core.clj

(ns hello-world.core
  (:use compojure.core
        ring.middleware.session
        ring.adapter.jetty))

(defroutes app
  (GET "/set-session" []
    {:body "set session"
     :session {:a-key "a value"}})
  (GET "/read-session" {s :session}
    {:body (str "session: " s)}))

(wrap! app :session)

(run-jetty app {:port 8080})

project.clj

(defproject hello-world "1.0.0-SNAPSHOT"
  :description "FIXME: write"
  :dependencies [[org.clojure/clojure "1.1.0"]
                 [org.clojure/clojure-contrib "1.1.0"]
                 [compojure "0.4.0-SNAPSHOT"]
                 [ring/ring-jetty-adapter "0.2.0"]])