在main函數(shù)里面調(diào)用了join，查看了join的源碼，調(diào)用join會(huì)轉(zhuǎn)換為調(diào)用wait(0)，比如下面的代碼

public static void main(String[] args) {
    Thread r1 = new Thread(new X("r1"))
    r1.start();
    try {
        r1.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println("Main finished");
}

但是，一般說A調(diào)用了wait則需要使用notify或者notifyAll喚醒線程A，而在join的代碼當(dāng)中并沒有看到這兩個(gè)函數(shù)對(duì)main線程的notify，那么main是怎么從join里面的wait(0)出來(lái)再進(jìn)行循環(huán)判斷的？（下面是join的源碼，調(diào)用join()，在join()內(nèi)部會(huì)直接調(diào)用join(0)，也就是下面這個(gè)函數(shù)）

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;
        }
    }
}