Java_并发线程_CompletionService

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1.CompletionService源码分析

CompletionService内部实现还是维护了一个可阻塞的队列，通过代理设计模式，从而操作队列。

    /**     * 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>>(); //新建一个完成队列    }

//通过submit提交Callable任务对象    public Future<V> submit(Callable<V> task) {        if (task == null) throw new NullPointerException();        RunnableFuture<V> f = newTaskFor(task);        executor.execute(new QueueingFuture(f));//线程池执行task对象        return f;    }

    /**     * FutureTask extension to enqueue upon completion     */    private class QueueingFuture extends FutureTask<Void> {        QueueingFuture(RunnableFuture<V> task) {            super(task, null);            this.task = task;        }        protected void done() {         completionQueue.add(task); //执行玩后将task返回对象放置于完成队列        }        private final Future<V> task;    }

//通过take方法取得Future对象    public Future<V> take() throws InterruptedException {        return completionQueue.take();    }

2.实例

public static void main(String[] args) {ExecutorService threadPool = Executors.newFixedThreadPool(3);CompletionService<Integer> completionService = new ExecutorCompletionService<Integer>(threadPool);//将任务添加至threadPool池中，但是只分配3个Thread对象for (int i = 1; i <= 10; i++) {final int seq = i;completionService.submit(new Callable<Integer>() {@Overridepublic Integer call() throws Exception {Thread.sleep(new Random().nextInt(5000));return seq;}});}for (int i = 0; i < 10; i++) {try {//completionService.take(), 至于call方法执行完成，take阻塞采用数据//future.get() 阻塞， 只有当call执行完成，System.out.println(completionService.take().get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}}

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