Executor不返回10个未来对象

Question

我有一个线程池为10的executor服务，我希望得到10条打印输出语句，它们之间的间隔为3秒，但我只收到一条打印输出语句。我传递了10作为参数，所以我希望运行10个线程。如何检索未来的10个对象？

public class Demo {
    private static final ExecutorService executor = Executors.newFixedThreadPool(10);

    public static void main (String[] args) throws ExecutionException, InterruptedException {

        ArrayList futureObjects = new ArrayList();

        Callable<Integer> task = () -> {
            try {
                TimeUnit.SECONDS.sleep(3);
                return 123;
            }
            catch (InterruptedException e) {
                throw new IllegalStateException("task interrupted", e);
            }
        };

        System.out.println("Before execution of threads");

        Future<Integer> future = executor.submit(task);

        Integer result = future.get();
        futureObjects.add(future.get());

        System.out.println("result: " + result);

        for(Object futures : futureObjects ){
            System.out.println("Futures in ArrayList: " + futures);
        }
    }

}

我得到的输出是：

线程执行前

结果：123

ArrayList中的期货：123

Answer 1

创建的执行器将尝试在10个线程中并行执行任务，但每个提交的任务将只执行一次。

Answer 2

你实际上只增加了一个任务

您需要同时提交多个任务（使用下面的选项1或选项2），以便实际使用线程池（使线程保持繁忙）。

您可以在下面查看代码的更新版本：

选项（1）：ExecutorService-invokeAll（）：

private static final ExecutorService executor = Executors.newFixedThreadPool(10);

    public static void main (String[] args) throws ExecutionException, InterruptedException {

        ArrayList futureObjects = new ArrayList();

        Callable<Integer> task = () -> {
            try {
                TimeUnit.MILLISECONDS.sleep(100);
                return 123;
            }
            catch (InterruptedException e) {
                throw new IllegalStateException("task interrupted", e);
            }
        };

        List<Callable<Integer>> callables = new ArrayList<>();
        callables.add(task);
        callables.add(task);
        callables.add(task);
        callables.add(task);
        //Add other tasks

        System.out.println("Before execution of threads");

        List<Future<Integer>> futures = executor.invokeAll(callables);

        for(Future future : futures ){
            System.out.println("Futures in ArrayList: " + future.get());
        }
    }

选项（2）：ExecutorService-提交（）：

private static final ExecutorService executor = Executors.newFixedThreadPool(10);

    public static void main (String[] args) throws ExecutionException, InterruptedException {

        ArrayList futureObjects = new ArrayList();

        Callable<Integer> task = () -> {
            try {
                TimeUnit.MILLISECONDS.sleep(100);
                return 123;
            }
            catch (InterruptedException e) {
                throw new IllegalStateException("task interrupted", e);
            }
        };

        List<Callable<Integer>> callables = new ArrayList<>();
        callables.add(task);
        callables.add(task);
        callables.add(task);
        callables.add(task);
        //Add other tasks

        List<Future<Integer>> futures = new ArrayList<>();
        System.out.println("Before execution of threads");

        for(Callable<Integer> callable : callables) {
            futures.add(executor.submit(callable));
        }

        for(Future future : futures ){
            System.out.println("Futures in ArrayList: " + future.get());
        }
    }

您可以在此处参考API