Java Thread Ping Pong example Issue

Question

I have seen the example of PingPong using synchronized keyword.Basicly the actual example is this:

public class PingPong implements Runnable {

    synchronized void hit(long n) throws InterruptedException {

        for (int i = 1; i < 3; i++)
            System.out.print(n + "-" + i + " ");
    }

    public static void main(String[] args) {
        new Thread(new PingPong()).start();
        new Thread(new PingPong()).start();
    }

    public void run() {
        try {
            Long id = Thread.currentThread().getId();

            hit(id);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Normally the hit method is not synchronized because the synchronized keyword will work only if there is one object.So the result can be like 8-1 9-1 8-2 9-2 or 8-1 9-1 9-2 8-2...(It's a randomly result).But in this example it give us all time 8-1 8-2 9-1 9-2 which is a little bit strange because that mean that the hit method is synchronized!!!. I have modified the code to check again that the hit method must be not sycnhronized so i add a Thread.sleep(1000) at the starting of the hit method it will be like

synchronized void hit(long n) throws InterruptedException {
        Thread.sleep(1000);
        for (int i = 1; i < 3; i++)
            System.out.print(n + "-" + i + " ");
    }

Now the code is giving random result each time i make an execution.

I'm really getting confusing!! Can some one help me in understanding this issue ?

Answer 1

You do indeed have two separate instances of PingPong, which would mean that there would be two separate monitor objects, which should mean that the threads are not being forced to run synchronously.

I think that you are probably running into thread scheduling behavior. On a single core CPU, the code may very well execute as you describe because the thread scheduler is never given a chance to take over.

If you add a sleep to your for loop:

synchronized void hit(long n) throws InterruptedException {

    for (int i = 1; i < 3; i++){
        System.out.print(n + "-" + i + " ");
        Thread.sleep(0);
    }
}

That should release the scheduler to run other threads. Note that the JVM doesn't provide any guarantees of how the scheduler will actually behave, but in my experience the above is sufficient.

If you are on a multi-core CPU, I'd expect it to work as you expected without the sleep() call.