Thread Synchronization Doubt?

[Joe San]

Hi Guys,

The following is from the K&B book. Page 720.

class ThreadA{
 public static void main(Stsinr args[]){
  ThreadB b = new ThreadB();
  b.start();
  synchronized(b){//Line 6
   try{
    System.out.println("Waiting for b to complete...");
    b.wait();
   }catch (InterruptedException e){}
    System.out.println("Total is: " + b.total);
  }
 }
}
 
class ThreadB extends Thread{
 int total;
  public void run(){
   for(int i = 0; i < 100; i++){
     total += i;
   }
   notify();
  }
 }
}

The explanation given as, "Notice in line 6 the code synchronizes itself with the object b- this is because in order to call wait() on the object. ThreadA must own a lock on b. For a thread to call wait() or notify(), the thread has to be the owner of the lock for that object.

So from my understanding, at line 6 which is in ThreadA, when we say synchronized(b), it means that ThreadA has locked ThreadB. Is that what I can infer?? This may sound dumb and stupid, but ranchers, please help me understand this.

Thanks in advance.

[Joe San]

Guys,

Anyone please help me with the above code please???

[John Johnson]

Yes, you are correct. When you say synchronized(b) you are locking onto the instance of ThreadB (not the class itself). So while your thread is in that synchronized block, no other thread can access synchronized methods of that b instance, however, they still can run un-synhronized methods of that class.

[Barry Gaunt]

Jothi, did you forget something in that ThreadB class? You can't just call notify like that. You are going to get an IllegalMonitorStateException in thread B.
[ December 16, 2006: Message edited by: Barry Gaunt ]

[Joe San]

Yes Barry,

I forgot the synchronized block.