How to expose a Java code variable in class module to the b4j main module?

Question

I have the following as a part of a Javacode in a class:

#if Java
   {
   //
   //....
        String msg ="blabla";
        ba.raiseEvent(null,"get_msg",msg);
   //.....
   //
   }

#end if

Is the ba.raiseEvent written correctly? How would I expose 'msg' variable to the main module of the b4j app and use it there ?