I'm not sure what exactly categorizes as "hot deploy" but how about erlang, elixir?

Rupy can switch code on the fly, meaning you can have users hitting your server and they will run the old code one request and new code the next. No other system I know of works like this, hot-deploying on a remote machine over HTTP seamlessly. Maybe erlang can do it but then again I can't write a 3D game in that language so Java still has the upper hand for my requirements.

Off the top of my head, if you write a custom MiddlewareChain class that exposed its own management port, you could inspect and modify the state of the chain at any time, checkboxes to disable middleware, change endpoints, insert debug middleware, etc.

I like this... definitely worth thinking about. Thanks, all!

Also, if you're talking about specifically replacing a source files (i.e. modules), one could hack around with https://docs.python.org/3/library/importlib.html#importlib.r.... I think it would work with enough indirection/factory functions.