ExecutorCompletionService

Class Overview

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);
    }
 

Summary

Public Constructors
	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.

Public Methods
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>	poll() Retrieves and removes the Future representing the next completed task or null if none are 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.

[Expand] Inherited Methods
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals(Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Is called before the object's memory is being reclaimed by the VM. final Class<? extends Object> getClass() Returns the unique instance of Class which represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.
From interface java.util.concurrent.CompletionService abstract 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. abstract Future<V> poll() Retrieves and removes the Future representing the next completed task or null if none are present. abstract Future<V> submit(Callable<V> task) Submits a value-returning task for execution and returns a Future representing the pending results of the task. abstract Future<V> submit(Runnable task, V result) Submits a Runnable task for execution and returns a Future representing that task. abstract Future<V> take() Retrieves and removes the Future representing the next completed task, waiting if none are yet present.