Why are you using an applet as your front end. I think
that you can go to an HTML front end accessing  an
Servlet/JSP and the JSP/Servlet does all the db
connections. So that you will need not worry about the
security isseus.

Alternatively you can make use of the https protocol
from your applet to send secure information to and
from your applet. If you need an advance level of
security please have a look at the JCE API's

Actually my web application is in asp. I want some visual display of
graphs in java applet and want the user to be able to see his/her
graphs in different ways like Bar chart, Line chart , Pie chart etc.
I could not find any such facality in ASP. Now I have made an applet
that can fetch live data from database  . But I want my applet to be
secure so that user can fetch his/her data but at the same time could
not be able to perfome some malicious operations on my database.