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Thread.join()

cThread.join()方法使当前线程阻塞，直到子线程cThread执行完毕后，当前线程才会恢复运行。

实现原理：

1. join()方法调用了join(0)

   public final void join() throws InterruptedException {
       join(0);
   }
   

2. join(long millis)是一个同步方法，最后会通过调用wait()方法挂起当前线程（即调用线程），直到其他线程调用子线程cThread的notify()或者notifyAll()方法

   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;
           }
       }
   }
   

3. 子线程run()执行完毕后，系统在关闭该子线程前，会调用其exit()方法，继而在ThreadGroup.threadTerminated(Thread t)中唤醒被阻塞的调用线程。

   /**
    * This method is called by the system to give a Thread
    * a chance to clean up before it actually exits.
    */
   private void exit() {
       if (group != null) {
           group.threadTerminated(this);//提心 ThreadGroup 当前线程已经被终止
           group = null;
       }
   
       //其他代码 ……
   }
   
   // ThreadGroup
   void threadTerminated(Thread t) {
       synchronized (this) {
           remove(t);
   
           if (nthreads == 0) {//线程组线程数为0时
               notifyAll();//唤醒所有等待中的线程
           }
           if (daemon && (nthreads == 0) &&
               (nUnstartedThreads == 0) && (ngroups == 0))
           {
               destroy();
           }
       }
   }