Added in API level 1

ScheduledExecutorService

public interface ScheduledExecutorService
implements ExecutorService

java.util.concurrent.ScheduledExecutorService


An ExecutorService that can schedule commands to run after a given delay, or to execute periodically.

The schedule methods create tasks with various delays and return a task object that can be used to cancel or check execution. The scheduleAtFixedRate and scheduleWithFixedDelay methods create and execute tasks that run periodically until cancelled.

Commands submitted using the Executor#execute(Runnable) and ExecutorService submit methods are scheduled with a requested delay of zero. Zero and negative delays (but not periods) are also allowed in schedule methods, and are treated as requests for immediate execution.

All schedule methods accept relative delays and periods as arguments, not absolute times or dates. It is a simple matter to transform an absolute time represented as a Date to the required form. For example, to schedule at a certain future date, you can use: schedule(task, date.getTime() - System.currentTimeMillis(), TimeUnit.MILLISECONDS). Beware however that expiration of a relative delay need not coincide with the current Date at which the task is enabled due to network time synchronization protocols, clock drift, or other factors.

The Executors class provides convenient factory methods for the ScheduledExecutorService implementations provided in this package.

Usage Example

Here is a class with a method that sets up a ScheduledExecutorService to beep every ten seconds for an hour:
 import static java.util.concurrent.TimeUnit.*;
 class BeeperControl {
   private final ScheduledExecutorService scheduler =
     Executors.newScheduledThreadPool(1);

   public void beepForAnHour() {
     Runnable beeper = () -> System.out.println("beep");
     ScheduledFuture<?> beeperHandle =
       scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
     Runnable canceller = () -> beeperHandle.cancel(false);
     scheduler.schedule(canceller, 1, HOURS);
   }
 }

Summary

Public methods

abstract <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.

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

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

abstract 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.

abstract 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.

Inherited methods

Public methods

schedule

Added in API level 1
public abstract 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 if the task cannot be scheduled for execution
NullPointerException if callable or unit is null

schedule

Added in API level 1
public abstract 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 if the task cannot be scheduled for execution
NullPointerException if command or unit is null

scheduleAtFixedRate

Added in API level 1
public abstract 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:

  • The task is explicitly cancelled via the returned future.
  • The executor terminates, also resulting in task cancellation.
  • An execution of the task throws an exception. In this case calling get on the returned future will throw ExecutionException, holding the exception as its cause.
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 if the task cannot be scheduled for execution
NullPointerException if command or unit is null
IllegalArgumentException if period less than or equal to zero

scheduleWithFixedDelay

Added in API level 1
public abstract 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:

  • The task is explicitly cancelled via the returned future.
  • The executor terminates, also resulting in task cancellation.
  • An execution of the task throws an exception. In this case calling get on the returned future will throw ExecutionException, holding the exception as its cause.
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 if the task cannot be scheduled for execution
NullPointerException if command or unit is null
IllegalArgumentException if delay less than or equal to zero