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