We can use the wrap-reload from ring.middleware.reload package. This reload the namespaces of modified files before each request.

;core.clj
(ns net.jsloop.server.core
    (:use [compojure.core :only [defroutes GET]]
          [compojure.handler :only [site]]
          [ring.middleware.reload :only [wrap-reload]]
          org.httpkit.server))
 
(defroutes app-routes
    (GET "/" [] "hello world"))
 
(defn -main []
    (run-server (wrap-reload (site #'app-routes)) {:port 8080})
    (println "Server started."))

;project.clj
(defproject server "1.0.0"
    :dependencies [[org.clojure/clojure "1.8.0"]
                   [ring "1.5.0"]
                   [compojure "1.5.1"]
                   [http-kit "2.2.0"]]
    :main net.jsloop.server.core)

Now start the server using:

lein run

Go to http://localhost:8080 and we will see "hello world". Modify the message in the core.clj, reload the browser page and we will see the new message. Note that we need to pass the route function as a var to wrap-reload using the #' reader macro for this to work.