Module java.base

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
      • poll

        public Future<V> poll​()
        Description copied from interface: CompletionService
        Retrieves and removes the Future representing the next completed task, or null if none are present.
        Specified by:
        poll in interface CompletionService<V>
        Returns:
        the Future representing the next completed task, or null if none are present
      • poll

        public Future<V> poll​(long timeout,
                              TimeUnit unit)
                       throws InterruptedException
        Description copied from interface: CompletionService
        Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.
        Specified by:
        poll in interface CompletionService<V>
        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
        Throws:
        InterruptedException - if interrupted while waiting