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.
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.
Extension notes: This class overrides
AbstractExecutorService
submit methods to generate
internal 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.
}
RunnableScheduledFuture decorateTask
protected RunnableScheduledFuture decorateTask(Callable callable,
RunnableScheduledFuture 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.
callable
- the submitted Callabletask
- the task created to execute the callable
- a task that can execute the callable
RunnableScheduledFuture decorateTask
protected RunnableScheduledFuture decorateTask(Runnable runnable,
RunnableScheduledFuture 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.
runnable
- the submitted Runnabletask
- the task created to execute the runnable
- a task that can execute the runnable
ScheduledFuture schedule
public ScheduledFuture schedule(Callable callable,
long delay,
TimeUnit unit)
Creates and executes a ScheduledFuture that becomes enabled after the
given delay.
- ScheduledFuture schedule in interface ScheduledExecutorService
callable
- the function to executedelay
- the time from now to delay executionunit
- the time unit of the delay parameter
- a ScheduledFuture that can be used to extract result or cancel
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.
- execute in interface Executor
- execute in interface ThreadPoolExecutor
command
- the task to execute
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.
- true if will continue after shutdown
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.
- true if will execute after shutdown
getQueue
public BlockingQueue 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.
- getQueue in interface ThreadPoolExecutor
schedule
public ScheduledFuture> schedule(Runnable command,
long delay,
TimeUnit unit)
Creates and executes a one-shot action that becomes enabled
after the given delay.
- schedule in interface ScheduledExecutorService
command
- the task to executedelay
- the time from now to delay executionunit
- the time unit of the delay parameter
- a ScheduledFuture representing pending completion of
the task and whose get() method will return
null upon completion
scheduleAtFixedRate
public 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.
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.
- scheduleAtFixedRate in interface ScheduledExecutorService
command
- the task to executeinitialDelay
- the time to delay first executionperiod
- the period between successive executionsunit
- the time unit of the initialDelay and period parameters
- a ScheduledFuture representing pending completion of
the task, and whose get() method will throw an
exception upon cancellation
scheduleWithFixedDelay
public 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. 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.
- scheduleWithFixedDelay in interface ScheduledExecutorService
command
- the task to executeinitialDelay
- the time to delay first executiondelay
- the delay between the termination of one
execution and the commencement of the nextunit
- the time unit of the initialDelay and delay parameters
- a ScheduledFuture representing pending completion of
the task, and whose get() method will throw an
exception upon cancellation
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.
value
- if true, continue after shutdown, else don't.
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.
value
- if true, execute after shutdown, else don't.
shutdown
public void shutdown()
Initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be accepted. 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.
- shutdown in interface ExecutorService
- shutdown in interface ThreadPoolExecutor
shutdownNow
public List 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.
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.
- shutdownNow in interface ExecutorService
- shutdownNow in interface ThreadPoolExecutor
- 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.