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JavaTM 2 Platform Std. Ed. v1.5.0

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>
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

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

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.Upon completion, this task may be taken or polled.

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 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
Throws:: NullPointerException - if executor or completionQueue are null

Method Detail