That's not possible. Only the webserver will know the IP of the client, and that doesn't mean it is really the client's IP (could be a proxy or NAT etc).
Your servlet can get it via class HttpServletRequest getRemoteAddr() and getRemoteHost().
Now Jetty, which comes with the newest GWT, seems to have a setting to not resolve the address (for speed) by default. But I can't seem to find this configuration setting to set this back.
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