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On the UI, of course. Do you mean to say that I can use some js libraries and do the highlighting on the client-side?

Yea, highlight.js is awesome and it's pretty easy to add new languages. Your code still looks like code, highlight handles breaking it down into html

Yes, if I don't get any good options in Java then I would go with either codemirror (used in the dev console in many browsers) or highlight.js