Use Java JARs within Clojure projects

I am creating a clojure wrapper for the excellent library from Pekka Enberg Falcon, which will provide Denarius with FIX protocol communication capabilities. The new project's name is, obviously, falcon-clj.

Although the ideal packaging would be to include Falcon as a Git submodule, I decided to pre-compile Denarius into a JAR stored into the /lib directory to avoid Leiningen complications in the starting phase and be able to focus on the implementation.

I compiled the library and pushed it into Github and then I received my new laptop, so I installed everything from scratch, including the JDK, and cloned everything back into the new laptop. The next stage would be to use the REPL to quickly get used to Falcon.

=> (import [falcon.fix Session Versions])
UnsupportedClassVersionError falcon/fix/Session : Unsupported major.minor version 51.0  java.lang.ClassLoader.defineClass1 (:-2)

So I digged up a little bit and found some interesting information here, which led me to here and then to correctly configure Eclipse (Aptana) to use an external JRE to run things.

Basically, you need to explicitly tell Eclipse to use the external JDK you are using. You can find the setting under Window -> Preferences -> Java -> Installed JREs.