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Android Reference

ExecutorCompletionService


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> 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);
     }
    • Method Summary

      Methods
      Modifier and Type Method and Description
      Future<V> poll()
      Retrieves and removes the Future representing the next completed task or null if none are present.
      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> submit(Callable<V> task)
      Submits a value-returning task for execution and returns a Future representing the pending results of the task.
      Future<V> submit(Runnable task, V result)
      Submits a Runnable task for execution and returns a Future representing that task.
      Future<V> take()
      Retrieves and removes the Future representing the next completed task, waiting if none are yet present.
    • 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 taskes 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
      • 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
      • 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


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