public class ExecutorCompletionService extends java.lang.Object implements CompletionService
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 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 |
poll()
Retrieves and removes the Future representing the next
completed task or null if none are present.
|
Future |
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 |
submit(Callable task)
Submits a value-returning task for execution and returns a Future
representing the pending results of the task.
|
Future |
submit(java.lang.Runnable task,
java.lang.Object result)
Submits a Runnable task for execution and returns a Future
representing that task.
|
Future |
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 usejava.lang.NullPointerException - if executor is nullpublic ExecutorCompletionService(Executor executor, BlockingQueue 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.java.lang.NullPointerException - if executor or completionQueue are nullpublic Future submit(Callable task)
CompletionServicesubmit in interface CompletionServicetask - the task to submitpublic Future submit(java.lang.Runnable task, java.lang.Object result)
CompletionServicesubmit in interface CompletionServicetask - the task to submitresult - the result to return upon successful completionpublic Future take() throws java.lang.InterruptedException
CompletionServicetake in interface CompletionServicejava.lang.InterruptedException - if interrupted while waitingpublic Future poll()
CompletionServicepoll in interface CompletionServicepublic Future poll(long timeout, TimeUnit unit) throws java.lang.InterruptedException
CompletionServicepoll in interface CompletionServicetimeout - how long to wait before giving up, in units of
unitunit - a TimeUnit determining how to interpret the
timeout parameterjava.lang.InterruptedException - if interrupted while waiting