//A service used to manage completed tasks, which encapsulates a queue internally. //It is an implementation of CompletionService public class ExecutorCompletionService<V> implements CompletionService<V> // first look at the constructor 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>>(); } 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; } //Submit task and return Future public Future<V> submit(Callable<V> task) { if (task == null) throw new NullPointerException(); RunnableFuture<V> f = newTaskFor(task); executor.execute(new QueueingFuture(f)); return f; } private RunnableFuture<V> newTaskFor(Callable<V> task) { if (aes == null) return new FutureTask<V>(task); else return aes.newTaskFor(task); } private class QueueingFuture extends FutureTask<Void> { QueueingFuture(RunnableFuture<V> task) { super(task, null); this.task = task; } //This sentence is the key, when the task is completed, it is added to the queue protected void done() { completionQueue.add(task); } private final Future<V> task; } //Submit Runnable task to return Future public Future<V> submit(Runnable task, V result) { if (task == null) throw new NullPointerException(); RunnableFuture<V> f = newTaskFor(task, result); executor.execute(new QueueingFuture(f)); 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); }
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