Trying To Cause Problems

[sean cotter]

I am playing around with threads trying to get a better understanding. In this case I am trying cause a variable to be corrupted. Unfortunately, is seems I am not sharing it but I do not understand why aren't I.

public class Test1 {

public static void main(String args[]){
	
		Thread tr;

		for (int i=0; i!=4; i++){
			tr=new Thread(new TT1(),new String("booThread"+ i));
			tr.start();
		}
	}
	
}
class TT1  extends Object implements Runnable {
	 int  num;
	public void run(){
		num=-1;
		Thread r=Thread.currentThread();
		System.out.println(r.getName() + " is starting!!");
		num++;
		System.out.println("The value of I is " + num);
		String name=r.getName();
		System.out.println("My thread name is " + name);
		while(true){
			synchronized(this){
				System.out.println("**** the value of i is: " + num + " - Name: " + name);
				try {
					r.sleep(((long)(Math.random()*2000)));
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
			num++;
		}
	}	
}

my output is:
booThread0 is starting!!
The value of I is 0
My thread name is booThread0
**** the value of i is: 0 - Name: booThread0
booThread1 is starting!!
The value of I is 0
My thread name is booThread1
**** the value of i is: 0 - Name: booThread1
booThread2 is starting!!
The value of I is 0
My thread name is booThread2
**** the value of i is: 0 - Name: booThread2
booThread3 is starting!!
The value of I is 0
My thread name is booThread3
**** the value of i is: 0 - Name: booThread3
**** the value of i is: 1 - Name: booThread2
**** the value of i is: 1 - Name: booThread3
**** the value of i is: 2 - Name: booThread2
**** the value of i is: 2 - Name: booThread3
**** the value of i is: 1 - Name: booThread0

Any ideas?

[Omar Al Kababji]

What variable is supposed to be shared???

and there is only one thread running (except the main thread) so how could a racing condition happen?

[sean cotter]

lines 7-9 create 4 threads with names booThread0 thru booThread3, these names can be seen in the output.
The variable being shared is num located on 17 and modified on lines 17, 20 and 34.
Every time a new thread starts the value of num is 0 for that specific thread, as stated by the ouput lines "The value of I is 0", and the fact the the last 4 lines of output say the values for num are 1,2,2,1. Once in the loop I am always incrementing so it should never go from 2 to 1.

How would you modify it so other threads are sharing the same version on num?

Thank you for your help.

[Omar Al Kababji]

use static variables, so the same instance is shared between all classes ;-)

[Henry Wong]

If you look at line 8, you will see that all four threads have there own copy of a TT1 instance. You will also notice that num is an instance variable, so all the four threads have there own copy of the num variable.

Two possible options to cause problems... (1) give all four threads the same TT1 instance, or (2) make the num variable a static variable.

Henry

[sean cotter]

Thank you Henry!! I am now able to visualize how this works in my head.

I changed the main() to:

	Thread tr;
	TT1 tt1 = new TT1();
	for (int i=0; i!=4; i++){
		tr=new Thread(tt1,new String("booThread"+ i));
		tr.start();
	}

Thanks again!