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Judging from http://socket.io under "Supported Transports" it could be several different transport protocols:
WebSocket, Adobe® Flash® Socket, AJAX long polling, AJAX multipart streaming, Forever Iframe, JSONP Polling
So, the question is, what is your server supporting?
From the perspective of Java clients the easiest would be WebSockets.
If you need a client side implementation, checkout java-socket.io.client I made it a few months ago and I believe some people have forked it and upgraded it to the latest version: https://github.com/clwillingham/java-socket.io.client