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I'm developing some polyglot Java + Clojure code using the REPL most of the time (in Eclipse with CounterClockwise and Maven). In this context, it's fine to just load the source in the REPL and start interacting with the code.

However, I will ultimately want to be able to distribute and deploy the code in .jar files or similar so that it can be run as a standalone application. In this context, the code needs to be executable as a Java application with the appropriate main(...) function. And I'd like it all to build cleanly from the Maven pom.xml file.

So far I have got code working so that I have a -main function that doesn't get called when I use Clojure at the REPL, but should get called when packaged/deployed as a jar. This seems to work, but I'm not sure if this represents the best approach.

Ideally I'd like everything to work smoothly in both:

  • The Counterclockwise REPL
  • A standard Maven build process which results in a fully packaged / deployed jar file

What are the best practices for setting up a Clojure code base so that it can easily be used both at the REPL and built a standalone application?

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The only thing which could interfere with the REPL experience is a top-level side-effecty statement which should not run when the namespace is required at the REPL and yet occurs in a namespace which one might want to have (possibly indirectly) required at the REPL. Clearly that's just a bad idea (if it seems like a useful arrangement in some way -- e.g. with complex hooks for Leiningen -- the namespace should probably be split into a namespace containing the definitions and a namespace with the side-effecty code).

The natural mode of running the app will depend on which of the following classes it falls into:

  1. main entry point in Clojure
    1. don't mind AOT compilation of Clojure code
    2. prefer to avoid AOT
  2. main entry point in Java (with Clojure stuff called through clojure.lang.RT)

For 1.1. the natural approach would be to use a :gen-class'd namespace as the Main-Class in the app's jar's manifest.mf. Then you can use java -jar (if it's an überjar / stand-alone jar) / java -cp <app's cp> app.main to launch it. Leiningen helps with this if you specify a :main property in project.clj; seeing how it's the most popular build tool and its choice to support this approach (to the best of my knowledge) attracts no criticism, one can probably call it idiomatic. Maven can do the same with the usual pom.xml incantation specifying main class.

For 1.2., you can do what Leiningen does: provide a wrapper calling clojure.main with something along the lines of -e "(use 'app.core)(-main)" as arguments.

For 2., there's nothing Clojure-specific about the launch.

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3 Comments

thanks Michal - very useful and great to see the enumeration of all the viable options!
No worries. Congrats on the [clojure] badge! :-)
Thanks.... though not sure if I deserve it yet, there still seems an awful lot to learn!!

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