ScheduledExecutorService

AI-generated Key Takeaways

• ScheduledExecutorService extends ExecutorService to allow scheduling tasks for delayed or periodic execution.

• It provides methods like schedule, scheduleAtFixedRate, and scheduleWithFixedDelay for controlling task execution timing.

• Tasks can be scheduled with relative delays and periods, using TimeUnit for specifying time units.

• The Executors class offers factory methods for creating ScheduledExecutorService instances.

• Scheduled tasks can be cancelled using the returned ScheduledFuture object.

public interface ScheduledExecutorService implements ExecutorService

Known Indirect Subclasses ScheduledThreadPoolExecutor ScheduledThreadPoolExecutor A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically.

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() {
     final Runnable beeper = new Runnable() {
       public void run() { System.out.println("beep"); }
     };
     final ScheduledFuture<?> beeperHandle =
       scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
     scheduler.schedule(new Runnable() {
       public void run() { beeperHandle.cancel(true); }
     }, 60 * 60, SECONDS);
   }
 }

Public Method Summary

abstract <V> ScheduledFuture<V>	schedule(Callable<V> callable, long delay, TimeUnit unit) Creates and executes a ScheduledFuture that becomes enabled after the given delay.
abstract ScheduledFuture<?>	schedule(Runnable command, long delay, TimeUnit unit) Creates and executes a one-shot action that becomes enabled after the given delay.
abstract 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.
abstract 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.

Inherited Method Summary

From interface java.util.concurrent.ExecutorService

abstract boolean	awaitTermination(long timeout, TimeUnit unit) Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
abstract <T> List<Future<T>>	invokeAll(Collection<? extends Callable<T>> tasks) Executes the given tasks, returning a list of Futures holding their status and results when all complete.
abstract <T> List<Future<T>>	invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) Executes the given tasks, returning a list of Futures holding their status and results when all complete or the timeout expires, whichever happens first.
abstract <T> T	invokeAny(Collection<? extends Callable<T>> tasks) Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do.
abstract <T> T	invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do before the given timeout elapses.
abstract boolean	isShutdown() Returns true if this executor has been shut down.
abstract boolean	isTerminated() Returns true if all tasks have completed following shut down.
abstract void	shutdown() Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
abstract 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.
abstract <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.
abstract <T> Future<T>	submit(Runnable task, T result) Submits a Runnable task for execution and returns a Future representing that task.
abstract Future<?>	submit(Runnable task) Submits a Runnable task for execution and returns a Future representing that task.

From interface java.util.concurrent.Executor

abstract void

execute(Runnable command) Executes the given command at some time in the future.

Public Methods

public abstract ScheduledFuture<V> schedule (Callable<V> callable, long delay, TimeUnit unit)

Creates and executes a ScheduledFuture that becomes enabled after the given delay.

Parameters

callable	the function to execute
delay	the time from now to delay execution
unit	the time unit of the delay parameter

Returns

• a ScheduledFuture that can be used to extract result or cancel

Throws

RejectedExecutionException	if the task cannot be scheduled for execution
NullPointerException	if callable is null

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

Creates and executes a one-shot action that becomes enabled after the given delay.

Parameters

command	the task to execute
delay	the time from now to delay execution
unit	the time unit of the delay parameter

Returns

• 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 is null

public abstract 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.

The sequence of task executions continues indefinitely until one of the following exceptional completions occur:

• The task is {@linkplain Future#cancel 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.

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	the task to execute
initialDelay	the time to delay first execution
period	the period between successive executions
unit	the time unit of the initialDelay and period parameters

Returns

• 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 is null
IllegalArgumentException	if period less than or equal to zero

public abstract 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.

The sequence of task executions continues indefinitely until one of the following exceptional completions occur:

• The task is {@linkplain Future#cancel 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.

Subsequent executions are suppressed. Subsequent calls to isDone() on the returned future will return true.

Parameters

command	the task to execute
initialDelay	the time to delay first execution
delay	the delay between the termination of one execution and the commencement of the next
unit	the time unit of the initialDelay and delay parameters

Returns

• 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 is null
IllegalArgumentException	if delay less than or equal to zero