I've seen from one of these other threads that you're calling this code from a
JSP file (as shown by stack trace element
at com.ibm._jsp._Translation._jspService(_Translation .java:158)).
Are you aware that calling this method from a JSP would return the
server's screen size, not the client's? You can only get the client screen size from an
applet, and that probably still requires a signed applet.
If you really do need to get the server's screen size, add
-Djava.awt.headless=false to your JVM options.