(def num-threads 50)

(def data (ref 0))

(defn hello [data]
  (doseq [_ (range 500)]
    (Thread/sleep (rand-int 1000))
    (dosync (ref-set data (inc @data)))
    (println @data)))

(def threads (for [_ (range num-threads)] (new Thread #(hello data))))

(doseq [t threads] (. t start))

(doseq [_ (range 500)]
  (Thread/sleep (rand-int 1000))
  (println "main thread"))