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On the web page it display: You have the access...
but I want to display PARAMETER NULL EXCEPTION. if app is null
This means that reg != null
so the code didn't even run the try/catch block because it didn't pass the (if reg == null) test
Now, I would like to know what does this line of code print in the console, obviously it printed something since it is failing the (if reg == null)
what I have typed so far really hasn't help with your issue, what's is the url to get to the "You have access..." page? that should tell you if the app request parameter is passing something to the servlet. One final thought is under what kind of condition that your app request parameter will be null, if you can recreate that than you should be able to see the exception