creating http based messenger

im trying to create a messenger that works in a office environment where there are restrictions and udp ports and other tcpip ports are blocked and ofcourse there are firewalls, so basically trying to make something working on http. does any one have any idea if this is possible and how??

and just in case .. can i have sumthing like this based on java? so that no one needs any s/w they can log to a website [where i can have my java code] and everyone chatting together like any chat!!