Reflection in Clojure

I recently got a question about dynamically loading and instantiating Java classes in Clojure. The goal was to check if a certain library was available on the classpath. If it was available it should be loaded and used, if not, a simpler default version would be used instead.

Given Clojure's dynamic nature, it would be reasonable to believe that this is a simple thing. Perhaps it is but, I didn't find an obvious solution to this problem in pure Clojure. Luckily, since Clojure runs on the JVM and interops nicely with Java, there is a pretty straight-forward way of solving this; finding and instantiating the classes needed aren't too hard to accomplish using Java and some good old reflection.

In the code example below the class we are looking for has a useful default constructor. If that's not the case, instantiating the class requires a little bit more work. But once you have a reference to that instance, using it from Clojure is just like using any other Java class from Clojure!

In the example below we prefer to call on Joda Time to provide us with the current date and time, but if the library is not in our classpath, we fall back to using java.util.Date.

(defn exists? [c]
  (let [loader (.getContextClassLoader (Thread/currentThread))]
    (try
      (Class/forName c false loader)
      true
      (catch ClassNotFoundException cnfe false))))

(defn print-java-date-now []
  (println (java.util.Date.)))

(defn print-joda-date-now []
  (let [clazz (Class/forName "org.joda.time.DateTime")]
    (println (.newInstance clazz))))

(if (exists? "org.joda.time.DateTime")
  (print-joda-date-now)
  (print-java-date-now))

Update 2012-10-22:

My colleague Mattias came up with this much nicer version of the exists? function using clojure.reflect/resolve-class:

(defn exists? [c] 
  (resolve-class (.getContextClassLoader (Thread/currentThread)) c))

Spring Break!

Last week the Citerus team spent a few fantastic days in Morocco. We try to take these trips somewhat regularly and over the years we have been to Italy, Czech Republic, and France among other places, but this trip was the first time we left Europe.

Citerus has been bringing quite a few new people on board over the last months, so this was a great opportunity to hang out with all our people for an extended time. Some for the first time as members of the Citerus team.

I must say I am amazed with the talents that have joined us for quite some time now. In fact, it blows me away. Spending time with my colleagues makes me feel old, slow and a bit stupid, and when building a great business, this is not a bad thing. I am very proud of being a part of this awesome group of people, you constantly learn new things, you are constantly challenged, and you always have fun.

Even though we bring our entire team together one day per month to share ideas and spend time together, doing this in new environments for an extended period really opens up a lot of great opportunities. The amazing views of the Atlas mountains as well as pool side work in Marrakech turned out to be perfect for creativity and focus. Look out for the new products we will be launching and exciting articles we will be publishing during the spring as a direct result of these last few days.

Feel like joining us? We’re hiring!

How Sad

How sad, no posts in over a year. Should we do something about that perhaps?