Assuming what you are wanting is to run a jetty server and have it load or reload your code when you change it from within emacs. While the advice already given is good, it may be more complex than you need when getting started. My advice is to take advantage of some of the templates out there for lein which will setup a default environment and workflow for you to begin with. You can then refine this default as you learn more until you get the workflow which suits you. My recommendation would be to start witht the default compojure template i.e.
lein new compojure my-project
This creates a bare bones project with the basic ring and compojure libraries and lein plugins as well as a simple dev profile.
Edit the src/my_project/handler.clj file and add the ring.middleware.reload middleware e.g.
(ns my-project.handler
(:require [compojure.core :refer :all]
[compojure.route :as route]
[ring.middleware.reload :refer [wrap-reload]]
[ring.middleware.defaults :refer [wrap-defaults site-defaults]]))
(defroutes app-routes
(GET "/" [] "Hello World")
(route/not-found "Not Found"))
(def app
(-> app-routes
wrap-reload
(wrap-defaults site-defaults)))
The wrap-reload middleware will cause your code to be reloaded when it is modified. You won't need to restart the jetty server for your code changes to take effect - just reload the page.
In a terminal run either
lein ring server
or
lein ring server-headless
This will start a jetty server listening on port 3000. Then from within emacs, you can just open a cider repl to use while writing your code. You won't need to restart the server process unless you make changes to your project.clj file. Same with the cider process.
Then, once your comfortable with this, look at the lein-ring documentation. There you will find information on how to setup a repl.clj file within the project. Once you do that, you will be able to do something like
lein repl
and then from within that repl, do something like
(start-server)
which will start the server. You can then switch to emacs and instead of running cider-jack-in, you can do a cider-connect, which will connect tot he already running repl rather than starting a second repl session. Later, if you decide to start also looking at clojurescript, you can look at some of the default templates for clojure+clojurescript apps. I quite like figwheel and use reagent a fair bit, so I also find the reagent template quite good.
There are quite a few lein templates out there and I find it really useful to just run them and have a look at what they do. I then tend to cherry pick the features/options I like.
