Be aware that updatePlus() will be called while the servlet is running, sticking whatever it returns into the action string. This will not cause updatePlus() to be called when the form is later submitted by the user.
No you can't call just any servlet method directly when the form is submitted. You need to set the action to the URL mapping of a servlet, and upon submit the doGet() or doPost() method of the servlet will be invoked.
you can call whole servlet (doGet or doPost) no need to call specific function and if you want to do so then go for Struts 2 !
or one more way is you can call that function from doGet or doPost Simple !!