| /* |
| * 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. |
| * |
| * <p> |
| * |
| * <b>Usage Examples.</b> |
| * |
| * 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: |
| * |
| * <pre> {@code |
| * 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); |
| * } |
| * }}</pre> |
| * |
| * 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: |
| * |
| * <pre> {@code |
| * 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<>(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); |
| * }}</pre> |
| */ |
| public class ExecutorCompletionService<V> implements CompletionService<V> { |
| private final Executor executor; |
| private final AbstractExecutorService aes; |
| private final BlockingQueue<Future<V>> completionQueue; |
| |
| /** |
| * FutureTask extension to enqueue upon completion. |
| */ |
| private static class QueueingFuture<V> extends FutureTask<Void> { |
| QueueingFuture(RunnableFuture<V> task, |
| BlockingQueue<Future<V>> completionQueue) { |
| super(task, null); |
| this.task = task; |
| this.completionQueue = completionQueue; |
| } |
| private final Future<V> task; |
| private final BlockingQueue<Future<V>> completionQueue; |
| protected void done() { completionQueue.add(task); } |
| } |
| |
| private RunnableFuture<V> newTaskFor(Callable<V> task) { |
| if (aes == null) |
| return new FutureTask<V>(task); |
| else |
| return aes.newTaskFor(task); |
| } |
| |
| private RunnableFuture<V> newTaskFor(Runnable task, V result) { |
| if (aes == null) |
| return new FutureTask<V>(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<Future<V>>(); |
| } |
| |
| /** |
| * 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 tasks cause |
| * them not to be retrievable. |
| * @throws NullPointerException if executor or completionQueue are {@code null} |
| */ |
| public ExecutorCompletionService(Executor executor, |
| BlockingQueue<Future<V>> 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<V> submit(Callable<V> task) { |
| if (task == null) throw new NullPointerException(); |
| RunnableFuture<V> f = newTaskFor(task); |
| executor.execute(new QueueingFuture<V>(f, completionQueue)); |
| return f; |
| } |
| |
| public Future<V> submit(Runnable task, V result) { |
| if (task == null) throw new NullPointerException(); |
| RunnableFuture<V> f = newTaskFor(task, result); |
| executor.execute(new QueueingFuture<V>(f, completionQueue)); |
| return f; |
| } |
| |
| public Future<V> take() throws InterruptedException { |
| return completionQueue.take(); |
| } |
| |
| public Future<V> poll() { |
| return completionQueue.poll(); |
| } |
| |
| public Future<V> poll(long timeout, TimeUnit unit) |
| throws InterruptedException { |
| return completionQueue.poll(timeout, unit); |
| } |
| |
| } |