Several possibilities. Among the easiest, you could have the service method of your servlet expect an invocation from the stand alone, i.e., the stand-alone would act as a client to the servlet. (If you don't want to alter the service method of your servlet, you could create a second servlet to handle this communication, and the two servlets could call methods on each other.) If you don't like this idea, you could use RMI or CORBA.

I assume that your stand-alone is not a Java program running in the same VM, which would make any sort of socket-based communication unnecessary.