java.util.concurrent

## Interface ScheduledExecutorService

- All Superinterfaces:
    Executor, ExecutorService
  All Known Implementing Classes:
    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(java.lang.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);
         }
       }

  Since:
    1.5
  Author:
    Doug Lea

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method and Description
    --- | ---
    `<V> ScheduledFuture<V>` | `schedule(Callable<V> callable, long delay, TimeUnit unit)` Creates and executes a ScheduledFuture that becomes enabled after the given delay.
    `ScheduledFuture<?>` | `schedule(Runnable command, long delay, TimeUnit unit)` Creates and executes a one-shot action that becomes enabled after the given delay.
    `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.
    `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.

    - ### Methods inherited from interface java.util.concurrent.ExecutorService

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

    - ### Methods inherited from interface java.util.concurrent.Executor

      `execute`

- - ### Method Detail

    - #### schedule

          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

    - #### schedule

          <V> 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

    - #### scheduleAtFixedRate

          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.
      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 task, and whose `get()` method will throw an exception upon cancellation
      Throws:
        `RejectedExecutionException` - if the task cannot be scheduled for execution
        `NullPointerException` - if command is null
        `IllegalArgumentException` - if period less than or equal to zero

    - #### scheduleWithFixedDelay

          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.
      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 task, and whose `get()` method will throw an exception upon cancellation
      Throws:
        `RejectedExecutionException` - if the task cannot be scheduled for execution
        `NullPointerException` - if command is null
        `IllegalArgumentException` - if delay less than or equal to zero

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