Shared Objects for Threads

Question

Hi,
Currently studying for SCJP exam and wondering is there a different between the following codes
public class Test extends Thread
static Object obj = new Object();
static int x, y;
public void run()
   synchronized(obj)
     for(;;)
      x++; y++; System.out.println(x+" "+y);
public static void main(String[] args)
   new Test().start();
   new Test().start();
}and
public class Test extends Thread
Object obj = new Object();
static int x, y;
public void run()
   synchronized(obj)
     for(;;)
      x++; y++; System.out.println(x+" "+y);
public static void main(String[] args)
   new Test().start();
   new Test().start();
In the first code example, is there a shared object between created threads and the second example, is their still a shared object between threads or is a new object created for each thread?
Cheers!

Answer 1

Hi,
In first case, you are acquiring lock on a static object that is of course shared (basic concept) by all instances(of Test). So, for any thread of any instance(of Test) to execute the synchronized block has to wait till any other thread of any other instance(of Test) comes out of synchronized block. It means, all the instances (of Test) are synchronized. In this scenario, you have infinite for loop within synchronized block and you are starting two threads. The thread which enters the synchronized block first will keep on executing and the other thread will never get chance to enter synchronized block.
In second case, as you are acquiring lock on an object, all the threads of a particular instance (of Test) will be synchronized. In this scenario, again you have infinite for loop within synchronized block and you are starting two threads on two different instances. Hence, both the threads will keep on executing in parallel.
I hope, you have already executed both the examples and observed output. And also hope that the above explanation will help you to understand the difference between two scenarios.
Note: Use System.out.println(Thread.currentThread().getName() + " " + x + " " + y); in your synchronized block to track which thread is printing the output.
Thanks,
Mrityunjoy