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 and 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 and 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 taskes 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 completionpublic Future<V> take() throws InterruptedException
CompletionServicetake in interface CompletionService<V>InterruptedException - if interrupted while waitingpublic Future<V> poll()
CompletionServicepoll in interface CompletionService<V>public 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 parameterInterruptedException - if interrupted while waiting