为什么主线程中的Thread.sleep（）会导致子线程无法关闭

Question

在下面的示例中，我有一个主程序，可以创建6个任务和6个线程（每个任务1个）。如您所见，代码非常简单。我创建并启动了6个线程然后我想通过共享变量/ flag shutdownIssued 来停止所有线程。

但是，我看到主程序和线程没有终止（它们继续运行，我必须强行终止它们）。但是如果我先注释掉Thread.sleep(1000)（即，在所有线程上调用shutdown()之前），该程序将按预期工作。

1：为什么在当前（主）线程上调用Thread.sleep()并不会导致线程停止（这意味着线程内的共享变量 shutdownIssued 不是真正通过主线程调用shutdown（）来更新。

2：当我第一次发表评论Thread.sleep(1000)时，为什么这会按预期工作？

示例代码：

package com.examples.threads;

import java.util.ArrayList;
import java.util.List;

public class ThreadExample {
    public static void main(String[] args) {

        //create 6 tasks and 6 threads to run them
        int numTasks = 6;
        List<Task> tasks = new ArrayList<Task>();
        List<Thread> threads = new ArrayList<Thread>();
        for (int i = 0; i < numTasks; i++) {
            tasks.add(new Task());
            Thread th = new Thread(tasks.get(i));
            threads.add(th);
            th.start();
        }

        //Wait for 2 seconds before issuing shutdown
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        //Now issue shutdown request to all the threads to stop execution of their assigned task
        for (int i = 0; i < numTasks; i++) {
            tasks.get(i).shutdown();
        }

        //Wait for 2 seconds after issuing shutdown
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        //Check if threads are still alive
        for (int i = 0; i < numTasks; i++) {
            System.out.println("Thread " + threads.get(i).getName() + " is alive = " + threads.get(i).isAlive());
        }

        System.out.println("finished shutdown of all the threads!!");
    }

    public static class Task implements Runnable {
        private boolean shutdownIssued = false;
        private String name;

        public void shutdown() {
            shutdownIssued = true;
            System.out.println("Issued shutdown to " + name);
        }

        public String getName() {
            return name;
        }

        public void run() {
            this.name = Thread.currentThread().getName();
            System.out.println("started thread " + name);
            int cnt = 0;
            while (!shutdownIssued) {
                //System.out.println("inside thread " + Thread.currentThread().getName());
                cnt++;
            }
            System.out.println("Finished thread " + name + ", count = " + cnt);
        }

        public boolean isShutdownIssues() {
            return shutdownIssued;
        }
    }

}