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. 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 Classes

Nested classes/interfaces declared in class java.util.concurrent.ThreadPoolExecutor

ThreadPoolExecutor.AbortPolicy, ThreadPoolExecutor.CallerRunsPolicy, ThreadPoolExecutor.DiscardOldestPolicy, ThreadPoolExecutor.DiscardPolicy

Constructors

Constructor Description
ScheduledThreadPoolExecutor​(int corePoolSize)

Creates a new ScheduledThreadPoolExecutor with the given core pool size.
ScheduledThreadPoolExecutor​(int corePoolSize, RejectedExecutionHandler handler)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
ScheduledThreadPoolExecutor​(int corePoolSize, ThreadFactory threadFactory)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
ScheduledThreadPoolExecutor​(int corePoolSize, ThreadFactory threadFactory, RejectedExecutionHandler handler)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Methods

Modifier and Type Method Description
protected <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.
void execute​(Runnable command)

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<Runnable> 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​(Runnable command, long delay, TimeUnit unit)

Submits a one-shot task that becomes enabled after the given delay.
<V> ScheduledFuture<V> schedule​(Callable<V> callable, long delay, TimeUnit unit)

Submits a value-returning one-shot task that becomes enabled after the given delay.
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.
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.
void setContinueExistingPeriodicTasksAfterShutdownPolicy​(boolean value)

Sets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown.
void setExecuteExistingDelayedTasksAfterShutdownPolicy​(boolean value)

Sets the policy on whether to execute existing delayed tasks even when this executor has been shutdown.
void setRemoveOnCancelPolicy​(boolean value)

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<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.
Future<?> submit​(Runnable task)

Submits a Runnable task for execution and returns a Future representing that task.
<T> Future<T> submit​(Runnable task, T result)

Submits a Runnable task for execution and returns a Future representing that task.
<T> Future<T> submit​(Callable<T> task)

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, beforeExecute, finalize, getActiveCount, getCompletedTaskCount, getCorePoolSize, getKeepAliveTime, getLargestPoolSize, getMaximumPoolSize, getPoolSize, getRejectedExecutionHandler, getTaskCount, getThreadFactory, isTerminating, prestartAllCoreThreads, prestartCoreThread, purge, remove, setCorePoolSize, setKeepAliveTime, setMaximumPoolSize, setRejectedExecutionHandler, setThreadFactory, terminated, toString

Methods declared in class java.util.concurrent.AbstractExecutorService

newTaskFor, newTaskFor

Methods declared in class java.lang.Object

clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods declared in interface java.util.concurrent.ExecutorService

awaitTermination, invokeAll, invokeAll, invokeAny, invokeAny, isShutdown, isTerminated

Constructors

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

Methods

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

© 1993, 2020, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ScheduledThreadPoolExecutor.html