求助，java join 的原理

Thread 类中，join 方法的实现：

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;
            }
        }
    }

比如：

	public static void main(String[] args) throws InterruptedException {
		System.out.println(Thread.currentThread().getName() + " at begin ");
		Thread t1 = new Thread(new Runnable(){
			public void run() {
				System.out.println(" Thread t1 stats");
				try {
					Thread.sleep(5000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println("Thread t1 ends");
			}
		});
		
		t1.start();
		t1.join();// t1调用join();t1会wait(0),那么t1是如何让main线程等待的呢
		
		System.out.println(Thread.currentThread().getName() + " at end ");
	}