OpenJDK 27 · java.util.concurrent.CompletionService · APIdia, the JavaDoc alternative

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().

Method Summary

Modifier and Type	Method and Description
public Future<V>	Returns: the Future representing the next completed task, or `null` if none are present poll() Retrieves and removes the Future representing the next completed task, or `null` if none are present.
public Future<V>	Returns: the Future representing the next completed task or `null` if the specified waiting time elapses before one is present poll(long how long to wait before giving up, in units of `unit` timeout, TimeUnit a `TimeUnit` determining how to interpret the `timeout` parameter 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.
public Future<V>	Returns: a Future representing pending completion of the task submit(Callable<V> the task to submit task) Submits a value-returning task for execution and returns a Future representing the pending results of the task.
public Future<V>	Returns: a Future representing pending completion of the task, and whose `get()` method will return the given result value upon completion submit(Runnable the task to submit task, V the result to return upon successful completion result) Submits a Runnable task for execution and returns a Future representing that task.
public Future<V>	Returns: the Future representing the next completed task take() Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Method Detail

poll back to summary

poll	back to summary
public Future<V> poll() Retrieves and removes the Future representing the next completed task, or `null` if none are present. Returns:Future<V> the Future representing the next completed task, or `null` if none are present

public Future<V> poll()

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

Returns:Future<V>: the Future representing the next completed task, or null if none are present

poll back to summary

poll	back to summary
public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException 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:long how long to wait before giving up, in units of `unit` unit:TimeUnit a `TimeUnit` determining how to interpret the `timeout` parameter Returns:Future<V> the Future representing the next completed task or `null` if the specified waiting time elapses before one is present Exceptions InterruptedException: if interrupted while waiting

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

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:long: how long to wait before giving up, in units of unit
unit:TimeUnit: a TimeUnit determining how to interpret the timeout parameter

Returns:Future<V>

the Future representing the next completed task or null if the specified waiting time elapses before one is present

Exceptions

InterruptedException:: if interrupted while waiting

submit	back to summary
public 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:Callable<V> the task to submit Returns:Future<V> a Future representing pending completion of the task Exceptions RejectedExecutionException: if the task cannot be scheduled for execution NullPointerException: if the task is null

submit

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public 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:Callable<V>: the task to submit

Returns:Future<V>

a Future representing pending completion of the task

Exceptions

RejectedExecutionException:: if the task cannot be scheduled for execution
NullPointerException:: if the task is null

submit back to summary

submit	back to summary
public 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:Runnable the task to submit result:V the result to return upon successful completion Returns:Future<V> a Future representing pending completion of the task, and whose `get()` method will return the given result value upon completion Exceptions RejectedExecutionException: if the task cannot be scheduled for execution NullPointerException: if the task is null

public 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:Runnable: the task to submit
result:V: the result to return upon successful completion

Returns:Future<V>

a Future representing pending completion of the task, and whose get() method will return the given result value upon completion

Exceptions

RejectedExecutionException:: if the task cannot be scheduled for execution
NullPointerException:: if the task is null

take	back to summary
public Future<V> take() throws InterruptedException Retrieves and removes the Future representing the next completed task, waiting if none are yet present. Returns:Future<V> the Future representing the next completed task Exceptions InterruptedException: if interrupted while waiting

take

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public Future<V> take() throws InterruptedException

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

Returns:Future<V>

the Future representing the next completed task

Exceptions

InterruptedException:: if interrupted while waiting

public Interface CompletionService<V>

Method Summary

Method Detail