Synchronization is not working as expected

[rakesh kadulkar]

Hi this is my code

class Reader1 extends Thread {
     Calculator c;
	 int no;

     public Reader1(Calculator calc,int no) {
        c = calc;
		c.no=no;

     }

     public void run() {
        synchronized(c) {
           try {
              System.out.println("Waiting for calculation...");
               c.wait ( ) ;
           } catch (InterruptedException e) {}
           System.out.println("Total is: " + c.total);
        }
     }

     public static void main(String [] args) {
        Calculator calculator = new Calculator();
        //new Reader1(calculator,20).start();
        //new Reader1(calculator,30).start();
        Reader1 ref = new Reader1(calculator,40);
		ref.start();
        calculator.start();
     }
  }

  class Calculator extends Thread {
     int total;
     int no;
     public void run() {
        synchronized(this) {
           for(int i=0;i<100;i++) {
              total += no;
           }
           
        }
     }
  }
    

The problem is that.
I am not calling notify/notifyAll method in the synchronized method of calculator class then too my program is completing giving me the right output.

I think the program should not come out until and unless i call notify/all method from the synchronized block in the calculator class.


I am not able to find what is the problem with the code.

kindly try to find the solution together

[Steve Luke]

Your problem is that you are extending Thread, then calling wait() on the Thread object. There is un-documented behavior regarding Thread and what happens when a Thread comes to an end. The part that is messing with your application is that when a Thread dies, it calls notifyAll() on the Thread object itself, so any wait()ers are woken up. This is not documented, so it can't be relied on - it is a side effect of some Thread implementation detail.

As a result, I use this as one of those reasons why it usually does NOT make sense to extend Thread. You should instead make both Reader1 and Calculator implement Runnable, not extend Thread. Then add start() methods which create new Threads and start them. Example:

class Calculator implements Runnable
{
    //...
    private Thread runningThread;

    public void start()
    {
        if (this.runningThread != null)
            throw new IllegalStateException("Calculator has already been started.  Cannot run Calculator twice.");

        this.runningThread = new Thread(this);
        this.runningThread.start();
    }

    //... The rest of your code as is

[Vijitha Kumara]

Congrats Steve & Good luck on your Bartender role

[rakesh kadulkar]

Thank you very much as my problem is solved very nicely.

And sorry i forgot my java basics.

Well steve congrats for the new role as a "Bartender".

All the best.

[Steve Luke]

Thanks guys :-)

Glad its working for you now.