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).
Write once, run anywhere, because there's nowhere to hide! - /. A.C.