java - 需要了解我的示例中的 wait() 和 notification() 行为吗？

Question

public class class_Q {
volatile boolean valueSet = false;
volatile int n;



synchronized int get()
{  
System.out.println("Now i am in get block and  valueset is : "+ valueSet  );
    if(!valueSet)
    {
        System.out.println("i am waiting in get block.....and releasing lock  ");
    try{
        wait();
    }catch(InterruptedException e)
        {
    System.out.println( "InterruptedException caught" );
        }
    }
    System.out.println( " value of n now in get block is  : " + n );
    valueSet=false;
    notify(); 
    return n; 
}


   synchronized void put(int n)
     {  
System.out.println(" Now i am in Put block and  valueset is : "+ valueSet);
if(valueSet) 
{
try
{
System.out.println("i am waiting in put block......and releasing lock. ");
wait();
}catch(InterruptedException e)
{
System.out.println( "InterruptedException caught" );
}
}
  this.n = n;
  valueSet = true;
  System.out.println( "the value of n now in put block is : " + n );
  notify();   
  }

}

 class Producer implements Runnable{
class_Q q;
Producer(class_Q q)
{
this.q = q;
new Thread( this, "Producer" ).start();
}
public void run()
{
    int i = 0;
    while(true)
    {
    q.put(i++);
    }
    }
} 


 class Consumer implements Runnable{
class_Q q;
 Consumer(class_Q q)
 {
    this.q = q;
    new Thread(this, "Consumer").start();
    }
  public void run()
  {
    while(true)
    {
        q.get();
        }
    }
  } 


  class PCFixed {
public static void main (String args[])
{
    class_Q q = new class_Q();
    new Producer(q);
    new Consumer(q);
    System.out.println( "Press Control-C to stop." );   
    }
} 

*OUTPUT**

现在我处于 get block 中，值集是: false

我正在等待 get block ......并释放锁

按 Control-C 停止。

现在我在 Put block 中，值集是: false

现在 put block 中 n 的值为:0

现在 get block 中 n 的值为:0

现在我处于 get block 中，值集是: false

我正在等待 get block ......并释放锁

现在我在 Put block 中，值集是: false

现在 put block 中 n 的值为:1

现在 get block 中 n 的值为:1

在输出的第六行之后，我期望 get() 线程唤醒(“notify()”)put() 线程。有人可以帮助我理解调用 get() 线程背后的逻辑(换句话说，为什么它在 get block 中？)

Answer 1

我已经重新格式化了您的代码并更改了日志消息，因此一切都应该更加清晰。

public class Test {
  static class class_Q {
    volatile boolean valueSet = false;
    volatile int n;

    synchronized int get() throws InterruptedException {
      System.out.println("get entering - valueSet=" + valueSet);
      // *** Changed from `if` to `while`
      while (!valueSet) {
        System.out.println("get waiting");
        wait();
      }
      // Clear to set the value.
      valueSet = false;
      // Tell any put waits to finish
      notify();
      System.out.println("get finished - n=" + n);
      return n;
    }

    synchronized void put(int n) throws InterruptedException {
      System.out.println("put entering - valueSet=" + valueSet);
      // *** Changed from `if` to `while`
      while (valueSet) {
        System.out.println("put waiting");
        wait();
      }
      this.n = n;
      valueSet = true;
      System.out.println("put finished - n=" + n);
      notify();
    }
  }

  static class Producer implements Runnable {
    class_Q q;

    Producer(class_Q q) {
      this.q = q;
    }

    public void run() {
      int i = 0;
      try {
        while (true) {
          q.put(i++);
          System.out.println("put(" + (i-1) + ")");
        }
      } catch (InterruptedException ex) {
        // Just exit the run loop and finish when interrupted.
      }
    }
  }

  static class Consumer implements Runnable {
    class_Q q;

    Consumer(class_Q q) {
      this.q = q;
    }

    public void run() {
      try {
        while (true) {
          int i;
          i = q.get();
          System.out.println("get(" + i + ")");
        }
      } catch (InterruptedException ex) {
        // Just exit the run loop and finish when interrupted.
      }
    }
  }

  public static void main(String args[]) {
    class_Q q = new class_Q();
    Thread producer = new Thread(new Producer(q));
    Thread consumer = new Thread(new Consumer(q));
    System.out.println("Press Control-C to stop.");
    producer.start();
    consumer.start();
  }
}

我也做了三项主要改变。我已经使中断机制只是退出你的线程。我已经让您的阻塞测试在阻塞状态下循环，而不仅仅是检查它们(while (x) 而不是 if (x))。我已经让你的线程不自动启动。

我认为如果您现在运行这段代码，流程应该会更清晰，您应该能够更好地理解正在发生的事情。请记住，System.out 是一个PrintWriter，因此可以进行缓冲。

我得到的输出是:

put entering - valueSet=falseput finished - n=0put(0)put entering - valueSet=trueput waitingPress Control-C to stop.get entering - valueSet=trueget finished - n=0put finished - n=1put(1)put entering - valueSet=trueput waitingget(0)get entering - valueSet=trueget finished - n=1put finished - n=2put(2)put entering - valueSet=trueput waitingget(1)get entering - valueSet=trueget finished - n=2get(2)get entering - valueSet=falseget waitingput finished - n=3get finished - n=3get(3)get entering - valueSet=falseget waitingput(3)put entering - valueSet=falseput finished - n=4get finished - n=4get(4)get entering - valueSet=falseget waitingput(4)...