CompletionService

  • CompletionService allows the submission of asynchronous tasks and retrieval of their results, decoupling production and consumption.

  • It manages a completion queue, often relying on a separate Executor for task execution.

  • Consumers can obtain completed tasks using take() (blocking), poll() (non-blocking), or poll(long, TimeUnit) (timed blocking).

  • submit() methods enable submitting tasks as either Runnable or Callable for execution.

  • Memory consistency is guaranteed: actions before submission happen-before task actions, which happen-before actions after successful take().
public interface CompletionService
Known Indirect Subclasses

A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks. Producers submit tasks for execution. Consumers take completed tasks and process their results in the order they complete. A CompletionService can for example be used to manage asynchronous I/O, in which tasks that perform reads are submitted in one part of a program or system, and then acted upon in a different part of the program when the reads complete, possibly in a different order than they were requested.

Typically, a CompletionService relies on a separate Executor to actually execute the tasks, in which case the CompletionService only manages an internal completion queue. The ExecutorCompletionService class provides an implementation of this approach.

Memory consistency effects: Actions in a thread prior to submitting a task to a CompletionService happen-before actions taken by that task, which in turn happen-before actions following a successful return from the corresponding take().

Public Method Summary

abstract Future<V>
poll(long timeout, TimeUnit unit)
Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.
abstract Future<V>
poll()
Retrieves and removes the Future representing the next completed task, or null if none are present.
abstract Future<V>
submit(Runnable task, V result)
Submits a Runnable task for execution and returns a Future representing that task.
abstract Future<V>
submit(Callable<V> task)
Submits a value-returning task for execution and returns a Future representing the pending results of the task.
abstract Future<V>
take()
Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Public Methods

public abstract Future<V> poll (long timeout, TimeUnit unit)

Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.

Parameters
timeout how long to wait before giving up, in units of unit
unit a TimeUnit determining how to interpret the timeout parameter
Returns
  • the Future representing the next completed task or null if the specified waiting time elapses before one is present
Throws
InterruptedException if interrupted while waiting

public abstract Future<V> poll ()

Retrieves and removes the Future representing the next completed task, or null if none are present.

Returns
  • the Future representing the next completed task, or null if none are present

public abstract Future<V> submit (Runnable task, V result)

Submits a Runnable task for execution and returns a Future representing that task. Upon completion, this task may be taken or polled.

Parameters
task the task to submit
result the result to return upon successful completion
Returns
  • a Future representing pending completion of the task, and whose get() method will return the given result value upon completion
Throws
RejectedExecutionException if the task cannot be scheduled for execution
NullPointerException if the task is null

public abstract Future<V> submit (Callable<V> task)

Submits a value-returning task for execution and returns a Future representing the pending results of the task. Upon completion, this task may be taken or polled.

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

public abstract Future<V> take ()

Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Returns
  • the Future representing the next completed task
Throws
InterruptedException if interrupted while waiting