1.1 --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
     1.2 +++ b/src/clojure/contrib/singleton.clj	Sat Aug 21 06:25:44 2010 -0400
     1.3 @@ -0,0 +1,54 @@
     1.4 +;;; singleton.clj: singleton functions
     1.5 +
     1.6 +;; by Stuart Sierra, http://stuartsierra.com/
     1.7 +;; April 14, 2009
     1.8 +
     1.9 +;; Copyright (c) Stuart Sierra, 2009. All rights reserved.  The use
    1.10 +;; and distribution terms for this software are covered by the Eclipse
    1.11 +;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
    1.12 +;; which can be found in the file epl-v10.html at the root of this
    1.13 +;; distribution.  By using this software in any fashion, you are
    1.14 +;; agreeing to be bound by the terms of this license.  You must not
    1.15 +;; remove this notice, or any other, from this software.
    1.16 +
    1.17 +
    1.18 +;; Change Log:
    1.19 +;;
    1.20 +;; April 14, 2009: added per-thread-singleton, renamed singleton to
    1.21 +;; global-singleton
    1.22 +;;
    1.23 +;; April 9, 2009: initial version
    1.24 +
    1.25 +
    1.26 +(ns 
    1.27 +  ^{:author "Stuart Sierra",
    1.28 +     :doc "Singleton functions"}
    1.29 +  clojure.contrib.singleton)
    1.30 +
    1.31 +(defn global-singleton
    1.32 +  "Returns a global singleton function.  f is a function of no
    1.33 +  arguments that creates and returns some object.  The singleton
    1.34 +  function will call f just once, the first time it is needed, and
    1.35 +  cache the value for all subsequent calls.
    1.36 +
    1.37 +  Warning: global singletons are often unsafe in multi-threaded code.
    1.38 +  Consider per-thread-singleton instead."
    1.39 +  [f]
    1.40 +  (let [instance (atom nil)
    1.41 +        make-instance (fn [_] (f))]
    1.42 +    (fn [] (or @instance (swap! instance make-instance)))))
    1.43 +
    1.44 +(defn per-thread-singleton
    1.45 +  "Returns a per-thread singleton function.  f is a function of no
    1.46 +  arguments that creates and returns some object.  The singleton
    1.47 +  function will call f only once for each thread, and cache its value
    1.48 +  for subsequent calls from the same thread.  This allows you to
    1.49 +  safely and lazily initialize shared objects on a per-thread basis.
    1.50 +
    1.51 +  Warning: due to a bug in JDK 5, it may not be safe to use a
    1.52 +  per-thread-singleton in the initialization function for another
    1.53 +  per-thread-singleton.  See
    1.54 +  http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5025230"
    1.55 +  [f]
    1.56 +  (let [thread-local (proxy [ThreadLocal] [] (initialValue [] (f)))]
    1.57 +    (fn [] (.get thread-local))))