Thread.join源码解析

/*
 *
 * Waits for this thread to die.
 *等待该线程终止的时间最长为 millis 毫秒。超时为 0 意味着要一直等下去。
 * 
 An invocation of this method behaves in exactly the same
 * way as the invocation
 *
 * 

 * {@linkplain #join(long) join}{@code (0)}
 * 
 * @throws  InterruptedException
 * 如果任何线程中断了当前线程。当抛出该异常时，当前线程的中断状态 被清除
 *          if any thread has interrupted the current thread. The
 *          interrupted status of the current thread is
 *          cleared when this exception is thrown.
 */
public final void join() throws InterruptedException {
    join(0);
}

/**
 * Waits at most {@code millis} milliseconds for this thread to
 * die. A timeout of {@code 0} means to wait forever.
 *
 * 
 This implementation uses a loop of {@code this.wait} calls
 * conditioned on {@code this.isAlive}. As a thread terminates the
 * {@code this.notifyAll} method is invoked. It is recommended that
 * applications not use {@code wait}, {@code notify}, or
 * {@code notifyAll} on {@code Thread} instances.
 *
 * @param  millis
 *         the time to wait in milliseconds
 *
 * @throws  IllegalArgumentException
 *          if the value of {@code millis} is negative
 *
 * @throws  InterruptedException
 *          if any thread has interrupted the current thread. The
 *          interrupted status of the current thread is
 *          cleared when this exception is thrown.
 */
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;
        }
    }
}

isAlive()方法：判断当前线程是否处于活动状态.(“活动状态”是指线程处于运行或者准备开始运行的状态。)

我的理解是：通过while (isAlive())判断线程是不是start（）了，其mills = 0，然后执行wait（0）；

可以看到在没有start（），直接join（）它的mills = 0，但是它的isAlive（）= false。