java.util.concurrent.ExecutorCompletionService<V>
A 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);
}
Summary
Public Constructors
Public Methods
clone,
equals,
finalize,
getClass,
hashCode,
notify,
notifyAll,
toString,
wait,
wait,
wait
Details
Public Constructors
public
ExecutorCompletionService(Executor executor)
Creates an ExecutorCompletionService using the supplied
executor for base task execution and a
LinkedBlockingQueue as a completion queue.
Parameters
executor
| the executor to use |
public
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.
Parameters
executor
| the executor to use |
completionQueue
| the queue to use as the completion queue
normally one dedicated for use by this service |
Public Methods
public
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.
public
Future<V>
poll()
Retrieves and removes the Future representing the next
completed task or null if none are present.
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.
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.
public
Future<V>
take()
Retrieves and removes the Future representing the next
completed task, waiting if none are yet present.