public class ScheduledThreadPoolExecutor extends ThreadPoolExecutor implements ScheduledExecutorService
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.
}ThreadPoolExecutor.AbortPolicy, ThreadPoolExecutor.CallerRunsPolicy,