Calling wait()/ notify() in run()

[Santiago Bravo]

Hi All,

I have the following code:

   
public class myThread extends Thread{
 
 String s="";
 public void run(){
 
 synchronized(s){ //Line 1
 try{
  wait(5); //Line 2
 System.out.println("Running");
 }
 catch(Exception e){}
 }
 }
 
public static void main(String[] args) {
new myThread().start(); 
 
}
 
}
 

When I run the above code I get no output. If I change Line 1 to synchronize on "this":

  synchronized(this){ //Line 1  

or change Line 2 to:

  s.wait(5); //Line 2  

I get the desired output - "Running".

What is happening in the original code and why does the print statement not execute? Is this due to the lock on the object being
synchronized which has to be called such as s.wait(5); instead of just wait(5); ?


Second question is, why can't we call wait(), notify(), notifyAll() in a static method? is this because there is no "this" in a static method?

Thanks

[Ankit Garg]

In the original code when you call wait, it is a call to wait on the this reference. Since the call to wait is not in synchronized context of this, so a IllegalMonitorStateException is thrown. Since the associated catch block does nothing when an exception is raised, so you see no output.

You are right that calling s.wait(5); will solve the problem as the code is synchronized on s. But that is not a perfect solution. The call to wait will return only because of timeout i.e. the 5 milliseconds will expire. The call will not return if you write s.wait() as no one notifies it.

So the correct code would be something like this

public class myThread extends Thread{
 
 String s="";
 public void run(){
 
    synchronized(s){ //Line 1
 try{
   s.wait(); //Line 2
   System.out.println("Running");
 }
 catch(Exception e){}
    }
 }
 
public static void main(String[] args) {
myThread mt = new myThread();
                mt.start();
                try{Thread.sleep(1000);}catch(Exception){}//sleep so that the run method will call wait before we notify it
                synchronized(mt.s)
                {
                   mt.s.notifyAll();
                }
}
 
}

you cannot call wait, notify and notifyAll in static methods because they are non-static methods in Object class and you need the this reference (which is like an object not a method) to access them...

[Edit: Removed the logical error that Santiago and Srilatha are referring below in this thread]
[ December 03, 2008: Message edited by: Ankit Garg ]

[Santiago Bravo]

aaah (lightbulb)

thanks alot Ankit, makes sense now

[Santiago Bravo]

Actually I do have a query about:

  mt.s.notifyAll();  

Is this being called in a synchonized context? Is this being achieved by the synchronized object 's'?


Thanks

[M Srilatha]

Is this being called in a synchonized context? Is this being achieved by the synchronized object 's'?

No this call is not in synchronized context. Hence it will throw runtime exception when you execute. (java.lang.IllegalMonitorStateException)

[Ankit Garg]

Originally posted by M Srilatha:


No this call is not in synchronized context. Hence it will throw runtime exception when you execute. (java.lang.IllegalMonitorStateException)

Oops! An error in my program . I am correcting it in the original post...

[Santiago Bravo]

fantastic, thanks for making it clearer

[meera kanekal]

I just want to clarify a point. When you have a class that extends Thread class, and when the start() is invoked does it automatically invoke the overridden run() ?
Thanks,
Meera

[Santiago Bravo]

Hi Meera,


Thread already implements Runnable


http://java.sun.com/j2se/1.5.0/docs/api/

So it implements the Runnable interface method run() but actually does nothing if you dont override the Thread run() method

Any subclass of Thread should override run()