/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; /** * A {@link CompletionService} that uses a supplied {@link Executor} * to execute tasks. This class arranges that submitted tasks are, * upon completion, placed on a queue accessible using {@code 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 {@code Result}, and would like to * run them concurrently, processing the results of each of them that * return a non-null value, in some method {@code use(Result r)}. You * could write this as: * *

 {@code
 * void solve(Executor e,
 *            Collection> solvers)
 *     throws InterruptedException, ExecutionException {
 *     CompletionService ecs
 *         = new ExecutorCompletionService(e);
 *     for (Callable 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: * *
 {@code
 * void solve(Executor e,
 *            Collection> solvers)
 *     throws InterruptedException {
 *     CompletionService ecs
 *         = new ExecutorCompletionService(e);
 *     int n = solvers.size();
 *     List> futures
 *         = new ArrayList>(n);
 *     Result result = null;
 *     try {
 *         for (Callable 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 f : futures)
 *             f.cancel(true);
 *     }
 *
 *     if (result != null)
 *         use(result);
 * }}
*/ public class ExecutorCompletionService implements CompletionService { private final Executor executor; private final AbstractExecutorService aes; private final BlockingQueue> completionQueue; /** * FutureTask extension to enqueue upon completion */ private class QueueingFuture extends FutureTask { QueueingFuture(RunnableFuture task) { super(task, null); this.task = task; } protected void done() { completionQueue.add(task); } private final Future task; } private RunnableFuture newTaskFor(Callable task) { if (aes == null) return new FutureTask(task); else return aes.newTaskFor(task); } private RunnableFuture newTaskFor(Runnable task, V result) { if (aes == null) return new FutureTask(task, result); else return aes.newTaskFor(task, result); } /** * Creates an ExecutorCompletionService using the supplied * executor for base task execution and a * {@link LinkedBlockingQueue} as a completion queue. * * @param executor the executor to use * @throws NullPointerException if executor is {@code null} */ public ExecutorCompletionService(Executor executor) { if (executor == null) throw new NullPointerException(); this.executor = executor; this.aes = (executor instanceof AbstractExecutorService) ? (AbstractExecutorService) executor : null; this.completionQueue = new LinkedBlockingQueue>(); } /** * Creates an ExecutorCompletionService using the supplied * executor for base task execution and the supplied queue as its * completion queue. * * @param executor the executor to use * @param 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 * {@code Queue.add} operations for completed taskes cause * them not to be retrievable. * @throws NullPointerException if executor or completionQueue are {@code null} */ public ExecutorCompletionService(Executor executor, BlockingQueue> completionQueue) { if (executor == null || completionQueue == null) throw new NullPointerException(); this.executor = executor; this.aes = (executor instanceof AbstractExecutorService) ? (AbstractExecutorService) executor : null; this.completionQueue = completionQueue; } public Future submit(Callable task) { if (task == null) throw new NullPointerException(); RunnableFuture f = newTaskFor(task); executor.execute(new QueueingFuture(f)); return f; } public Future submit(Runnable task, V result) { if (task == null) throw new NullPointerException(); RunnableFuture f = newTaskFor(task, result); executor.execute(new QueueingFuture(f)); return f; } public Future take() throws InterruptedException { return completionQueue.take(); } public Future poll() { return completionQueue.poll(); } public Future poll(long timeout, TimeUnit unit) throws InterruptedException { return completionQueue.poll(timeout, unit); } }