Java线程join（）混乱

Question

我试图按照此示例以了解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");
      }
   }
}

我了解到T1.join（）例如使主线程等待T1完成以继续其流程。 我在这里正确吗？

因此，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.

我无法解释...

为什么在“ 5”字符之后打印柜台印刷品？ 我可能不太了解加入问题...

Answer 1

您启动一个线程（A），该线程又启动了另一个线程（B）。

您等待线程A完成-这几乎是立即执行的，因为它除了启动线程B外几乎没有做其他任何事情-但线程B仍在独立运行。

Answer 2

正如@Micheal所说，您的主线程未加入“ desire”线程，也许这段代码可以帮助您进行说明。 它有助于主线程与内部线程连接。

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 void joinInsideThread() throws InterruptedException {
       if (t != null){
           t.join();
       }
   }
}

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);

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

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

Answer 3

我正在尝试...了解join（）方法

t.join()非常容易理解。 它所做的只是等待线程t死亡。 它什么也没做。 最特别的是，它不会对t线程执行任何操作。

调用t.join()不会执行任何操作，如果t已经完成，则立即返回。 否则，它会做什么，它不会返回，直到t 完成 。