OpenJDK 1.23 · java.util.concurrent.ScheduledThreadPoolExecutor

A ThreadPoolExecutor that can additionally schedule commands to run after a given delay, or to execute periodically. This class is preferable to java.util.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.

When a submitted task is cancelled before it is run, execution is suppressed. By default, such a cancelled task is not automatically removed from the work queue until its delay elapses. While this enables further inspection and monitoring, it may also cause unbounded retention of cancelled tasks. To avoid this, use setRemoveOnCancelPolicy to cause tasks to be immediately removed from the work queue at time of cancellation.

Successive executions of a periodic task scheduled via scheduleAtFixedRate or scheduleWithFixedDelay do not overlap. While different executions may be performed by different threads, the effects of prior executions happen-before those of subsequent ones.

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. Additionally, it is almost never a good idea to set corePoolSize to zero or use allowCoreThreadTimeOut because this may leave the pool without threads to handle tasks once they become eligible to run.

As with ThreadPoolExecutor, if not otherwise specified, this class uses Executors#defaultThreadFactory as the default thread factory, and ThreadPoolExecutor.AbortPolicy as the default rejected execution handler.

Extension Notes

This class overrides the execute and submit methods to generate internal ScheduledFuture 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.
}

Nested and Inner Type Summary

Modifier and Type	Class and Description
pack-priv static class	ScheduledThreadPoolExecutor.DelayedWorkQueue Specialized delay queue.
private class	ScheduledThreadPoolExecutor.ScheduledFutureTask<V>

Field Summary

Modifier and Type	Field and Description
private volatile boolean	continueExistingPeriodicTasksAfterShutdown False if should cancel/suppress periodic tasks on shutdown.
private static final long	DEFAULT_KEEPALIVE_MILLIS The default keep-alive time for pool threads.
private volatile boolean	executeExistingDelayedTasksAfterShutdown False if should cancel non-periodic not-yet-expired tasks on shutdown.
pack-priv volatile boolean	removeOnCancel True if ScheduledFutureTask.cancel should remove from queue.
private static final AtomicLong	sequencer Sequence number to break scheduling ties, and in turn to guarantee FIFO order among tied entries.

Constructor Summary

Access	Constructor and Description
public	ScheduledThreadPoolExecutor(int the number of threads to keep in the pool, even if they are idle, unless `allowCoreThreadTimeOut` is set corePoolSize) Creates a new `ScheduledThreadPoolExecutor` with the given core pool size.
public	ScheduledThreadPoolExecutor(int the number of threads to keep in the pool, even if they are idle, unless `allowCoreThreadTimeOut` is set corePoolSize, ThreadFactory the factory to use when the executor creates a new thread threadFactory) Creates a new `ScheduledThreadPoolExecutor` with the given initial parameters.
public	ScheduledThreadPoolExecutor(int the number of threads to keep in the pool, even if they are idle, unless `allowCoreThreadTimeOut` is set corePoolSize, RejectedExecutionHandler the handler to use when execution is blocked because the thread bounds and queue capacities are reached handler) Creates a new `ScheduledThreadPoolExecutor` with the given initial parameters.
public	ScheduledThreadPoolExecutor(int the number of threads to keep in the pool, even if they are idle, unless `allowCoreThreadTimeOut` is set corePoolSize, ThreadFactory the factory to use when the executor creates a new thread threadFactory, RejectedExecutionHandler the handler to use when execution is blocked because the thread bounds and queue capacities are reached handler) Creates a new `ScheduledThreadPoolExecutor` with the given initial parameters.

Method Summary

Modifier and Type	Method and Description
pack-priv boolean	canRunInCurrentRunState(RunnableScheduledFuture<?> task) Returns true if can run a task given current run state and run-after-shutdown parameters.
protected < the type of the task's result V> RunnableScheduledFuture<V>	Returns: a task that can execute the runnable decorateTask(Runnable the submitted Runnable runnable, RunnableScheduledFuture<V> the task created to execute the runnable task) Modifies or replaces the task used to execute a runnable.
protected < the type of the task's result V> RunnableScheduledFuture<V>	Returns: a task that can execute the callable decorateTask(Callable<V> the submitted Callable callable, RunnableScheduledFuture<V> the task created to execute the callable task) Modifies or replaces the task used to execute a callable.
private void	delayedExecute(RunnableScheduledFuture<?> the task task) Main execution method for delayed or periodic tasks.
public void	execute(Runnable the runnable task command) Overrides java.util.concurrent.ThreadPoolExecutor.execute. Implements java.util.concurrent.Executor.execute. Executes `command` with zero required delay.
public boolean	Returns: `true` if will continue after shutdown getContinueExistingPeriodicTasksAfterShutdownPolicy() Gets the policy on whether to continue executing existing periodic tasks even when this executor has been `shutdown`.
public boolean	Returns: `true` if will execute after shutdown getExecuteExistingDelayedTasksAfterShutdownPolicy() Gets the policy on whether to execute existing delayed tasks even when this executor has been `shutdown`.
public BlockingQueue<Runnable>	Returns: the task queue getQueue() Overrides java.util.concurrent.ThreadPoolExecutor.getQueue. Returns the task queue used by this executor.
public boolean	Returns: `true` if cancelled tasks are immediately removed from the queue getRemoveOnCancelPolicy() Gets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation.
pack-priv void	onShutdown() Overrides java.util.concurrent.ThreadPoolExecutor.onShutdown. Cancels and clears the queue of all tasks that should not be run due to shutdown policy.
private long	overflowFree(long delay) Constrains the values of all delays in the queue to be within Long.MAX_VALUE of each other, to avoid overflow in compareTo.
pack-priv void	reExecutePeriodic(RunnableScheduledFuture<?> the task task) Requeues a periodic task unless current run state precludes it.
public ScheduledFuture<?>	schedule(Runnable the task to execute command, long the time from now to delay execution delay, TimeUnit the time unit of the delay parameter unit) Implements java.util.concurrent.ScheduledExecutorService.schedule. Submits a one-shot task that becomes enabled after the given delay.
public <V> ScheduledFuture<V>	schedule(Callable<V> the function to execute callable, long the time from now to delay execution delay, TimeUnit the time unit of the delay parameter unit) Implements java.util.concurrent.ScheduledExecutorService.schedule. Submits a value-returning one-shot task that becomes enabled after the given delay.
public ScheduledFuture<?>	scheduleAtFixedRate(Runnable the task to execute command, long the time to delay first execution initialDelay, long the period between successive executions period, TimeUnit the time unit of the initialDelay and period parameters unit) Implements java.util.concurrent.ScheduledExecutorService.scheduleAtFixedRate. 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.
public ScheduledFuture<?>	scheduleWithFixedDelay(Runnable the task to execute command, long the time to delay first execution initialDelay, long the delay between the termination of one execution and the commencement of the next delay, TimeUnit the time unit of the initialDelay and delay parameters unit) Implements java.util.concurrent.ScheduledExecutorService.scheduleWithFixedDelay. 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.
public void	setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean if `true`, continue after shutdown, else don't value) Sets the policy on whether to continue executing existing periodic tasks even when this executor has been `shutdown`.
public void	setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean if `true`, execute after shutdown, else don't value) Sets the policy on whether to execute existing delayed tasks even when this executor has been `shutdown`.
public void	setRemoveOnCancelPolicy(boolean if `true`, remove on cancellation, else don't value) Sets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation.
public void	shutdown() Overrides java.util.concurrent.ThreadPoolExecutor.shutdown. Implements java.util.concurrent.ExecutorService.shutdown. Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
public List<Runnable>	Returns: list of tasks that never commenced execution. Each element of this list is a `ScheduledFuture`. For tasks submitted via one of the `schedule` methods, the element will be identical to the returned `ScheduledFuture`. For tasks submitted using `execute`, the element will be a zero-delay `ScheduledFuture`. shutdownNow() Overrides java.util.concurrent.ThreadPoolExecutor.shutdownNow. Implements java.util.concurrent.ExecutorService.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.
public Future<?>	submit(Runnable the task to submit task) Overrides java.util.concurrent.AbstractExecutorService.submit. Implements java.util.concurrent.ExecutorService.submit. Submits a Runnable task for execution and returns a Future representing that task.
public <T> Future<T>	submit(Runnable the task to submit task, T the result to return result) Overrides java.util.concurrent.AbstractExecutorService.submit. Implements java.util.concurrent.ExecutorService.submit. Submits a Runnable task for execution and returns a Future representing that task.
public <T> Future<T>	submit(Callable<T> the task to submit task) Overrides java.util.concurrent.AbstractExecutorService.submit. Implements java.util.concurrent.ExecutorService.submit. Submits a value-returning task for execution and returns a Future representing the pending results of the task.
private long	triggerTime(long delay, TimeUnit unit) Returns the nanoTime-based trigger time of a delayed action.
pack-priv long	triggerTime(long delay) Returns the nanoTime-based trigger time of a delayed action.

public Class ScheduledThreadPoolExecutor

Nested and Inner Type Summary

Field Summary

Constructor Summary

Method Summary