join为啥会阻塞主线程？

import java.util.concurrent.TimeUnit;

/**
 * @author ：jiaolian
 * @date ：Created in 2021-02-28 21:43
 * @description：join测试
 * @modified By：
 * 公众号:叫练
 */
public class JoinTest {

    public static void main(String[] args) throws InterruptedException {
        Thread threadA = new Thread(()->{
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName()+":想先执行");
        },"线程A");
        //开启一个线程A
        threadA.start();
        //主线程会持有子线程的锁,子线程还没开始主线程就阻塞了,等待子线程结束后通知;
        threadA.join();
        System.out.println(Thread.currentThread().getName()+ "线程执行");
    }
}

import java.util.concurrent.TimeUnit;

/**
 * @author ：jiaolian
 * @date ：Created in 2021-02-28 21:43
 * @description：join测试
 * @modified By：
 * 公众号:叫练
 */
public class JoinCodeTest {

    public static void main(String[] args) throws InterruptedException {

        MyThread threadA = new MyThread("线程A");
        //开启一个线程A
        threadA.start();
        //主线程会持有子线程的锁,子线程还没开始主线程就阻塞了,等待子线程结束后通知;
        threadA.join2(0);
        System.out.println(Thread.currentThread().getName()+ "线程执行");
    }

    private static class MyThread extends Thread {

        public MyThread(String name) {
            super(name);
        }

        @Override
        public void run() {
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName()+":想先执行");
        }

        //复制Thread源码中的join方法测试阻塞的是线程A还是main线程？
        public final synchronized void join2(long millis)
                throws InterruptedException {
            long base = System.currentTimeMillis();
            long now = 0;

            if (millis < 0) {
                throw new IllegalArgumentException("timeout value is negative");
            }

            if (millis == 0) {
                while (isAlive()) {
                    //虽然调用者是线程A，但真正执行阻塞的是main线程！
                    System.out.println(Thread.currentThread().getName()+"会阻塞");
                    wait(0);
                }
            } else {
                while (isAlive()) {
                    long delay = millis - now;
                    if (delay <= 0) {
                        break;
                    }
                    wait(delay);
                    now = System.currentTimeMillis() - base;
                }
            }
        }
    }
}