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<V>
submit(Runnable task, V result)
Submits a Runnable task for execution and returns a Future representing that task.
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>
take()
Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Inherited Method Summary

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
Throws
NullPointerException if executor is null

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. This queue is treated as unbounded -- failed attempted Queue.add operations for completed tasks cause them not to be retrievable.
Throws
NullPointerException if executor or completionQueue are null

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.

Parameters
timeout how long to wait before giving up, in units of unit
unit a TimeUnit determining how to interpret the timeout parameter
Returns
  • the Future representing the next completed task or null if the specified waiting time elapses before one is present

public