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;;; 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))))
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