多线程等待任务结束的几种方法

比如，主线程创建线程池，提交n个任务，想让主线程在任务全部结束之后再继续做其他的事情。

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1、使用awaitTermination方法

public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(3);
        int i = 0;
        AtomicInteger result = new AtomicInteger(0);
        while (i < 10) {
            i++;
            executor.execute(() -> {
                //每执行一次，对result加1
                System.out.println(result.addAndGet(1));
            });
        }
        System.out.println("调用shutdown()方法时，result的值为：" + result.get());
        executor.shutdown();
        try {
            //等待所有任务结束，最多等待30分钟
            executor.awaitTermination(30, TimeUnit.MINUTES);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("计算完成，result的值为：" + result.get() + "，可以继续处理其他事情了");
    }

boolean awaitTermination(long timeout, TimeUnit unit)的作用：

Blocks until all tasks have completed execution after a shutdown request,
or the timeout occurs, or the current thread is interrupted,
whichever happens first.

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2、借助工具类CountDownLatch

public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(3);
        CountDownLatch latch = new CountDownLatch(10);
        AtomicInteger result = new AtomicInteger(0);
        for (int i = 1; i <= 10; i++) {
            executor.execute(() -> {
                //do something...
                System.out.println(result.addAndGet(1));
                //then
                latch.countDown();
            });
        }
        System.out.println("调用shutdown()方法时，result的值为：" + result.get());
        executor.shutdown();
        try {
            latch.await();//等待任务结束：其实是等待latch值为0
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("计算完成，result的值为：" + result.get() + "，可以继续处理其他事情了");
}

适用于知道任务的数量，因为CountDownLatch在创建时就要指定其大小，并且不能重新初始化。

//TODO：CountDownLatch是否会禁止指令重排序？(从官方的例子看，会！有待研究)

public void example() {
        CountDownLatch doneSignal = new CountDownLatch(10);
        Executor e = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 10; ++i) // create and start threads
            e.execute(new WorkerRunnable(doneSignal, i));
        try {
            doneSignal.await();           // wait for all to finis
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }
    }

    class WorkerRunnable implements Runnable {
        private final CountDownLatch doneSignal;
        private final int i;

        WorkerRunnable(CountDownLatch doneSignal, int i) {
            this.doneSignal = doneSignal;
            this.i = i;
        }

        public void run() {
            doWork(i);
            doneSignal.countDown();
        }

        void doWork(int i) {
        }
    }

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其他

//TODO