequent executions are suppressed. Otherwise, the task will only terminate via cancellation or termination of the executor.
        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 task, and whose get() method will throw an exception upon cancellation
        Throws:
        RejectedExecutionException - if the task cannot be scheduled for execution
        NullPointerException - if command 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 <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
        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
        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, these tasks will only terminate upon shutdownNow or after setting the policy 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, these tasks will only terminate upon shutdownNow or after setting the policy 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
      • 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.

        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.

        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, including those tasks submitted using execute, which are for scheduling purposes used as the basis of a zero-delay ScheduledFuture.
      • getQueue

        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 execute which are for scheduling purposes used as the basis of 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


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