I havent tried compiling your code in my PC. But just some suggestions.
a) I think doGet used to be the default method that would be invoked. So doGet and doPost should have the same implementation if you want to ensure that your code works as you intend to .
b) Try printing the PreparedStatement stat before running the update to see what statement is actually appearing. Look for syntax errors by trying to run that in any SQL editor
From your post I think the servlet's method doPost() gets invoked correctly. However the problem might your SQL statement.
Does your table have auto increment primary key? If so, you should not pass a value for that column in the statement.
I followed your suggestions and kept processRequest(request, response); as the last line of dopost() this time i got an exception ...I am not sure which value is Null right now
type Exception report
description The server encountered an internal error () that prevented it from fulfilling this request.
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.9 logs.