ExecutorCompletionService

public class ExecutorCompletionService extends Object
implements CompletionService<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<>(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);
 }

Public Constructor Summary

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.

Public Method Summary

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