Класс ScheduledThreadPoolExecutor

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.

Примечания к расширению: Этот класс переопределяет методы execute и submit, чтобы генерировать внутренние объекты ScheduledFuture для управления задержками и планированием задач. Для сохранения функциональности, любые дальнейшие переопределения этих методов в подклассах должны вызывать версии суперкласса, что фактически отключает дополнительную настройку задач. Однако, этот класс предоставляет альтернативные защищенные методы расширения decorateTask (по одной версии для Runnable и Callable), которые можно использовать для настройки конкретных типов задач, используемых для выполнения команд, введённых через execute, submit, schedule, scheduleAtFixedRate, и scheduleWithFixedDelay. По умолчанию, ScheduledThreadPoolExecutor использует тип задачи, расширяющий FutureTask. Однако, это можно изменить или заменить, используя подклассы в форме:

 
 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.
 }