edu.emory.mathcs.backport.java.util.concurrent

Class ExecutorCompletionService

public class ExecutorCompletionService extends Object implements CompletionService

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);
   }
 
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 completionQueue)
Creates an ExecutorCompletionService using the supplied executor for base task execution and the supplied queue as its completion queue.
Method Summary
Futurepoll()
Futurepoll(long timeout, TimeUnit unit)
Futuresubmit(Callable task)
Futuresubmit(Runnable task, Object result)
Futuretake()

Constructor Detail

ExecutorCompletionService

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

ExecutorCompletionService

public ExecutorCompletionService(Executor executor, BlockingQueue 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 taskes cause them not to be retrievable.

Throws: NullPointerException if executor or completionQueue are null

Method Detail

poll

public Future poll()

poll

public Future poll(long timeout, TimeUnit unit)

submit

public Future submit(Callable task)

submit

public Future submit(Runnable task, Object result)

take

public Future take()