A UDP echo server in Clojure. http://jvns.ca/blog/2013/10/09/day-7-an-echo-server-in-clojure/

echo-server.clj

(ns echo-server
  (:import (java.net InetAddress DatagramPacket DatagramSocket)))

(def udp-server (ref nil))

(def port 12345)

(defn localhost [] (. InetAddress getLocalHost))

(defn message [text]
  (new DatagramPacket (. text getBytes) (. text length) (localhost) port ))

(defn send-message [text]
  (.send @udp-server (message text)))

(defn create-udp-server []
  (DatagramSocket. port))

(defn start-udp-server []
  (dosync (ref-set udp-server (create-udp-server))))

(defn stop-udp-server []
  (.close @udp-server))

(start-udp-server)

(defn empty-message [n] 
  (new DatagramPacket (byte-array n) n))



(while true (do
    (def orig-packet (empty-message 1024))
    (.receive @udp-server orig-packet)
    (.send @udp-server orig-packet)
    (println ( new String (.getData orig-packet) "UTF-8"))
  )
)