You need to run the java command with a security manager and policy set
on the command line. Something like:

  java -Djava.security.manager -Djava.security.policy=someURL SomeApp

You need to create a policy file with the permissions you want to give
the application.

Command line and policy syntax:

http://java.sun.com/javase/6/docs/technotes/guides/security/PolicyFiles.html

Details of permissions:

http://java.sun.com/javase/6/docs/technotes/guides/security/permissions.html

Obviously you don't want to give your application write permission on
your policy file.

Tom Hawtin