# Interface ScheduledExecutorService

All Superinterfaces:
  `AutoCloseable, Executor, ExecutorService`

All Known Implementing Classes:
  `ForkJoinPool, ScheduledThreadPoolExecutor`

public interface ScheduledExecutorService extends ExecutorService
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);
      }
    }

Since:
  1.5

## Method Summary

Modifier and Type
Method
Description
`ScheduledFuture<?>`
`schedule(Runnable command, long delay, TimeUnit unit)`
Submits a one-shot task that becomes enabled after the given delay.
`<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.
`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.
`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.

### Methods declared in interface Executor

`execute`

### Methods declared in interface ExecutorService

`awaitTermination, close, invokeAll, invokeAll, invokeAny, invokeAny, isShutdown, isTerminated, shutdown, shutdownNow, submit, submit, submit`

## Method Details

### schedule

ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
Submits a one-shot task 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 or unit is null

### schedule

<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.

Type Parameters:
  `V` - the type of the callable's result
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 or unit is null

### scheduleAtFixedRate

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` - 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 or unit is null
  `IllegalArgumentException` - if period less than or equal to zero

### scheduleWithFixedDelay

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` - 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 or unit is null
  `IllegalArgumentException` - if delay less than or equal to zero