我不明白为什么要显式调用executorService.shutdown()来终止executorService。如果我不调用shutdown()，那么JVM不会自行终止。
我的程序出了什么问题，或者我缺少了什么概念？

public class ExecutorServiceExample {

    public static class Task1 implements Runnable {

        @Override
        public void run() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("Message from Task1 :"
                    + Thread.currentThread().getName());
        }

    }

    public static class Task2 implements Runnable {

        @Override
        public void run() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Message from Task2 :"
                    + Thread.currentThread().getName());
        }

    }

    public static class Task3 implements Runnable {

        @Override
        public void run() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Message from Task3 :"
                    + Thread.currentThread().getName());
        }

    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);

        Future future1 = executorService.submit(new Task1());
        Future future2 = executorService.submit(new Task2());
        Future future3 = executorService.submit(new Task3());

    }

}

输出：
来自Task 2的消息：pool-1-thread-2
来自Task 1的消息：pool-1-thread-1
来自Task 3的消息：pool-1-thread-3
JVM还活着。如果我调用shutdown（），那么只有JVM会死。