Wait and notify sample does not work

[Celina Paul]

The following wait and notify sample does not work.
Please can somebody let me know what is wrong ?

 
public class TestWaitNotify implements Runnable
{
Object[] obj = null;
 
public TestWaitNotify()
{
obj = new Object[5];
for(int i=0;i<5;i++)
{
obj[i] = new Object();
}
}
 
public void waitFor(int objNum)
{
synchronized(obj[objNum])
{
try{
obj[objNum].wait();
System.out.println("here after being notified");
}
catch(Exception e)
{
System.out.println("error");
}
}
}
 
public void notifyTo(int objNum)
{
synchronized(obj[objNum])
{
obj[objNum].notifyAll();
System.out.println("Notified all the waiting threads");
}
}
 
public static void main(String[] args)
{
Thread t1 = new Thread(new TestWaitNotify());
Thread t2 = new Thread(new TestWaitNotify());
 
t1.setName("t1");
t2.setName("t2");
 
t1.start();
t2.start();
}
 
public void run()
{
if(Thread.currentThread().getName().equals("t1"))
{
waitFor(0);
}
 
if(Thread.currentThread().getName().equals("t2"))
{
notifyTo(0);
}
}
 
 
}
 

The output that I see is -
"Notified all the waiting threads"

and then it hangs for ever
[ August 03, 2004: Message edited by: Celina Paul ]

[Ernest Friedman-Hill]

This program assumes that waitFor(0) will be called before notifyTo(0), but there's nothing at all that guarantees this will be the case. As written, the two methods could be called in either order, because there's no guarantee which of the two threads will start running first.

If notifyTo() is called first, then the program will hang, as the waitFor() will never return.

[Celina Paul]

Hi Ernest,

Thanks for the reply.

After reading your comment, I changed the code, so that the Thread t2
sleeps for 10 seconds before calling notify().

 
try{
 
if(Thread.currentThread().getName().equals("t2"))
{
Thread.sleep(10000);
notifyTo(0);
}
 
}
catch(Exception e)
{
System.out.println(e.getMessage());
}
 
 

This should take care that wait() gets called since the other thread is not sleeping. I still see the same result.

I am expecting to see after the wait is called -
here after being notified

Let me know what is going on here...

[Ernest Friedman-Hill]

OK, well, now that you mention it, there is another problem: each Thread has its own TestWaitNotify object, and each TestWaitNotify object has its own array of five Objects -- so that the Object that wait() is called on is never notified; it's another object altogether that is notified, but nothing is waiting.

To fix just use one single TestWaitNotify object to pass to both Thread constructors.
[ August 03, 2004: Message edited by: Ernest Friedman-Hill ]

[Celina Paul]

Great ! It works !

Thanks Ernest,
It also cleared few things on the way...