Using a Spring controller here is no different than sending information from an applet to any server-side component. The thing to remember is that the applet is running in the user's browser, on a different machine from the server. You could use regular distributed programming approaches, like sockets/RMI/WebServices/etc. but since you explicitly want to talk to a servlet, a better approach may be to just have the applet do a URLConnection back to the server and send the object as text content to the servlet (if your object can be serialized as text).
-Nate
Write once, run anywhere, because there's nowhere to hide! - /. A.C.