java多线程实现主线程等待子线程执行完问题

本文介绍两种主线程等待子线程的实现方式，以5个子线程来说明：

1、使用Thread的join()方法，join()方法会阻塞主线程继续向下执行。

2、使用Java.util.concurrent中的CountDownLatch，是一个倒数计数器。初始化时先设置一个倒数计数初始值，每调用一次countDown()方法，倒数值减一，他的await()方法会阻塞当前进程，直到倒数至0。

public class AppMain {

    public static void main(String[] args) throws InterruptedException {
        // TODO Auto-generated method stub
        long start = System.currentTimeMillis() ;
        System.out.println();
        ArrayList<Thread> arrayList = new ArrayList<>();
        for(int i = 0;i<100;i++)
        {
            myThread m1 =  new myThread();
                    m1.start();
                    m1.join();
            myThread2 m2 = new myThread2();
                    m2.start();
                    m2.join();
        }
        long end = System.currentTimeMillis();
        long endure = end-start;
        System.out.println("耗时："+endure);
    }

}

输出：

Thread-172 :1111 : 111
Thread-173 :2222 : 222
Thread-174 :1111 : 111
Thread-175 :2222 : 222
Thread-176 :1111 : 111
Thread-177 :2222 : 222
Thread-178 :1111 : 111
Thread-179 :2222 : 222
Thread-180 :1111 : 111
Thread-181 :2222 : 222
Thread-182 :1111 : 111
Thread-183 :2222 : 222
Thread-184 :1111 : 111
Thread-185 :2222 : 222
Thread-186 :1111 : 111
Thread-187 :2222 : 222
Thread-188 :1111 : 111
Thread-189 :2222 : 222
Thread-190 :1111 : 111
Thread-191 :2222 : 222
Thread-192 :1111 : 111
Thread-193 :2222 : 222
Thread-194 :1111 : 111
Thread-195 :2222 : 222
Thread-196 :1111 : 111
Thread-197 :2222 : 222
Thread-198 :1111 : 111
Thread-199 :2222 : 222
耗时：68

如果不采用join()方法的话，会先执行完main线程，子线程继续执行。则不会正确打印出耗时时间

输出如下：

Thread-181 :2222 : 222
Thread-182 :1111 : 111
Thread-183 :2222 : 222
Thread-184 :1111 : 111
Thread-185 :2222 : 222
Thread-186 :1111 : 111
Thread-187 :2222 : 222
Thread-188 :1111 : 111
Thread-189 :2222 : 222
Thread-190 :1111 : 111

耗时：68----------------------------------会发现主线程先执行完了
Thread-191 :2222 : 222
Thread-192 :1111 : 111
Thread-193 :2222 : 222
Thread-194 :1111 : 111
Thread-195 :2222 : 222
Thread-196 :1111 : 111
Thread-197 :2222 : 222
Thread-198 :1111 : 111
Thread-199 :2222 : 222