threads_and_locks.clj

dev> (let [count (volatile! 0)
           t1 (Thread. #(dotimes [_ 1000] (vswap! count inc)))
           t2 (Thread. #(dotimes [_ 1000] (vswap! count inc)))]
       (.start t1)
       (.start t2)
       (.join t1)
       (.join t2)
       @count)
1577
dev> (let [count (atom 0)
           t1 (Thread. #(dotimes [_ 1000] (swap! count inc)))
           t2 (Thread. #(dotimes [_ 1000] (swap! count inc)))]
       (.start t1)
       (.start t2)
       (.join t1)
       (.join t2)
       @count)
2000
dev> (let [answer-ready? (volatile! false)
           answer (volatile! 0)
           t1 (Thread. #(do (vreset! answer-ready? true)
                            (vreset! answer 42)))
           t2 (Thread. #(if @answer-ready?
                          (println "The meaning of life is:" @answer)
                          (println "I don't know the answer")))]
       (.start t1)
       (.start t2)
       (.join t1)
       (.join t2))
The meaning of life is: 42
nil
dev> (let [answer-ready? (ref false)
           answer (ref 0)
           t1 (Thread. #(dosync (ref-set answer-ready? true)
                                (ref-set answer 42)))
           t2 (Thread. #(if @answer-ready?
                          (println "The meaning of life is:" @answer)
                          (println "I don't know the answer")))]
       (.start t1)
       (.start t2)
       (.join t1)
       (.join t2))
I don't know the answer
nil

cf. Seven Concurrency Models in Seven Weeks