Clojure STM ( dosync ) x Java synchronize block

Question

What is the difference between Clojure STM (dosync) approach and Java synchronize Block?

I'm reading the code below from "The sleeping barber" problem. (http://www.bestinclass.dk/index.clj/2009/09/scala-vs-clojure-round-2-concurrency.html)

(defn the-shop [a]  
  (print "[k] entering shop" a)  
  (dosync     
    (if (< (count @queue) seats)  
      (alter queue conj a)  
      (print "[s] turning away customer" a))))

To avoid race conditions, dosync is used, so i ask myself "What is the difference (STM) from Java synchronize block" ? Will it block this critical code ?

Thanks in advance ! Dantas