t.join()方法只会使主线程进入等待池并等待t线程执行完毕后才会被唤醒。join源码中，只会调用wait方法，并没有在结束时调用notify，这是因为线程在die的时候会自动调用自身的notifyAll方法，来释放所有的资源和锁。

1 使用案例

public static void main(String[] args) {
    Runnable runnable = new Runnable() {
			@Override
			public void run() {
				System.out.println("子线程执行");
			}
		};
    Thread thread1 = new Thread(runnable);
    Thread thread2 = new Thread(runnable);
    thread1.start();
    thread2.start();
    try {
        //主线程开始等待子线程thread1，thread2
        thread1.join();
        thread2.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    //等待两个线程都执行完（不活动）了，才执行下行打印
    System.out.println("执行完毕");
}

2 源码分析

  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);           //join(0)等同于wait(0)，即wait无限时间直到被notify
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }