RMI WebSocket provides a library for remote method invocations between a browser and a Jetty Web Server using the HTML5 WebSocket. The idea is to enable tight method-level integration between the user interface and the server so that patterns such as MVP (Model-View-Presenter) can be applied in a Web environment. The method-level communications between the browser code and the server code allows the developer to think about the Web page and server-side components as objects in a UML world. Details such as the over-the-wire protocols in WebSockets are abstracted away in the process.
msg-rpc provides bidirectional RPC support over a simple message interface such as WebSocket. Tere are WebSocket examples based on socket.io and sockjs. It just has a simple sendMessage/onMessage interface, with no other dependencies. It supports not only normal RPC, but also RPC Service, which makes RPC pushable (i.e. the server can also send a request to the client and expect some kind of response).