Module java.base
Package java.util.concurrent

# Class ScheduledThreadPoolExecutor

java.lang.Object
java.util.concurrent.AbstractExecutorService
java.util.concurrent.ThreadPoolExecutor
java.util.concurrent.ScheduledThreadPoolExecutor

All Implemented Interfaces:
  `AutoCloseable`, `Executor`, `ExecutorService`, `ScheduledExecutorService`

public class ScheduledThreadPoolExecutor extends ThreadPoolExecutor implements ScheduledExecutorService
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. 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(boolean)` 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.
     }

Since:
  1.5

- ## Nested Class Summary

  ## Nested classes/interfaces declared in class java.util.concurrent.ThreadPoolExecutor

  `ThreadPoolExecutor.AbortPolicy, ThreadPoolExecutor.CallerRunsPolicy, ThreadPoolExecutor.DiscardOldestPolicy, ThreadPoolExecutor.DiscardPolicy`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ScheduledThreadPoolExecutor(int corePoolSize)`
  Creates a new `ScheduledThreadPoolExecutor` with the given core pool size.
  `ScheduledThreadPoolExecutor(int corePoolSize, RejectedExecutionHandler handler)`
  Creates a new `ScheduledThreadPoolExecutor` with the given initial parameters.
  `ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory)`
  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.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected <V> RunnableScheduledFuture<V>`
  `decorateTask(Runnable runnable, RunnableScheduledFuture<V> task)`
  Modifies or replaces the task used to execute a runnable.
  `protected <V> RunnableScheduledFuture<V>`
  `decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task)`
  Modifies or replaces the task used to execute a callable.
  `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.
  `boolean`
  `getRemoveOnCancelPolicy()`
  Gets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation.
  `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.
  `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`
  `setRemoveOnCancelPolicy(boolean value)`
  Sets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation.
  `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.
  `Future<?>`
  `submit(Runnable task)`
  Submits a Runnable task for execution and returns a Future representing that task.
  `<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.

  ### Methods declared in class java.util.concurrent.ThreadPoolExecutor

  `afterExecute, allowCoreThreadTimeOut, allowsCoreThreadTimeOut, awaitTermination, beforeExecute, finalize, getActiveCount, getCompletedTaskCount, getCorePoolSize, getKeepAliveTime, getLargestPoolSize, getMaximumPoolSize, getPoolSize, getRejectedExecutionHandler, getTaskCount, getThreadFactory, isShutdown, isTerminated, isTerminating, prestartAllCoreThreads, prestartCoreThread, purge, remove, setCorePoolSize, setKeepAliveTime, setMaximumPoolSize, setRejectedExecutionHandler, setThreadFactory, terminated, toString`

  ### Methods declared in class java.util.concurrent.AbstractExecutorService

  `invokeAll, invokeAll, invokeAny, invokeAny, newTaskFor, newTaskFor`

  ### Methods declared in class java.lang.Object

  `clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface java.util.concurrent.ExecutorService

  `awaitTermination, close, invokeAll, invokeAll, invokeAny, invokeAny, isShutdown, isTerminated`

- ## Constructor Details

  - ### ScheduledThreadPoolExecutor

    public ScheduledThreadPoolExecutor(int corePoolSize)
    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`

  - ### ScheduledThreadPoolExecutor

    public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory)
    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

  - ### ScheduledThreadPoolExecutor

    public ScheduledThreadPoolExecutor(int corePoolSize, RejectedExecutionHandler handler)
    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

  - ### ScheduledThreadPoolExecutor

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

- ## Method Details

  - ### decorateTask

    protected <V> RunnableScheduledFuture<V> decorateTask(Runnable runnable, RunnableScheduledFuture<V> task)
    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.
    Type Parameters:
      `V` - the type of the task's result
    Parameters:
      `runnable` - the submitted Runnable
      `task` - the task created to execute the runnable
    Returns:
      a task that can execute the runnable
    Since:
      1.6

  - ### decorateTask

    protected <V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task)
    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.
    Type Parameters:
      `V` - the type of the task's result
    Parameters:
      `callable` - the submitted Callable
      `task` - the task created to execute the callable
    Returns:
      a task that can execute the callable
    Since:
      1.6

  - ### schedule

    public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
    Description copied from interface: `ScheduledExecutorService`
    Submits a one-shot task that becomes enabled after the given delay.
    Specified by:
      `schedule` in interface `ScheduledExecutorService`
    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

    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit)
    Description copied from interface: `ScheduledExecutorService`
    Submits a value-returning one-shot task that becomes enabled after the given delay.
    Specified by:
      `schedule` in interface `ScheduledExecutorService`
    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

    public 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.
    - Method `shutdown()` is called and the policy on whether to continue after shutdown is not set true, or method `shutdownNow()` is called; 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.

    Specified by:
      `scheduleAtFixedRate` in interface `ScheduledExecutorService`
    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

    public 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.
    - Method `shutdown()` is called and the policy on whether to continue after shutdown is not set true, or method `shutdownNow()` is called; 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`.
    Specified by:
      `scheduleWithFixedDelay` in interface `ScheduledExecutorService`
    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

  - ### execute

    public void execute(Runnable command)
    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 `Future.get()`.

    Specified by:
      `execute` in interface `Executor`
    Overrides:
      `execute` in class `ThreadPoolExecutor`
    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` - if `command` is null

  - ### submit

    public Future<?> submit(Runnable task)
    Description copied from interface: `ExecutorService`
    Submits a Runnable task for execution and returns a Future representing that task. The Future's `get` method will return `null` upon successful completion.
    Specified by:
      `submit` in interface `ExecutorService`
    Overrides:
      `submit` in class `AbstractExecutorService`
    Parameters:
      `task` - the task to submit
    Returns:
      a Future representing pending completion of the task
    Throws:
      `RejectedExecutionException` - if the task cannot be scheduled for execution
      `NullPointerException` - if the task is null

  - ### submit

    public <T> Future<T> submit(Runnable task, T result)
    Description copied from interface: `ExecutorService`
    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.
    Specified by:
      `submit` in interface `ExecutorService`
    Overrides:
      `submit` in class `AbstractExecutorService`
    Type Parameters:
      `T` - the type of the result
    Parameters:
      `task` - the task to submit
      `result` - the result to return
    Returns:
      a Future representing pending completion of the task
    Throws:
      `RejectedExecutionException` - if the task cannot be scheduled for execution
      `NullPointerException` - if the task is null

  - ### submit

    public <T> Future<T> submit(Callable<T> task)
    Description copied from interface: `ExecutorService`
    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.

    Specified by:
      `submit` in interface `ExecutorService`
    Overrides:
      `submit` in class `AbstractExecutorService`
    Type Parameters:
      `T` - the type of the task's result
    Parameters:
      `task` - the task to submit
    Returns:
      a Future representing pending completion of the task
    Throws:
      `RejectedExecutionException` - if the task cannot be scheduled for execution
      `NullPointerException` - if the task is null

  - ### setContinueExistingPeriodicTasksAfterShutdownPolicy

    public void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value)
    Sets the policy on whether to continue executing existing periodic tasks even when this executor has been `shutdown`. In this case, executions will continue until `shutdownNow` or the policy is set to `false` when already shutdown. This value is by default `false`.
    Parameters:
      `value` - if `true`, continue after shutdown, else don't
    See Also:
      - `getContinueExistingPeriodicTasksAfterShutdownPolicy()`

  - ### getContinueExistingPeriodicTasksAfterShutdownPolicy

    public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy()
    Gets the policy on whether to continue executing existing periodic tasks even when this executor has been `shutdown`. In this case, executions will continue until `shutdownNow` or the policy is set to `false` when already shutdown. This value is by default `false`.
    Returns:
      `true` if will continue after shutdown
    See Also:
      - `setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean)`

  - ### setExecuteExistingDelayedTasksAfterShutdownPolicy

    public void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value)
    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:
      - `getExecuteExistingDelayedTasksAfterShutdownPolicy()`

  - ### getExecuteExistingDelayedTasksAfterShutdownPolicy

    public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy()
    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:
      - `setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean)`

  - ### setRemoveOnCancelPolicy

    public void setRemoveOnCancelPolicy(boolean value)
    Sets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation. This value is by default `false`.
    Parameters:
      `value` - if `true`, remove on cancellation, else don't
    Since:
      1.7
    See Also:
      - `getRemoveOnCancelPolicy()`

  - ### getRemoveOnCancelPolicy

    public boolean getRemoveOnCancelPolicy()
    Gets the policy on whether cancelled tasks should be immediately removed from the work queue at time of cancellation. This value is by default `false`.
    Returns:
      `true` if cancelled tasks are immediately removed from the queue
    Since:
      1.7
    See Also:
      - `setRemoveOnCancelPolicy(boolean)`

  - ### shutdown

    public void shutdown()
    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.

    Specified by:
      `shutdown` in interface `ExecutorService`
    Overrides:
      `shutdown` in class `ThreadPoolExecutor`
    Throws:
      `SecurityException` - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold `RuntimePermission``("modifyThread")`, or the security manager's `checkAccess` method denies access.

  - ### shutdownNow

    public 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. These tasks are drained (removed) from the task queue upon return from this method.

    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 interrupts tasks via `Thread.interrupt()`; any task that fails to respond to interrupts may never terminate.

    Specified by:
      `shutdownNow` in interface `ExecutorService`
    Overrides:
      `shutdownNow` in class `ThreadPoolExecutor`
    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`.
    Throws:
      `SecurityException` - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold `RuntimePermission``("modifyThread")`, or the security manager's `checkAccess` method denies access.

  - ### getQueue

    public BlockingQueue<Runnable> getQueue()
    Returns the task queue used by this executor. Access to the task queue is intended primarily for debugging and monitoring. This queue may be in active use. Retrieving the task queue does not prevent queued tasks from executing.

    Each element of this queue 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`.

    Iteration over this queue is not guaranteed to traverse tasks in the order in which they will execute.

    Overrides:
      `getQueue` in class `ThreadPoolExecutor`
    Returns:
      the task queue