Java线程join()混淆

Question

我试图按照这个例子来理解join()方法:

class PrintDemo {

   public void printCount() {

      try {

         for(int i = 5; i > 0; i--) {
            System.out.println("Counter   ---   "  + i );
         }
      } catch (Exception e) {
         System.out.println("Thread  interrupted.");
      }
   }
}

class ThreadDemo extends Thread {
   private Thread t;
   private String threadName;
   PrintDemo  PD;

   ThreadDemo(String name,  PrintDemo pd) {
      threadName = name;
      PD = pd;
   }

   public void run() {

      synchronized(PD) {
         PD.printCount();
      }
      System.out.println("Thread " +  threadName + " exiting.");
   }

   public void start () {
      System.out.println("Starting " +  threadName );

      if (t == null) {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
}

public class TestThread {

   public static void main(String args[]) {
      PrintDemo PD = new PrintDemo();

      ThreadDemo T1 = new ThreadDemo("Thread - 1 ", PD);
      ThreadDemo T2 = new ThreadDemo("Thread - 2 ", PD);

      T1.start();
      T2.start();

      // wait for threads to end
      try {
         T1.join();
         T2.join();
      } catch (Exception e) {
         System.out.println("Interrupted");
      }
   }
}

据我所知,T1.join()使主线程等待T1完成以继续其流程.我在这里纠正吗？

所以,我修改了这样的代码:

      System.out.println("1");
      T1.start();
      System.out.println("2");
      T2.start();

      // wait for threads to end
      try {
         System.out.println("3");
         T1.join();
         System.out.println("4");
         T2.join();
         System.out.println("5");
      } catch (Exception e) {
         System.out.println("Interrupted");
      }

为了尝试并遵循完整的流程.

这就是我每次都得到的:

1
Starting Thread - 1 
2
Starting Thread - 2 
3
4
5
Counter   ---   5
Counter   ---   4
Counter   ---   3
Counter   ---   2
Counter   ---   1
Thread Thread - 1  exiting.
Counter   ---   5
Counter   ---   4
Counter   ---   3
Counter   ---   2
Counter   ---   1
Thread Thread - 2  exiting.

而且我无法解释它......

为什么计数器打印在"5"字符后打印？我可能不太了解加入问题......

Answer 1

你启动一个线程(A),它启动另一个线程(B).

你等待线程A完成 - 这几乎是立即的,因为它除了启动线程B之外几乎没有做任何事情 - 但是线程B仍然是独立运行的.