AutoCloseable
, Executor
, ExecutorService
, ScheduledExecutorService
public class ScheduledThreadPoolExecutor extends ThreadPoolExecutor implements 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.
}
ThreadPoolExecutor.AbortPolicy, ThreadPoolExecutor.CallerRunsPolicy, ThreadPoolExecutor.DiscardOldestPolicy, ThreadPoolExecutor.DiscardPolicy
Constructor | Description |
---|---|
ScheduledThreadPoolExecutor |
Creates a new ScheduledThreadPoolExecutor with the given core pool size. |
ScheduledThreadPoolExecutor |
Creates a new ScheduledThreadPoolExecutor with the given initial parameters. |
ScheduledThreadPoolExecutor |
Creates a new ScheduledThreadPoolExecutor with the given initial parameters. |
ScheduledThreadPoolExecutor |
Creates a new ScheduledThreadPoolExecutor with the given initial parameters. |
Modifier and Type | Method | Description |
---|---|---|
protected <V> RunnableScheduledFuture |
decorateTask |
Modifies or replaces the task used to execute a runnable. |
protected <V> RunnableScheduledFuture |
decorateTask |
Modifies or replaces the task used to execute a callable. |
void |
execute |
Executes command with zero required delay. |
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 . |
BlockingQueue |
getQueue() |
Returns the task queue used by this executor. |
boolean |
getRemoveOnCancelPolicy() |
Gets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation. |
ScheduledFuture |
schedule |
Submits a one-shot task that becomes enabled after the given delay. |
<V> ScheduledFuture |
schedule |
Submits a value-returning one-shot task that becomes enabled after the given delay. |
ScheduledFuture |
scheduleAtFixedRate |
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. |
ScheduledFuture |
scheduleWithFixedDelay |
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. |
void |
setContinueExistingPeriodicTasksAfterShutdownPolicy |
Sets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown . |
void |
setExecuteExistingDelayedTasksAfterShutdownPolicy |
Sets the policy on whether to execute existing delayed tasks even when this executor has been shutdown . |
void |
setRemoveOnCancelPolicy |
Sets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation. |
void |
shutdown() |
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. |
List |
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. |
Future |
submit |
Submits a Runnable task for execution and returns a Future representing that task. |
<T> Future |
submit |
Submits a Runnable task for execution and returns a Future representing that task. |
<T> Future |
submit |
Submits a value-returning task for execution and returns a Future representing the pending results of the task. |
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, toString
invokeAll, invokeAll, invokeAny, invokeAny, newTaskFor, newTaskFor
clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait
awaitTermination, close, invokeAll, invokeAll, invokeAny, invokeAny, isShutdown, isTerminated
public ScheduledThreadPoolExecutor(int corePoolSize)
ScheduledThreadPoolExecutor
with the given core pool size.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is setIllegalArgumentException
- if corePoolSize < 0
public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory)
ScheduledThreadPoolExecutor
with the given initial parameters.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is setthreadFactory
- the factory to use when the executor creates a new threadIllegalArgumentException
- if corePoolSize < 0
NullPointerException
- if threadFactory
is nullpublic ScheduledThreadPoolExecutor(int corePoolSize, RejectedExecutionHandler handler)
ScheduledThreadPoolExecutor
with the given initial parameters.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is sethandler
- the handler to use when execution is blocked because the thread bounds and queue capacities are reachedIllegalArgumentException
- if corePoolSize < 0
NullPointerException
- if handler
is nullpublic ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory, RejectedExecutionHandler handler)
ScheduledThreadPoolExecutor
with the given initial parameters.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is 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 reachedIllegalArgumentException
- if corePoolSize < 0
NullPointerException
- if threadFactory
or handler
is nullprotected <V> RunnableScheduledFuture<V> decorateTask(Runnable runnable, RunnableScheduledFuture<V> task)
V
- the type of the task's resultrunnable
- the submitted Runnabletask
- the task created to execute the runnableprotected <V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task)
V
- the type of the task's resultcallable
- the submitted Callabletask
- the task created to execute the callablepublic ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
ScheduledExecutorService
schedule
in interface ScheduledExecutorService
command
- the task to executedelay
- the time from now to delay executionunit
- the time unit of the delay parameterget()
method will return null
upon completionRejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if command or unit is nullpublic <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit)
ScheduledExecutorService
schedule
in interface ScheduledExecutorService
V
- the type of the callable's resultcallable
- the function to executedelay
- the time from now to delay executionunit
- the time unit of the delay parameterRejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if callable or unit is nullpublic ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit)
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:
shutdown()
is called and the policy on whether to continue after shutdown is not set true, or method shutdownNow()
is called; also resulting in task cancellation. get
on the returned future will throw ExecutionException
, holding the exception as its cause. 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.
scheduleAtFixedRate
in interface ScheduledExecutorService
command
- the task to executeinitialDelay
- the time to delay first executionperiod
- the period between successive executionsunit
- the time unit of the initialDelay and period parametersget()
method will never return normally, and will throw an exception upon task cancellation or abnormal termination of a task execution.RejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if command or unit is nullIllegalArgumentException
- if period less than or equal to zeropublic ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit)
The sequence of task executions continues indefinitely until one of the following exceptional completions occur:
shutdown()
is called and the policy on whether to continue after shutdown is not set true, or method shutdownNow()
is called; also resulting in task cancellation. get
on the returned future will throw ExecutionException
, holding the exception as its cause. isDone()
on the returned future will return true
.scheduleWithFixedDelay
in interface ScheduledExecutorService
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 parametersget()
method will never return normally, and will throw an exception upon task cancellation or abnormal termination of a task execution.RejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if command or unit is nullIllegalArgumentException
- if delay less than or equal to zeropublic void execute(Runnable command)
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()
.
execute
in interface Executor
execute
in class ThreadPoolExecutor
command
- the task to executeRejectedExecutionException
- at discretion of RejectedExecutionHandler
, if the task cannot be accepted for execution because the executor has been shut downNullPointerException
- if command
is nullpublic Future<?> submit(Runnable task)
ExecutorService
get
method will return null
upon successful completion.submit
in interface ExecutorService
submit
in class AbstractExecutorService
task
- the task to submitRejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is nullpublic <T> Future<T> submit(Runnable task, T result)
ExecutorService
get
method will return the given result upon successful completion.submit
in interface ExecutorService
submit
in class AbstractExecutorService
T
- the type of the resulttask
- the task to submitresult
- the result to returnRejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is nullpublic <T> Future<T> submit(Callable<T> task)
ExecutorService
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.
submit
in interface ExecutorService
submit
in class AbstractExecutorService
T
- the type of the task's resulttask
- the task to submitRejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is nullpublic void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value)
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
.value
- if true
, continue after shutdown, else don'tpublic boolean getContinueExistingPeriodicTasksAfterShutdownPolicy()
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
.true
if will continue after shutdownpublic void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value)
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
.value
- if true
, execute after shutdown, else don'tpublic boolean getExecuteExistingDelayedTasksAfterShutdownPolicy()
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
.true
if will execute after shutdownpublic void setRemoveOnCancelPolicy(boolean value)
false
.value
- if true
, remove on cancellation, else don'tpublic boolean getRemoveOnCancelPolicy()
false
.true
if cancelled tasks are immediately removed from the queuepublic void shutdown()
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.
shutdown
in interface ExecutorService
shutdown
in class ThreadPoolExecutor
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.public List<Runnable> shutdownNow()
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.
shutdownNow
in interface ExecutorService
shutdownNow
in class ThreadPoolExecutor
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
.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.public BlockingQueue<Runnable> getQueue()
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.
getQueue
in class ThreadPoolExecutor
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Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/ScheduledThreadPoolExecutor.html