Class ScheduledThreadPoolExecutor
- All Implemented Interfaces:
AutoCloseable,Executor,ExecutorService,ScheduledExecutorService
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
-
Nested Class Summary
Nested classes/interfaces declared in class java.util.concurrent.ThreadPoolExecutor
ThreadPoolExecutor.AbortPolicy, ThreadPoolExecutor.CallerRunsPolicy, ThreadPoolExecutor.DiscardOldestPolicy, ThreadPoolExecutor.DiscardPolicy -
Constructor Summary
ConstructorsConstructorDescriptionScheduledThreadPoolExecutor(int corePoolSize) Creates a newScheduledThreadPoolExecutorwith the given core pool size.ScheduledThreadPoolExecutor(int corePoolSize, RejectedExecutionHandler handler) Creates a newScheduledThreadPoolExecutorwith the given initial parameters.ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory) Creates a newScheduledThreadPoolExecutorwith the given initial parameters.ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory, RejectedExecutionHandler handler) Creates a newScheduledThreadPoolExecutorwith the given initial parameters. -
Method Summary
Modifier and TypeMethodDescriptionprotected <V> RunnableScheduledFuture<V> decorateTask(Runnable runnable, RunnableScheduledFuture<V> task) Modifies or replaces the task used to execute a runnable.protected <V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task) Modifies or replaces the task used to execute a callable.voidExecutescommandwith zero required delay.booleanGets the policy on whether to continue executing existing periodic tasks even when this executor has beenshutdown.booleanGets the policy on whether to execute existing delayed tasks even when this executor has beenshutdown.getQueue()Returns the task queue used by this executor.booleanGets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation.Submits a one-shot task that becomes enabled after the given delay.<V> ScheduledFuture<V> Submits a value-returning one-shot task that becomes enabled after the given delay.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 afterinitialDelay, theninitialDelay + period, theninitialDelay + 2 * period, and so on.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.voidsetContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value) Sets the policy on whether to continue executing existing periodic tasks even when this executor has beenshutdown.voidsetExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value) Sets the policy on whether to execute existing delayed tasks even when this executor has beenshutdown.voidsetRemoveOnCancelPolicy(boolean value) Sets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation.voidshutdown()Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.Future<?> Submits a Runnable task for execution and returns a Future representing that task.<T> Future<T> Submits a Runnable task for execution and returns a Future representing that task.<T> Future<T> Submits a value-returning task for execution and returns a Future representing the pending results of the task.Methods declared in class java.util.concurrent.ThreadPoolExecutor
afterExecute, allowCoreThreadTimeOut, allowsCoreThreadTimeOut, awaitTermination, beforeExecute, finalize, getActiveCount, getCompletedTaskCount, getCorePoolSize, getKeepAliveTime, getLargestPoolSize, getMaximumPoolSize, getPoolSize, getRejectedExecutionHandler, getTaskCount, getThreadFactory, isShutdown, isTerminated, isTerminating, prestartAllCoreThreads, prestartCoreThread, purge, remove, setCorePoolSize, setKeepAliveTime, setMaximumPoolSize, setRejectedExecutionHandler, setThreadFactory, terminated, toStringMethods declared in class java.util.concurrent.AbstractExecutorService
invokeAll, invokeAll, invokeAny, invokeAny, newTaskFor, newTaskForMethods declared in class java.lang.Object
clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, waitMethods declared in interface java.util.concurrent.ExecutorService
awaitTermination, close, invokeAll, invokeAll, invokeAny, invokeAny, isShutdown, isTerminated
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Constructor Details
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ScheduledThreadPoolExecutor
public ScheduledThreadPoolExecutor(int corePoolSize) Creates a newScheduledThreadPoolExecutorwith the given core pool size.- Parameters:
corePoolSize- the number of threads to keep in the pool, even if they are idle, unlessallowCoreThreadTimeOutis set- Throws:
IllegalArgumentException- ifcorePoolSize < 0
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ScheduledThreadPoolExecutor
Creates a newScheduledThreadPoolExecutorwith the given initial parameters.- Parameters:
corePoolSize- the number of threads to keep in the pool, even if they are idle, unlessallowCoreThreadTimeOutis setthreadFactory- the factory to use when the executor creates a new thread- Throws:
IllegalArgumentException- ifcorePoolSize < 0NullPointerException- ifthreadFactoryis null
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ScheduledThreadPoolExecutor
Creates a newScheduledThreadPoolExecutorwith the given initial parameters.- Parameters:
corePoolSize- the number of threads to keep in the pool, even if they are idle, unlessallowCoreThreadTimeOutis sethandler- the handler to use when execution is blocked because the thread bounds and queue capacities are reached- Throws:
IllegalArgumentException- ifcorePoolSize < 0NullPointerException- ifhandleris null
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ScheduledThreadPoolExecutor
public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory, RejectedExecutionHandler handler) Creates a newScheduledThreadPoolExecutorwith the given initial parameters.- Parameters:
corePoolSize- the number of threads to keep in the pool, even if they are idle, unlessallowCoreThreadTimeOutis setthreadFactory- the factory to use when the executor creates a new threadhandler- the handler to use when execution is blocked because the thread bounds and queue capacities are reached- Throws:
IllegalArgumentException- ifcorePoolSize < 0NullPointerException- ifthreadFactoryorhandleris null
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Method Details
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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 Runnabletask- 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 Callabletask- the task created to execute the callable- Returns:
- a task that can execute the callable
- Since:
- 1.6
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schedule
Description copied from interface:ScheduledExecutorServiceSubmits a one-shot task that becomes enabled after the given delay.- Specified by:
schedulein interfaceScheduledExecutorService- Parameters:
command- the task to executedelay- the time from now to delay executionunit- the time unit of the delay parameter- Returns:
- a ScheduledFuture representing pending completion of
the task and whose
get()method will returnnullupon completion - Throws:
RejectedExecutionException- if the task cannot be scheduled for executionNullPointerException- if command or unit is null
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schedule
Description copied from interface:ScheduledExecutorServiceSubmits a value-returning one-shot task that becomes enabled after the given delay.- Specified by:
schedulein interfaceScheduledExecutorService- Type Parameters:
V- the type of the callable's result- Parameters:
callable- the function to executedelay- the time from now to delay executionunit- 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 executionNullPointerException- if callable or unit is null
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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 afterinitialDelay, theninitialDelay + period, theninitialDelay + 2 * period, and so on.The sequence of task executions continues indefinitely until one of the following exceptional completions occur:
- The task is explicitly cancelled via the returned future.
- Method
shutdown()is called and the policy on whether to continue after shutdown is not set true, or methodshutdownNow()is called; also resulting in task cancellation. - An execution of the task throws an exception. In this case
calling
geton the returned future will throwExecutionException, holding the exception as its cause.
isDone()on the returned future will returntrue.If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.
- Specified by:
scheduleAtFixedRatein interfaceScheduledExecutorService- Parameters:
command- the task to executeinitialDelay- the time to delay first executionperiod- the period between successive executionsunit- 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 executionNullPointerException- if command or unit is nullIllegalArgumentException- if period less than or equal to zero
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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:
- The task is explicitly cancelled via the returned future.
- Method
shutdown()is called and the policy on whether to continue after shutdown is not set true, or methodshutdownNow()is called; also resulting in task cancellation. - An execution of the task throws an exception. In this case
calling
geton the returned future will throwExecutionException, holding the exception as its cause.
isDone()on the returned future will returntrue.- Specified by:
scheduleWithFixedDelayin interfaceScheduledExecutorService- Parameters:
command- the task to executeinitialDelay- the time to delay first executiondelay- the delay between the termination of one execution and the commencement of the nextunit- 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 executionNullPointerException- if command or unit is nullIllegalArgumentException- if delay less than or equal to zero
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execute
Executescommandwith zero required delay. This has effect equivalent toschedule(command, 0, anyUnit). Note that inspections of the queue and of the list returned byshutdownNowwill access the zero-delayedScheduledFuture, not thecommanditself.A consequence of the use of
ScheduledFutureobjects is thatafterExecuteis always called with a null secondThrowableargument, even if thecommandterminated abruptly. Instead, theThrowablethrown by such a task can be obtained viaFuture.get().- Specified by:
executein interfaceExecutor- Overrides:
executein classThreadPoolExecutor- Parameters:
command- the task to execute- Throws:
RejectedExecutionException- at discretion ofRejectedExecutionHandler, if the task cannot be accepted for execution because the executor has been shut downNullPointerException- ifcommandis null
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submit
Description copied from interface:ExecutorServiceSubmits a Runnable task for execution and returns a Future representing that task. The Future'sgetmethod will returnnullupon successful completion.- Specified by:
submitin interfaceExecutorService- Overrides:
submitin classAbstractExecutorService- Parameters:
task- the task to submit- Returns:
- a Future representing pending completion of the task
- Throws:
RejectedExecutionException- if the task cannot be scheduled for executionNullPointerException- if the task is null
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submit
Description copied from interface:ExecutorServiceSubmits a Runnable task for execution and returns a Future representing that task. The Future'sgetmethod will return the given result upon successful completion.- Specified by:
submitin interfaceExecutorService- Overrides:
submitin classAbstractExecutorService- Type Parameters:
T- the type of the result- Parameters:
task- the task to submitresult- the result to return- Returns:
- a Future representing pending completion of the task
- Throws:
RejectedExecutionException- if the task cannot be scheduled for executionNullPointerException- if the task is null
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submit
Description copied from interface:ExecutorServiceSubmits a value-returning task for execution and returns a Future representing the pending results of the task. The Future'sgetmethod 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
Executorsclass includes a set of methods that can convert some other common closure-like objects, for example,PrivilegedActiontoCallableform so they can be submitted.- Specified by:
submitin interfaceExecutorService- Overrides:
submitin classAbstractExecutorService- 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 executionNullPointerException- if the task is null
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setContinueExistingPeriodicTasksAfterShutdownPolicy
public void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value) Sets the policy on whether to continue executing existing periodic tasks even when this executor has beenshutdown. In this case, executions will continue untilshutdownNowor the policy is set tofalsewhen already shutdown. This value is by defaultfalse.- Parameters:
value- iftrue, continue after shutdown, else don't- See Also:
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getContinueExistingPeriodicTasksAfterShutdownPolicy
public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy()Gets the policy on whether to continue executing existing periodic tasks even when this executor has beenshutdown. In this case, executions will continue untilshutdownNowor the policy is set tofalsewhen already shutdown. This value is by defaultfalse.- Returns:
trueif will continue after shutdown- See Also:
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setExecuteExistingDelayedTasksAfterShutdownPolicy
public void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value) Sets the policy on whether to execute existing delayed tasks even when this executor has beenshutdown. In this case, these tasks will only terminate uponshutdownNow, or after setting the policy tofalsewhen already shutdown. This value is by defaulttrue.- Parameters:
value- iftrue, execute after shutdown, else don't- See Also:
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getExecuteExistingDelayedTasksAfterShutdownPolicy
public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy()Gets the policy on whether to execute existing delayed tasks even when this executor has beenshutdown. In this case, these tasks will only terminate uponshutdownNow, or after setting the policy tofalsewhen already shutdown. This value is by defaulttrue.- Returns:
trueif will execute after shutdown- See Also:
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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 defaultfalse.- Parameters:
value- iftrue, remove on cancellation, else don't- Since:
- 1.7
- See Also:
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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 defaultfalse.- Returns:
trueif cancelled tasks are immediately removed from the queue- Since:
- 1.7
- See Also:
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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
awaitTerminationto do that.If the
ExecuteExistingDelayedTasksAfterShutdownPolicyhas been setfalse, existing delayed tasks whose delays have not yet elapsed are cancelled. And unless theContinueExistingPeriodicTasksAfterShutdownPolicyhas been settrue, future executions of existing periodic tasks will be cancelled.- Specified by:
shutdownin interfaceExecutorService- Overrides:
shutdownin classThreadPoolExecutor
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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
awaitTerminationto 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:
shutdownNowin interfaceExecutorService- Overrides:
shutdownNowin classThreadPoolExecutor- Returns:
- list of tasks that never commenced execution.
Each element of this list is a
ScheduledFuture. For tasks submitted via one of theschedulemethods, the element will be identical to the returnedScheduledFuture. For tasks submitted usingexecute, the element will be a zero-delayScheduledFuture.
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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 theschedulemethods, the element will be identical to the returnedScheduledFuture. For tasks submitted usingexecute, the element will be a zero-delayScheduledFuture.Iteration over this queue is not guaranteed to traverse tasks in the order in which they will execute.
- Overrides:
getQueuein classThreadPoolExecutor- Returns:
- the task queue
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