Default Clojure Handler for web page

handler.clj

;; block 1
(ns sessiontest.handler
  (:require [compojure.core :refer :all]
            [compojure.route :as route]
            [ring.middleware.defaults :refer [wrap-defaults site-defaults]]))
;; block 2
;; this is the route part
(defroutes app-routes
  (GET "/" [] "Hello World")             ;; for /
  (route/not-found "Not Found"))         ;; 404
;; block 3
(def app
  (wrap-defaults app-routes site-defaults))