Wait() IllegalMonitorStateException

[Thiyagarajan Kamala]

class BigExten{

void notifyEveryone(){
Thread.currentThread().notify();
}

public static void main(String args[]) throws Exception{
synchronized(System.out){
Thread.currentThread().wait();
}
}
}


The above program throws IllegalMonitorStateException can someone explain ?.

[Nitish Bangera]

The main thread should hold a lock on Thread.currentThread and not on System.out. Before calling a wait on an object, the thread should acquire that object's lock.

[Thiyagarajan Kamala]

Hi Nitsh,
class BigExten{

public static void main(String args[]) throws Exception{
synchronized(Thread.currentThread()){
Thread.currentThread().wait();
System.out.println("done");
}
Thread.currentThread().notify();
}
}


I modified to lock on Thread.CurrentThread() but notify is not happening .....
Appreciate your response.
Thanks.

[Sebastian Janisch]

How do you suppose to call notify when the code halts at the wait statement ;) ...

Also, please put your code between code tags.

[Nitish Bangera]

If the main thread is waiting means its not proceeding any further means it won't reach the notify at all.

[Thiyagarajan Kamala]

Hi Guys,
I modified the wait() method to wait on the someother object (say G not current thread)...Is the current thread then free to go and execute notify or the main thread itself will go to waiting state..Please clarify .
Thanks.

[Nitish Bangera]

If you are calling a wait on an object then the thread should acquire the lock on that object means synchronized on that object. So if you are waiting on G then it should synchronized on G and also while notifying, it should be synchronized on G and then call the notify.

[Thiyagarajan Kamala]

 class BigExten {
    G g = new G();

    void notifyEveryone(G g) {
        synchronized (g) {
          g.notify();
        }
    }

    public static void main(String args[]) throws Exception {
        BigExten bigExten = new BigExten();
        synchronized (bigExten.g) {
            bigExten.g.wait();
            System.out.println("done");
        }

        bigExten.notifyEveryone(bigExten.g);
    }
}

class G {}
   

Evenafter synchronizing on g in both wait and notify call the notification is not happening.Can you give the detail on where Iam wrong.

[Nitish Bangera]

hmmmm here the main thread wait on an object BigExten.g so it will wait. Then if the main thread is waiting how will it reach bigExten.notifyEveryone(bigExten.g) which is there in the same thread?

[Akanksha Mittal]

I tried to modify the above code thinking that if notify is called from a different thread then the wait would interrupted but I was wrong.
Have a look at the code.

 public class BigExtn {   
    G g = new G();   
  
    void notifyEveryone(G g) {   
        synchronized (g) {   
          g.notifyAll();  
         }   
    }   
  
    public static void main(String args[]) throws Exception {   
        BigExtn bigExten = new BigExtn();
        Thread thr = new Thread(){ 
        	public void run()
        	{ 	
        		try{
        			System.out.println("going to sleep");
        			sleep(1000);
        		}catch(Exception e){}
        		
        		BigExtn bigExten = new BigExtn();
        		System.out.println("Going to notify");
        		bigExten.notifyEveryone(bigExten.g);
        		System.out.println("from thread");
        		
        	}
         };
        thr.start();
        synchronized (bigExten.g) { 
        	System.out.println("started");
            try{
        	bigExten.g.wait(10000);
            }catch (InterruptedException e)
            {
            	System.out.println("done");
            }
        }  
        System.out.println("notified from main");
        bigExten.notifyEveryone(bigExten.g);  
        
    }   
}   
  
class G {}   
        

The output comes as follows:
going to sleep
started
Going to notify
from thread
notified from main

I suspected "done" would be printed after being notified but its not.
Can anyone tell me what is the basic thing I am missing here?

[Ankit Garg]

I suspected "done" would be printed after being notified but its not.

I can't understand two things in your code.

1. There are two different bigExten references in your main method. So the notifyEveryone method is called with a different object than the one on which main method synchronizes. I've marked it here

 public class BigExtn {   
    G g = new G();   
  
    void notifyEveryone(G g) {   
        synchronized (g) {   
          g.notifyAll();  
         }   
    }   
  
    public static void main(String args[]) throws Exception {   
        BigExtn bigExten = new BigExtn();         <-----
        Thread thr = new Thread(){ 
        	public void run()
        	{ 	
        		try{
        			System.out.println("going to sleep");
        			sleep(1000);
        		}catch(Exception e){}
        		
        		BigExtn bigExten = new BigExtn();   <------
        		System.out.println("Going to notify");
        		bigExten.notifyEveryone(bigExten.g);   <----- different object from the one on line (1)
        		System.out.println("from thread");
        		
        	}
         };
        thr.start();
        synchronized (bigExten.g) {           <--------(1)
        	System.out.println("started");
            try{
        	bigExten.g.wait(10000);
            }catch (InterruptedException e)
            {
            	System.out.println("done");
            }
        }  
        System.out.println("notified from main");
        bigExten.notifyEveryone(bigExten.g);  
        
    }   
}   
  
class G {}   

2. Even if you notified the correct thread, why would there be an InterruptedException?? Notifying threads waiting on an object doesn't throw an exception in the target threads. Instead the wait method call in the waiting threads return normally. InterruptedException is only thrown if a waiting thread is interrupted using the Thread.interrupt() method...

[Akanksha Mittal]

Thanks Ankit.
I think its clear now.