On Sat, Nov 27, 2010 at 7:50 PM, Mike Meyer
<mwm-keyword-googlegroups.620...@mired.org> wrote:
> My simple web app
> (http://blog.mired.org/2010/11/x10-controller-in-clojure.html) has
> some stuff that needs to happen just once (in this case, opening the
> serial port). It's not clear how to get this to happen using ring. If
> I do it inside my ring handler, then it gets run on every request, and
> I have to check to make sure it's not run multiple times. If I run it
> outside the handler, then it gets run when I do "lein uberwar", which
> is simply wrong.
>
> When the deployment platform activates the war would seem to be the
> right time to run this ("war load time"?). So maybe this is a question
> that depends on the deployment platform, or war? However, a quick
> google search didn't turn up anything that looked interesting.
>
> Anyone got suggestions on how to set up code to be run when Jetty (or
> tomcat, or ...)?
If it were an ordinary application, -main would be the obvious place
(or some module-init function eventually called from there). In this
case, though, I think your best bet may actually be a Java
lazily-initialized singleton that you access from Clojure, if
performance is an issue, or else the Clojure equivalent:
(defmacro defsingleton [name init-sexp]
`(def ~name
(let [singleton-uninitialized# (Object.)
a# (atom singleton-uninitialized#)]
(fn []
(let [x# @a#]
(if (= x# singleton-uninitialized#)
(do
(reset! a# ~init-sexp)
@a#)
x#))))))
user=> (defsingleton foo (do (println "foo initialized") 7))
#'user/foo
user=> (foo)
foo initialized
7
user=> (foo)
7
You can also abuse futures:
user=> (def foo (future (do (println "foo initialized") 7)))
#'user/foo
user=> @foo
foo initialized
7
user=> @foo
7
or delay/force, or even lazy-seq:
user=> (def foo (drop 1 (take 2 (iterate (fn [_] (do (println "foo
initialized") 7)) nil))))
#'user/foo
user=> (first foo)
foo initialized
7
user=> (first foo)
7
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
