public class ExecutorCompletionService<V> extends Object implements CompletionService<V>
CompletionService that uses a supplied Executor
to execute tasks. This class arranges that submitted tasks are,
upon completion, placed on a queue accessible using take.
The class is lightweight enough to be suitable for transient use
when processing groups of tasks.
Usage Examples.
Suppose you have a set of solvers for a certain problem, each
returning a value of some type Result, and would like to
run them concurrently, processing the results of each of them that
return a non-null value, in some method use(Result r). You
could write this as:
void solve(Executor e,
Collection<Callable<Result>> solvers)
throws InterruptedException, ExecutionException {
CompletionService<Result> ecs
= new ExecutorCompletionService<Result>(e);
for (Callable<Result> s : solvers)
ecs.submit(s);
int n = solvers.size();
for (int i = 0; i < n; ++i) {
Result r = ecs.take().get();
if (r != null)
use(r);
}
}
Suppose instead that you would like to use the first non-null result
of the set of tasks, ignoring any that encounter exceptions,
and cancelling all other tasks when the first one is ready:
void solve(Executor e,
Collection<Callable<Result>> solvers)
throws InterruptedException {
CompletionService<Result> ecs
= new ExecutorCompletionService<Result>(e);
int n = solvers.size();
List<Future<Result>> futures
= new ArrayList<Future<Result>>(n);
Result result = null;
try {
for (Callable<Result> s : solvers)
futures.add(ecs.submit(s));
for (int i = 0; i < n; ++i) {
try {
Result r = ecs.take().get();
if (r != null) {
result = r;
break;
}
} catch (ExecutionException ignore) {}
}
}
finally {
for (Future<Result> f : futures)
f.cancel(true);
}
if (result != null)
use(result);
}| Constructor | Description |
|---|---|
ExecutorCompletionService(Executor executor) |
Creates an ExecutorCompletionService using the supplied
executor for base task execution and a
LinkedBlockingQueue as a completion queue. |
ExecutorCompletionService(Executor executor,
BlockingQueue<Future<V>> completionQueue) |
Creates an ExecutorCompletionService using the supplied
executor for base task execution and the supplied queue as its
completion queue.
|
| Modifier and Type | Method | Description |
|---|---|---|
Future<V> |
poll() |
Retrieves and removes the Future representing the next
completed task, or
null if none are present. |
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.
|
Future<V> |
submit(Callable<V> task) |
Submits a value-returning task for execution and returns a Future
representing the pending results of the task.
|
Future<V> |
submit(Runnable task,
V result) |
Submits a Runnable task for execution and returns a Future
representing that task.
|
Future<V> |
take() |
Retrieves and removes the Future representing the next
completed task, waiting if none are yet present.
|
public ExecutorCompletionService(Executor executor)
LinkedBlockingQueue as a completion queue.executor - the executor to useNullPointerException - if executor is nullpublic ExecutorCompletionService(Executor executor, BlockingQueue<Future<V>> completionQueue)
executor - the executor to usecompletionQueue - the queue to use as the completion queue
normally one dedicated for use by this service. This
queue is treated as unbounded -- failed attempted
Queue.add operations for completed tasks cause
them not to be retrievable.NullPointerException - if executor or completionQueue are nullpublic Future<V> submit(Callable<V> task)
CompletionServicesubmit in interface CompletionService<V>task - the task to submitpublic Future<V> submit(Runnable task, V result)
CompletionServicesubmit in interface CompletionService<V>task - the task to submitresult - the result to return upon successful completionget() method will return the given
result value upon completionpublic Future<V> take() throws InterruptedException
CompletionServicetake in interface CompletionService<V>InterruptedException - if interrupted while waitingpublic Future<V> poll()
CompletionServicenull if none are present.poll in interface CompletionService<V>null if none are presentpublic Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException
CompletionServicepoll in interface CompletionService<V>timeout - how long to wait before giving up, in units of
unitunit - a TimeUnit determining how to interpret the
timeout parameternull if the specified waiting time elapses
before one is presentInterruptedException - if interrupted while waiting Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright © 1993, 2026, Oracle and/or its affiliates. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.
Scripting on this page tracks web page traffic, but does not change the content in any way.