Package java.util.concurrent

Class ExecutorCompletionService<V>

java.lang.Object
java.util.concurrent.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);
 }

  • Constructor Summary

    Constructors
    Constructor Description
    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.

  • Method Summary

    Modifier and Type Method 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​(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.

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Constructor Details

    • 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 Details

    • 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

    • take

      public Future<V> take() throws InterruptedException
      Description copied from interface: CompletionService
      Retrieves and removes the Future representing the next completed task, waiting if none are yet present.
      Specified by:
      take in interface CompletionService<V>
      Returns:
      the Future representing the next completed task
      Throws:
      InterruptedException - if interrupted while waiting

    • 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