;;; singleton.clj: singleton functions

 

 ;; by Stuart Sierra, http://stuartsierra.com/

 ;; April 14, 2009

 

 ;; Copyright (c) Stuart Sierra, 2009. All rights reserved.  The use

 ;; and distribution terms for this software are covered by the Eclipse

 ;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)

 ;; which can be found in the file epl-v10.html at the root of this

 ;; distribution.  By using this software in any fashion, you are

 ;; agreeing to be bound by the terms of this license.  You must not

 ;; remove this notice, or any other, from this software.

 

 

 ;; Change Log:

 ;;

 ;; April 14, 2009: added per-thread-singleton, renamed singleton to

 ;; global-singleton

 ;;

 ;; April 9, 2009: initial version

 

 

 (ns 

   ^{:author "Stuart Sierra",

      :doc "Singleton functions"}

   clojure.contrib.singleton)

 

 (defn global-singleton

   "Returns a global singleton function.  f is a function of no

   arguments that creates and returns some object.  The singleton

   function will call f just once, the first time it is needed, and

   cache the value for all subsequent calls.

 

   Warning: global singletons are often unsafe in multi-threaded code.

   Consider per-thread-singleton instead."

   [f]

   (let [instance (atom nil)

         make-instance (fn [_] (f))]

     (fn [] (or @instance (swap! instance make-instance)))))

 

 (defn per-thread-singleton

   "Returns a per-thread singleton function.  f is a function of no

   arguments that creates and returns some object.  The singleton

   function will call f only once for each thread, and cache its value

   for subsequent calls from the same thread.  This allows you to

   safely and lazily initialize shared objects on a per-thread basis.

 

   Warning: due to a bug in JDK 5, it may not be safe to use a

   per-thread-singleton in the initialization function for another

   per-thread-singleton.  See

   http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5025230"

   [f]

   (let [thread-local (proxy [ThreadLocal] [] (initialValue [] (f)))]

     (fn [] (.get thread-local))))