juju on local server + wrapping my own application server custom built wtih charm

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http://serverfault.com – I'm interested in juju before starting with it i would like to ask a few questions. Can juju be only be used with ubuntu? is it possible to use it with CentOS? I have my own application server which i wrote, can i wrap it in a charm and deploy it on my local juju cloud? (I do not publish ofcourse my charm to the openstack because its a propriety server code, if yes can someone direct me maybe to a link of how i can achieve such a thing?) my server is built with java. Is ubuntu server + full capabilities of juju always be free of charge? Which technologies would you recommend me to write my a (HowTos)