Thread.join方法的疑问

Question

join的方法是使主线程阻塞，可是从源码看哪里看出是主线程阻塞而不是子线程呢？

wait()哪里说命了是主线程。还有isAlive哪里说明了是判断主线程的存活状态。

public static void main(String[] agrs){
    Thread a = new Thread();
    a.join();
    System.out.pring("最后执行");
}

public final synchronized void join(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()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

Answer 1

wait最终会转换成不同操作系统的进程阻塞的调用。也就是当线程失去cpu调度的时候，wait就会挂起，直到重新获得cpu。