JQ+ Mock - Threads - Question ID: 988384270281

[Shanel Jacob]

Hi all, this is from JQ+ Mock Question ID: 988384270281

The answer given is: Here, there are two threads. But when one thread enters run(), it accquires lock on obj and enters into infinite loop. Now, the second thread also enters run but has to wait as it is also trying to accquire the same lock (obj is a static field). So, in affect, only the thread that gets the lock first, keeps on running. The second thread never gets a chance. so the values printed are sequencial, x and y are always printed as equal and they get incremented by 1 on each line.

01. public class Test extends Thread
02. {
03. 
04.  static Object obj = new Object(); // try removing static
05.  static int x, y;
06. 
07.  public void run()
08.  {
09.    synchronized(obj)
10.    {
11.      for( ; ; ) 
12.      {
13.       x++; y++; System.out.println(Thread.currentThread().getName()+" "+x+" "+y); // if static on line4 removed, both threads are shown
14.      }
15.    }
16.  }
17. 
18.  public static void main(String[] args)
19.  {
20.    new Test().start();
21.    new Test().start();
22.  }
23. 
24. } 

However, when I tried removing the "static" on line 04, I could see that both threads are shown in the println on line 13. why is it that "x" and "y" are still incrementing by 1 on each line (which suggests to me that syncronization still works) but both threads are shown eventhough we have an infinite loop (which suggests to me that syncronization doesn't seem to work). The screen output I got is:

Thread-0 1234 1234
Thread-0 1235 1235
Thread-1 1236 1236
Thread-1 1237 1237

instead of:

Thread-0 1234 1234
Thread-0 1235 1235
Thread-0 1236 1236
Thread-0 1237 1237

[ August 07, 2006: Message edited by: Shanel Jacob ]

[Kishore Balla]

In the first case (when you have used static), class lock has to be obtained to enter into synchronized block.

And in second case (without static), its object lock and you have two seperate objects. So, both threads didn't interfere with each other's execution.
[ August 07, 2006: Message edited by: Kishore Balla ]

[Kishore Balla]

~ edited ~

[repeated]
[ August 07, 2006: Message edited by: Kishore Balla ]