/modules/singleton/src/main/clojure/clojure/contrib/singleton.clj
Clojure | 54 lines | 28 code | 10 blank | 16 comment | 1 complexity | 280f63fa124434387857082d1e2572dc MD5 | raw file
- ;;; 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))))