The easiest singleton around

Iv just finished watching stuart-clojure-presentation where the issue of singletons is mentioned (global & thread local ones), a note from the audience mentions delay as a solution, REPL-ing a bit shows how this works:

user=> (defn singleton-factory []
         (println "creating object")
         (+ 1 2))
#'user/singleton-factory
user=> (def singleton (delay (singleton-factory)))
#'user/singleton
user=> (defn usage []
         (force singleton))
#'user/usage
user=> (usage)
creating object
3
user=> (usage)
3

Stuart does mention contrib singleton which has also per-thread-singleton.

user=> (use 'clojure.contrib.singleton)
nil
user=> (def t-singleton (per-thread-singleton singleton-factory)) 
#'user/t-singleton
user=> (defn use-twice [] (+ 1 (t-singleton)) (+ 1 (t-singleton)))
#'user/use-twice
user=> (defn use-twice-no-singleton [] (+ 1 (singleton-factory)) (+ 1 (singleton-factory)))
#'user/use-twice-no-singleton
user=> (. (Thread. use-twice) start) ; each Clojure Fn implements Runnable
nil
user=> creating object ; REPL prints on the wrong line

user=> (. (Thread. use-twice-no-singleton) start)
user=> creating object ; same thing
creating object

user=> 

This should come useful when working with all those Java mutable objects.