How to genclass a servlet

mistakes — cgrand, 4 June 2008 @ 15 h 45 min
This is an old post, refers to for details on how gen-class works now.

Although generating a servlet class is a canonical example of using Clojure’s genclass I had never do this — I’ve used a handmade ClojureServlet which predates genclass.
Today, as I was starting a new project, I decided to get rid of this legacy ClojureServlet and to generate a brand new one.

Right at the end of genclass.clj there’s the following example:

(clojure/gen-and-save-class "/Users/rich/dev/clojure/gen/" 
 :extends javax.servlet.http.HttpServlet)

I changed the path, created the directory structure (gen-and-save-class doesn’t create the org/clojure/ directories) and ran this code (NB: servlet-api.jar must be on the classpath). Et voilà, org/clojure/ClojureServlet.class!

Now to the Clojure code, I created org/clojure/ClojureServlet.clj to define the methods and launched Tomcat to test the servlet. It crashed in the init method.
In org/clojure/ClojureServlet.clj I have:

(in-ns 'org.clojure.ClojureServlet)
(clojure/refer 'clojure)

(defn init [this config]

Cause: init does not call super.init(ServletConfig) as GenericServlet mandates it. Hopefully GenericServlet provides an init() which is easier to override, so I changed the code to:

(in-ns 'org.clojure.ClojureServlet)
(clojure/refer 'clojure)

(defn init-void [this]
  ;censored, again

which override only the init method taking no arguments. The bug was fixed.

To override a specific signature, define a var named methodName-firstArgClassName-secondClassName... (e.g. init-ServletConfig) or methodName-void if no arguments.


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