Threading issues - Best way to debug

Hey everyone... I have a small server program that I wrote in Java. Its module based where each thread that is created runs through a list of modules and returns the output. It works just fine and I am doing a little stress test using Siege. I set it up running 50 concurrent users with no delay and take millions of requests with no errors at all.
However, when I move it up to 125 concurrent users, I start getting NullPointerExceptions on a fairly routine basis (inside of one of the modules). The server is still running with no issues, just with that many users it appears to have a bug.
I was wondering what the best way figure out why this would be happening. Or is this kind of thing to be expected and I should just implement some method of throttling for the threads. I plan to do that eventually, just trying to get the bugs out first.
Thanks in advance!

I think you are seeing the NPE concurrent scenario because of the assumptions you code is making about how JVM should behave. You code may be making assumption of "happens-before" pattern which may seem valid when you look at the code, but in fact are not.
Take a look at this presentation that outlines the Java Memory model and defines the contract between concurrent programs and JVM.
http://developers.sun.com/learning/javaoneonline/j1sessn.jsp?sessn=TS-1630&yr=2006&track=coreplatform
Throttling the method is a bad idea (and a cop out). How would you arrive at the optimal number threads that can safely access your program ? Will your code be portable when it is deployed on a computer with different computing power than what you are testing on ?
Hope this helps.
-Prashant

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