Question: I would like to implement a robust IPC solution between a single JVM applet and a C++ application running on the same machine. What is the best approach for doing so?

Any suggestions will be greatly appreciated! Thanks!

Answer: 
sockets are about your best (only reasonable?) choice. although if you are running an applet, you will have to deal with access permissions issues (signing the applet will probably solve these).
Sockets as a transport protocol.

JSON as a message protocol, with optional length header or trailing zero if required.