ScheduledThreadPoolExecutor

public class ScheduledThreadPoolExecutor
extends ThreadPoolExecutor implements ScheduledExecutorService

java.lang.Object
   ↳ java.util.concurrent.AbstractExecutorService
     ↳ java.util.concurrent.ThreadPoolExecutor
       ↳ java.util.concurrent.ScheduledThreadPoolExecutor


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.

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.
 }

Summary

Public constructors

ScheduledThreadPoolExecutor(int corePoolSize)

Creates a new ScheduledThreadPoolExecutor with the given core pool size.

ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

ScheduledThreadPoolExecutor(int corePoolSize, RejectedExecutionHandler handler)

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.

Public methods

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)

Submits a value-returning one-shot task that becomes enabled after the given delay.

ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)

Submits a 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(Callable<T> task)

Submits a value-returning task for execution and returns a Future representing the pending results of the task.

<T> Future<T> submit(Runnable task, T result)

Submits a Runnable task for execution and returns a Future representing that task.

Protected methods

<V> RunnableScheduledFuture<V> decorateTask(Runnable runnable, RunnableScheduledFuture<V> task)

Modifies or replaces the task used to execute a runnable.

<V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task)

Modifies or replaces the task used to execute a callable.

Inherited methods

Public constructors

ScheduledThreadPoolExecutor

Added in API level 1
public ScheduledThreadPoolExecutor (int corePoolSize)

Creates a new ScheduledThreadPoolExecutor with the given core pool size.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set

Throws
IllegalArgumentException if corePoolSize < 0

ScheduledThreadPoolExecutor

Added in API level 1
public ScheduledThreadPoolExecutor (int corePoolSize, 
                ThreadFactory threadFactory)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set

threadFactory ThreadFactory: the factory to use when the executor creates a new thread

Throws
IllegalArgumentException if corePoolSize < 0
NullPointerException if threadFactory is null

ScheduledThreadPoolExecutor

Added in API level 1
public ScheduledThreadPoolExecutor (int corePoolSize, 
                RejectedExecutionHandler handler)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set

handler RejectedExecutionHandler: 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

Added in API level 1
public ScheduledThreadPoolExecutor (int corePoolSize, 
                ThreadFactory threadFactory, 
                RejectedExecutionHandler handler)

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set

threadFactory ThreadFactory: the factory to use when the executor creates a new thread

handler RejectedExecutionHandler: 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

Public methods

execute

Added in API level 1
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.

Parameters
command Runnable: 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

getContinueExistingPeriodicTasksAfterShutdownPolicy

Added in API level 1
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
boolean true if will continue after shutdown

getExecuteExistingDelayedTasksAfterShutdownPolicy

Added in API level 1
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
boolean true if will execute after shutdown

getQueue

Added in API level 1
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.

Returns
BlockingQueue<Runnable> the task queue

getRemoveOnCancelPolicy

Added in API level 21
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
boolean true if cancelled tasks are immediately removed from the queue

schedule

Added in API level 1
public ScheduledFuture<V> schedule (Callable<V> callable, 
                long delay, 
                TimeUnit unit)

Submits a value-returning one-shot task that becomes enabled after the given delay.

Parameters
callable Callable: the function to execute

delay long: the time from now to delay execution

unit TimeUnit: the time unit of the delay parameter

Returns
ScheduledFuture<V> a ScheduledFuture that can be used to extract result or cancel

Throws
RejectedExecutionException
NullPointerException

schedule

Added in API level 1
public ScheduledFuture<?> schedule (Runnable command, 
                long delay, 
                TimeUnit unit)

Submits a one-shot task that becomes enabled after the given delay.

Parameters
command Runnable: the task to execute

delay long: the time from now to delay execution

unit TimeUnit: the time unit of the delay parameter

Returns
ScheduledFuture<?> a ScheduledFuture representing pending completion of the task and whose get() method will return null upon completion

Throws
RejectedExecutionException
NullPointerException

scheduleAtFixedRate

Added in API level 1
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.

Parameters
command Runnable: the task to execute

initialDelay long: the time to delay first execution

period long: the period between successive executions

unit TimeUnit: the time unit of the initialDelay and period parameters

Returns
ScheduledFuture<?> 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
NullPointerException
IllegalArgumentException

scheduleWithFixedDelay

Added in API level 1
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.

Parameters
command Runnable: the task to execute

initialDelay long: the time to delay first execution

delay long: the delay between the termination of one execution and the commencement of the next

unit TimeUnit: the time unit of the initialDelay and delay parameters

Returns
ScheduledFuture<?> 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
NullPointerException
IllegalArgumentException

setContinueExistingPeriodicTasksAfterShutdownPolicy

Added in API level 1
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 boolean: if true, continue after shutdown, else don't

setExecuteExistingDelayedTasksAfterShutdownPolicy

Added in API level 1
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 boolean: if true, execute after shutdown, else don't

setRemoveOnCancelPolicy

Added in API level 21
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 boolean: if true, remove on cancellation, else don't

shutdown

Added in API level 1
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.

shutdownNow

Added in API level 1
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.

Returns
List<Runnable> 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.

submit

Added in API level 1
public Future<?> submit (Runnable task)

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.

Parameters
task Runnable: the task to submit

Returns
Future<?> a Future representing pending completion of the task

Throws
RejectedExecutionException
NullPointerException

submit

Added in API level 1
public Future<T> submit (Callable<T> task)

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.

Parameters
task Callable: the task to submit

Returns
Future<T> a Future representing pending completion of the task

Throws
RejectedExecutionException
NullPointerException

submit

Added in API level 1
public Future<T> submit (Runnable task, 
                T result)

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.

Parameters
task Runnable: the task to submit

result T: the result to return

Returns
Future<T> a Future representing pending completion of the task

Throws
RejectedExecutionException
NullPointerException

Protected methods

decorateTask

Added in API level 9
protected 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.

Parameters
runnable Runnable: the submitted Runnable

task RunnableScheduledFuture: the task created to execute the runnable

Returns
RunnableScheduledFuture<V> a task that can execute the runnable

decorateTask

Added in API level 9
protected 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.

Parameters
callable Callable: the submitted Callable

task RunnableScheduledFuture: the task created to execute the callable

Returns
RunnableScheduledFuture<V> a task that can execute the callable