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, set setRemoveOnCancelPolicy(boolean) to true, which causes tasks to be immediately removed from the work queue at time of cancellation.

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

Nested Classes

Nested classes/interfaces inherited from class java.util.concurrent.ThreadPoolExecutor

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

Constructors

Constructor and 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 and Description
protected <V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task)

Modifies or replaces the task used to execute a callable.
protected <V> RunnableScheduledFuture<V> decorateTask(Runnable runnable, RunnableScheduledFuture<V> task)

Modifies or replaces the task used to execute a runnable.
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.
<V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit)

Creates and executes a ScheduledFuture that becomes enabled after the given delay.
ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)

Creates and executes a one-shot action that becomes enabled after the given delay.
ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit)

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(Runnable command, long initialDelay, long delay, TimeUnit unit)

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.
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.
<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.
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.

Methods inherited from 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, toString

Methods inherited from class java.util.concurrent.AbstractExecutorService

invokeAll, invokeAll, invokeAny, invokeAny, newTaskFor, newTaskFor

Methods inherited from class java.lang.Object

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

Methods inherited from 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

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

schedule

public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                       long delay,
                                       TimeUnit unit)

Description copied from interface: ScheduledExecutorService

Creates and executes a ScheduledFuture 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 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 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, 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
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.

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.
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. 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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https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledThreadPoolExecutor.html