Clojure: Casting to a Java class…or not!
I have a bit of Java code for working out the final destination of a URL assuming that there might be one redirect which looks like this:
private String resolveUrl(String url) { try { HttpURLConnection con = (HttpURLConnection) (new URL(url).openConnection()); con.setInstanceFollowRedirects(false); con.connect(); int responseCode = con.getResponseCode(); if (String.valueOf(responseCode).startsWith("3")) { return con.getHeaderField("Location"); } } catch (IOException e) { return url; } return url; }
I need to cast to HttpURLConnection on the first line so that I can make the call to setInstanceFollowRedirects which isn’t available on URLConnection.
I wanted to write some similar code in Clojure and my first thought was that I needed to work out how to do the cast, which I didn’t know how to do.
I then remembered that Clojure is actually dynamically typed so there isn’t any need – as long as the object has the method that we want to call on it everything will be fine.
In this case we end up with the following code:
(defn resolve-url [url] (let [con (.. (new URL url) openConnection)] (doall (.setInstanceFollowRedirects con false) (.connect con)) (if (.startsWith (str (.getResponseCode con)) "3") (.getHeaderField con "Location") url)))
Which can be simplified to this:
(defn resolve-url [url] (let [con (doto (.. (new URL url) openConnection) (.setInstanceFollowRedirects false) (.connect))] (if (.startsWith (str (.getResponseCode con)) "3") (.getHeaderField con "Location") url)))
