ScheduledThreadPoolExecutor

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.

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.
 }

Public Constructor Summary

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 Method Summary

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.
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 Method Summary

<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 Method Summary