老师，根据你总结的，同步非静态方法，锁的是当前对象的实例对象，理解 t.join() 应该是锁的 t 对象，但实际中，为什么join方法锁的又是主线程对象，调用的主线程的wait(0)呢？
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;
        }
    }
}