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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.
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:
ScheduledThreadPoolExecutor (intcorePoolSize)
Creates a new ScheduledThreadPoolExecutor with the
given core pool size.
boolean
getContinueExistingPeriodicTasksAfterShutdownPolicy ()
Gets the policy on whether to continue executing existing
periodic tasks even when this executor has been shutdown.
boolean
getExecuteExistingDelayedTasksAfterShutdownPolicy ()
Gets the policy on whether to execute existing delayed
tasks even when this executor has been shutdown.
boolean
getRemoveOnCancelPolicy ()
Gets the policy on whether cancelled tasks should be immediately
removed from the work queue at time of cancellation.
ScheduledFuture ?
scheduleAtFixedRate (Runnablecommand,
longinitialDelay,
longperiod,
TimeUnitunit)
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.
ScheduledFuture ?
scheduleWithFixedDelay (Runnablecommand,
longinitialDelay,
longdelay,
TimeUnitunit)
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.
setContinueExistingPeriodicTasksAfterShutdownPolicy (booleanvalue)
Sets the policy on whether to continue executing existing
periodic tasks even when this executor has been shutdown.
setExecuteExistingDelayedTasksAfterShutdownPolicy (booleanvalue)
Sets the policy on whether to execute existing delayed
tasks even when this executor has been shutdown.
setRemoveOnCancelPolicy (booleanvalue)
Sets the policy on whether cancelled tasks should be immediately
removed from the work queue at time of cancellation.
shutdown ()
Initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be accepted.
ScheduledThreadPoolExecutor
publicScheduledThreadPoolExecutor (intcorePoolSize)
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
publicScheduledThreadPoolExecutor (intcorePoolSize,
ThreadFactorythreadFactory)
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
publicScheduledThreadPoolExecutor (intcorePoolSize,
RejectedExecutionHandlerhandler)
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
publicScheduledThreadPoolExecutor (intcorePoolSize,
ThreadFactorythreadFactory,
RejectedExecutionHandlerhandler)
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
decorateTask
protected V RunnableScheduledFuture V decorateTask (Runnablerunnable,
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 tasks result
Parameters:
runnable - the submitted Runnable
task - the task created to execute the runnable
Returns:
a task that can execute the runnable
Since:
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 tasks result
Parameters:
callable - the submitted Callable
task - the task created to execute the callable
Returns:
a task that can execute the callable
Since:
schedule
publicScheduledFuture ? schedule (Runnablecommand,
longdelay,
TimeUnitunit)
Description copied from interface:ScheduledExecutorService
Creates and executes a one-shot action that becomes enabled
after the given delay.
Specified by:
schedulein interfaceScheduledExecutorService
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
schedule
public V ScheduledFuture V schedule (Callable V callable,
longdelay,
TimeUnitunit)
Description copied from interface:ScheduledExecutorService
Creates and executes a ScheduledFuture that becomes enabled after the
given delay.
Specified by:
schedulein interfaceScheduledExecutorService
Type Parameters:
V - the type of the callables 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 is null
scheduleAtFixedRate
publicScheduledFuture ? scheduleAtFixedRate (Runnablecommand,
longinitialDelay,
longperiod,
TimeUnitunit)
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.
The sequence of task executions continues indefinitely until
one of the following exceptional completions occur:
The task is explicitly cancelled
via the returned future.
The executor terminates, also resulting in task cancellation.
An execution of the task throws an exception. In this case
calling get on the returned future will
throw ExecutionException.
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:
scheduleAtFixedRatein interfaceScheduledExecutorService
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 futures 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 is null
IllegalArgumentException - if period less than or equal to zero
scheduleWithFixedDelay
publicScheduledFuture ? scheduleWithFixedDelay (Runnablecommand,
longinitialDelay,
longdelay,
TimeUnitunit)
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.
The sequence of task executions continues indefinitely until
one of the following exceptional completions occur:
The task is explicitly cancelled
via the returned future.
The executor terminates, also resulting in task cancellation.
An execution of the task throws an exception. In this case
calling get on the returned future will
throw ExecutionException.
Subsequent executions are suppressed. Subsequent calls to
isDone() on the returned future will
return true.
Specified by:
scheduleWithFixedDelayin interfaceScheduledExecutorService
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 futures 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 is null
IllegalArgumentException - if delay less than or equal to zero
execute
publicvoidexecute (Runnablecommand)
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:
executein interfaceExecutor
Overrides:
executein classThreadPoolExecutor
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
publicFuture ? submit (Runnabletask)
Description copied from interface:ExecutorService
Submits a Runnable task for execution and returns a Future
representing that task. The Futures get method will
return null upon 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 execution
NullPointerException - if the task is null
submit
public T Future T submit (Runnabletask,
Tresult)
Description copied from interface:ExecutorService
Submits a Runnable task for execution and returns a Future
representing that task. The Futures get method 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 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
Futures get method will return the tasks 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:
submitin interfaceExecutorService
Overrides:
submitin classAbstractExecutorService
Type Parameters:
T - the type of the tasks 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
publicvoidsetContinueExistingPeriodicTasksAfterShutdownPolicy (booleanvalue)
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 dont
See Also:
getContinueExistingPeriodicTasksAfterShutdownPolicy()
getContinueExistingPeriodicTasksAfterShutdownPolicy
publicbooleangetContinueExistingPeriodicTasksAfterShutdownPolicy ()
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
publicvoidsetExecuteExistingDelayedTasksAfterShutdownPolicy (booleanvalue)
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 dont
See Also:
getExecuteExistingDelayedTasksAfterShutdownPolicy()
getExecuteExistingDelayedTasksAfterShutdownPolicy
publicbooleangetExecuteExistingDelayedTasksAfterShutdownPolicy ()
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
publicvoidsetRemoveOnCancelPolicy (booleanvalue)
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 dont
Since:
See Also:
getRemoveOnCancelPolicy()
publicbooleangetRemoveOnCancelPolicy ()
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:
See Also:
setRemoveOnCancelPolicy(boolean)
publicvoidshutdown ()
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:
shutdownin interfaceExecutorService
Overrides:
shutdownin classThreadPoolExecutor
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 managers checkAccess method
denies access.
shutdownNow
publicList 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:
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 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 managers checkAccess method
denies access.
getQueue
publicBlockingQueue 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:
getQueuein classThreadPoolExecutor
Returns:
the task queue
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