How to automatically reload Clojure code?

I have watched the presentation. And in there they use some hot swap Clojure technology that reloads code whenever changes are made into source files. I can run the code, but I can not observe the effect of auto reload. How is it possible to reload the code?

the source code.

Solution

There are many ways to reload code, depending on the situation:

Emacs/CIDER shortcuts to reload a file. Probably Rich is doing something similar. Also see this: How to reload a clojure file in REPL
watch files and reload the code (figwheel for frontend development does that, test plugins as well as CIDER shortcuts specifically for running tests, boot has a watch task...)
a middleware for the web server you use (ring for example has ring.middleware.reload/wrap-reload for this, pedestal comes with one as well, other webservers like yada play well with component)
a component workflow works also, there is an example of an auto-reloadable system with holy-grail