public class

ScheduledThreadPoolExecutor

extends ThreadPoolExecutor
implements ScheduledExecutorService
java.lang.Object
   ↳ java.util.concurrent.AbstractExecutorService
     ↳ java.util.concurrent.ThreadPoolExecutor
       ↳ java.util.concurrent.ScheduledThreadPoolExecutor

Class Overview

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

Successive executions of a 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.

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 implements RunnableScheduledFuture { ... 

   protected  RunnableScheduledFuture decorateTask(
                Runnable r, RunnableScheduledFuture task) {
       return new CustomTask(r, task);
   }

   protected  RunnableScheduledFuture decorateTask(
                Callable c, RunnableScheduledFuture task) {
       return new CustomTask(c, task);
   }
   // ... add constructors, etc.
 }}

Summary

Public Constructors
ScheduledThreadPoolExecutor(int corePoolSize)
Creates a new ScheduledThreadPoolExecutor with the given core pool size.
ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory)
Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
ScheduledThreadPoolExecutor(int corePoolSize, RejectedExecutionHandler handler)
Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory, RejectedExecutionHandler handler)
Creates a new ScheduledThreadPoolExecutor with the given initial parameters.
Public Methods
void execute(Runnable command)
Executes command with zero required delay.
boolean getContinueExistingPeriodicTasksAfterShutdownPolicy()
Gets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown.
boolean getExecuteExistingDelayedTasksAfterShutdownPolicy()
Gets the policy on whether to execute existing delayed tasks even when this executor has been shutdown.
BlockingQueue<Runnable> getQueue()
Returns the task queue used by this executor.
ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
Creates and executes a one-shot action that becomes enabled after the given delay.
<V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit)
Creates and executes a ScheduledFuture 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.
void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value)
Sets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown.
void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value)
Sets the policy on whether to execute existing delayed tasks even when this executor has been shutdown.
void shutdown()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
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.
<T> Future<T> submit(Runnable task, T result)
Submits a Runnable task for execution and returns a Future representing that task.
<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.
Future<?> submit(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task.
Protected Methods
<V> RunnableScheduledFuture<V> decorateTask(Runnable runnable, RunnableScheduledFuture<V> task)
Modifies or replaces the task used to execute a runnable.
<V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task)
Modifies or replaces the task used to execute a callable.
[Expand]
Inherited Methods
From class java.util.concurrent.ThreadPoolExecutor
From class java.util.concurrent.AbstractExecutorService
From class java.lang.Object
From interface java.util.concurrent.Executor
From interface java.util.concurrent.ExecutorService
From interface java.util.concurrent.ScheduledExecutorService

Public Constructors

public ScheduledThreadPoolExecutor (int corePoolSize)

Since: API Level 1

Creates a new ScheduledThreadPoolExecutor with the given core pool size.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
Throws
IllegalArgumentException if corePoolSize < 0

public ScheduledThreadPoolExecutor (int corePoolSize, ThreadFactory threadFactory)

Since: API Level 1

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
threadFactory the factory to use when the executor creates a new thread
Throws
IllegalArgumentException if corePoolSize < 0
NullPointerException if threadFactory is null

public ScheduledThreadPoolExecutor (int corePoolSize, RejectedExecutionHandler handler)

Since: API Level 1

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
handler the handler to use when execution is blocked because the thread bounds and queue capacities are reached
Throws
IllegalArgumentException if corePoolSize < 0
NullPointerException if handler is null

public ScheduledThreadPoolExecutor (int corePoolSize, ThreadFactory threadFactory, RejectedExecutionHandler handler)

Since: API Level 1

Creates a new ScheduledThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
threadFactory the factory to use when the executor creates a new thread
handler the handler to use when execution is blocked because the thread bounds and queue capacities are reached
Throws
IllegalArgumentException if corePoolSize < 0
NullPointerException if threadFactory or handler is null

Public Methods

public void execute (Runnable command)

Since: API Level 1

Executes command with zero required delay. This has effect equivalent to schedule(command, 0, anyUnit). Note that inspections of the queue and of the list returned by shutdownNow will access the zero-delayed ScheduledFuture, not the command itself.

A consequence of the use of ScheduledFuture objects is that afterExecute is always called with a null second Throwable argument, even if the command terminated abruptly. Instead, the Throwable thrown by such a task can be obtained via get().

Parameters
command the task to execute
Throws
RejectedExecutionException at discretion of RejectedExecutionHandler, if the task cannot be accepted for execution because the executor has been shut down
NullPointerException

public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy ()

Since: API Level 1

Gets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown. In this case, these tasks will only terminate upon shutdownNow or after setting the policy to false when already shutdown. This value is by default false.

Returns
  • true if will continue after shutdown
See Also

public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy ()

Since: API Level 1

Gets the policy on whether to execute existing delayed tasks even when this executor has been shutdown. In this case, these tasks will only terminate upon shutdownNow, or after setting the policy to false when already shutdown. This value is by default true.

Returns
  • true if will execute after shutdown
See Also

public BlockingQueue<Runnable> getQueue ()

Since: API Level 1

Returns the task queue used by this executor. Each element of this queue is a ScheduledFuture, including those tasks submitted using execute which are for scheduling purposes used as the basis of a zero-delay ScheduledFuture. Iteration over this queue is not guaranteed to traverse tasks in the order in which they will execute.

Returns
  • the task queue

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

Since: API Level 1

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
NullPointerException

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

Since: API Level 1

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
NullPointerException

public ScheduledFuture<?> scheduleAtFixedRate (Runnable command, long initialDelay, long period, TimeUnit unit)

Since: API Level 1

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
NullPointerException
IllegalArgumentException

public ScheduledFuture<?> scheduleWithFixedDelay (Runnable command, long initialDelay, long delay, TimeUnit unit)

Since: API Level 1

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
NullPointerException
IllegalArgumentException

public void setContinueExistingPeriodicTasksAfterShutdownPolicy (boolean value)

Since: API Level 1

Sets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown. In this case, these tasks will only terminate upon shutdownNow or after setting the policy to false when already shutdown. This value is by default false.

Parameters
value if true, continue after shutdown, else don't.
See Also

public void setExecuteExistingDelayedTasksAfterShutdownPolicy (boolean value)

Since: API Level 1

Sets the policy on whether to execute existing delayed tasks even when this executor has been shutdown. In this case, these tasks will only terminate upon shutdownNow, or after setting the policy to false when already shutdown. This value is by default true.

Parameters
value if true, execute after shutdown, else don't.
See Also

public void shutdown ()

Since: API Level 1

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down.

This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

If the ExecuteExistingDelayedTasksAfterShutdownPolicy has been set false, existing delayed tasks whose delays have not yet elapsed are cancelled. And unless the ContinueExistingPeriodicTasksAfterShutdownPolicy has been set true, future executions of existing periodic tasks will be cancelled.

Throws
SecurityException

public List<Runnable> shutdownNow ()

Since: API Level 1

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that.

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. This implementation cancels tasks via interrupt(), so any task that fails to respond to interrupts may never terminate.

Returns
  • list of tasks that never commenced execution. Each element of this list is a ScheduledFuture, including those tasks submitted using execute, which are for scheduling purposes used as the basis of a zero-delay ScheduledFuture.
Throws
SecurityException

public Future<T> submit (Runnable task, T result)

Since: API Level 1

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return the given result upon successful completion.

Parameters
task the task to submit
result the result to return
Returns
  • a Future representing pending completion of the task
Throws
RejectedExecutionException
NullPointerException

public Future<T> submit (Callable<T> task)

Since: API Level 1

Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future's get method will return the task's result upon successful completion.

If you would like to immediately block waiting for a task, you can use constructions of the form result = exec.submit(aCallable).get();

Note: The Executors class includes a set of methods that can convert some other common closure-like objects, for example, PrivilegedAction to Callable form so they can be submitted.

Parameters
task the task to submit
Returns
  • a Future representing pending completion of the task
Throws
RejectedExecutionException
NullPointerException

public Future<?> submit (Runnable task)

Since: API Level 1

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.

Parameters
task the task to submit
Returns
  • a Future representing pending completion of the task
Throws
RejectedExecutionException
NullPointerException

Protected Methods

protected RunnableScheduledFuture<V> decorateTask (Runnable runnable, RunnableScheduledFuture<V> task)

Since: API Level 9

Modifies or replaces the task used to execute a runnable. This method can be used to override the concrete class used for managing internal tasks. The default implementation simply returns the given task.

Parameters
runnable the submitted Runnable
task the task created to execute the runnable
Returns
  • a task that can execute the runnable

protected RunnableScheduledFuture<V> decorateTask (Callable<V> callable, RunnableScheduledFuture<V> task)

Since: API Level 9

Modifies or replaces the task used to execute a callable. This method can be used to override the concrete class used for managing internal tasks. The default implementation simply returns the given task.

Parameters
callable the submitted Callable
task the task created to execute the callable
Returns
  • a task that can execute the callable