【Android笔记】Thread类中关于join()方法的源码分析

1、join()方法的作用：

例如有一个线程对象为Thread1，在main()方法中调用Thread1.join()方法可使得当前线程(即主线程)阻塞，而执行Thread1线程。

2、源码分析(以上面的例子为例)

/**

 * Waits at most {@code millis} milliseconds for this thread to
 * die. A timeout of {@code 0} means to wait forever.
 *
 * <p> 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  (等待时间为毫秒)
 *         the time to wait in milliseconds
 *
 * @throws  IllegalArgumentException (当 millis 为负数时抛出此异常)

 *          if the value of {@code millis} is negative
 *
 * @throws  InterruptedException (如果有任何线程中断了当前线程（当前线程为main线程），抛出此异常，当前线程的中断状态将被清除。该异常由源码中的wait()抛出)

 *          if any thread has interrupted the current thread. The
 *          <i>interrupted status</i> of the current thread is
 *          cleared when this exception is thrown.
 */

//这段代码是由main线程执行的
public final void join(long millis) throws InterruptedException {
        synchronized(lock) { //这里的lock是在Thread1中创建的，源码为 private final Object lock=new Object();
//main线程获取对象lock的锁
        long base = System.currentTimeMillis();//获取系统当前时间毫秒值
        long now = 0;    //表示等待了多少时间

        if (millis < 0) { //抛出 millis 为负数的异常
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {     //毫秒值为零，意味着等待到Thread1结束
            while (isAlive()) { //当Thread1线程还活着时
                lock.wait(0);    //使得拥有lock对象的锁的线程(main线程)停止，并使得当前线程释放锁，那什么时候调用notifyAll呢？这个答案在 jvm源码中，在此不细述
            }                     //中断异常也是由wait()方法抛出
        } else {   //否则，等待millis
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;        // 等待时间结束，跳出了join()方法，main线程继续执行
                }
                lock.wait(delay);   //延时等待
                now = System.currentTimeMillis() - base; //更新now
            }
        }
        }
    }