Module java.base
Package java.util.concurrent

Class ExecutorCompletionService<V>

  • All Implemented Interfaces:
    CompletionService<V>
    public class ExecutorCompletionService<V>
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> cs
       = new ExecutorCompletionService<>(e);
   solvers.forEach(cs::submit);
   for (int i = solvers.size(); i > 0; i--) {
     Result r = cs.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> cs
       = new ExecutorCompletionService<>(e);
   int n = solvers.size();
   List<Future<Result>> futures = new ArrayList<>(n);
   Result result = null;
   try {
     solvers.forEach(solver -> futures.add(cs.submit(solver)));
     for (int i = n; i > 0; i--) {
       try {
         Result r = cs.take().get();
         if (r != null) {
           result = r;
           break;
         }
       } catch (ExecutionException ignore) {}
     }
   } finally {
     futures.forEach(future -> future.cancel(true));
   }

   if (result != null)
     use(result);
 }

    Since:
    1.5

    • 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<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

    • Method Detail

      • submit

        public Future<V> submit​(Callable<V> task)
        Description copied from interface: CompletionService
        Submits a value-returning task for execution and returns a Future representing the pending results of the task. Upon completion, this task may be taken or polled.
        Specified by:
        submit in interface CompletionService<V>
        Parameters:
        task - the task to submit
        Returns:
        a Future representing pending completion of the task
        Throws:
        RejectedExecutionException - if the task cannot be scheduled for execution
        NullPointerException - if the task is null

      • submit

        public Future<V> submit​(Runnable task,
                        V result)
        Description copied from interface: CompletionService
        Submits a Runnable task for execution and returns a Future representing that task. Upon completion, this task may be taken or polled.
        Specified by:
        submit in interface CompletionService<V>
        Parameters:
        task - the task to submit
        result - the result to return upon successful completion
        Returns:
        a Future representing pending completion of the task, and whose get() method will return the given result value upon completion
        Throws:
        RejectedExecutionException - if the task cannot be scheduled for execution
        NullPointerException - if the task is null