Session Expired on google app engine(GAE)

I write a small vaadin app and deployed it onto google GAP, when I pressed one of two combox button, it show

Session Expired
Take note of any unsaved data, and click here to continue.

but if it run on local host, it’ OK, how do I deal with this bug?

Maybe one of the classes you have declared within you application does not implement

Sounds like you are not using the GAEApplicationServlet instead of ApplicationServlet. Google App Engine requires some special handling regarding the session and this is implemented in GAEApplicationServlet