Java多线程-线程池ExecutorService.shutdown什么时候执行

public class SemaphoreTest extends Thread {

    Semaphore position;

    private int id;

    public SemaphoreTest(int i, Semaphore s) {

        this.id = i;

        this.position = s;

    }

    public void run() {

        try {

            if (position.availablePermits() > 0) {

                System.out.println("顾客[" + id + "]进入厕所，有空位");

            } else {

                System.out.println("顾客[" + id + "]进入厕所，没空位，排队");

            }

            position.acquire(); 

            System.out.println("【" + id + "】acquire坑位");

            Thread.sleep((int) (Math.random() * 1000));

            System.out.println("【" + id + "】完毕release");

            position.release(); 

        }

        catch (Exception e) {

            e.printStackTrace();

        }

    }



    public static void main(String args[]) {

        ExecutorService pool = Executors.newCachedThreadPool();

        Semaphore position = new Semaphore(2);    // 初始化两个空位

        for (int i = 0; i < 5; i++) {

            pool.submit(new SemaphoreTest(i, position));

        }

        System.out.println("开始释放线程池资源");

        pool.shutdown();

        System.out.println("完成释放线程池资源");

        position.acquireUninterruptibly(2);  

        System.out.println("如厕完毕，清理厕所");

        position.release(2);  

    }

}