(:gen-class), not only the purpose, but the WHY?

Community Center Beginners
1

Hi Clojureverse,

I’m on a learning-path and picking up various examples and tutorials to get me started, besides following two excellent books, ‘Getting Clojure’ and ‘Web development with Clojure’. Then I found the following article with the associated GitHub repo, and found it useful for learning and exploring.

I noticed this little statement in the namespace declaration of the core class .

(:gen-class)

Using doc and also reading it on ClojureDoc I roughly get the idea what :gen-class is. But I don’t really understand why this is useful for this particular little script? Do I make a premature assumption when I see this as a design-practice, not really necessary? If so, what is the principle?

2

It all depends on how you plan to start the Clojure application.

If you intend to launch it from Java directly, you need to declare a :gen-class main, so that a Java Main class is created which Java can use to bootstrap your Clojure code.

If you instead use the Clojure CLI clj to launch it, you don’t need the gen-class.

It doesn’t seem the linked article talks about how they were starting the server app, but since they have a gen-class, my guess is they are doing it through java.

Edit: The github page mentions using lein to run it with the run task. The lein run task also doesn’t need the gen-class. Both clj and lein use clojure.main to bootstrap your Clojure app. So neither needs the gen-class.

1 Like
3

Thanks for clarifying. I have been running the server from both terminal lein and the IntelliJ REPL with no issues. So, I’ll just leave it as is,… and test it from a Java project as well. I just wanna make sure I don’t pick up any useless habits and idioms.

Regards, Henning

1 Like
4

The thing is you’re learning and doing practice. In a real production scenario, you would know how your server is deployed and what will kick-start it on the host. Based on that, you’d know if you need to gen-class or not your -main entry point.

I’ll try and detail things a bit more.

First off, Clojure JVM is actually Java.

So in the world of Java and the JVM. If you want to create a program, you need a public Main class which has a public static main method on it. Once you have that, you can call java on your Main class and the JVM will start and run the Java program.

java -cp classes myjavaapp.Main

A Clojure program IS a Java program. Thus to start a Clojure program you can do the same thing you’d do to start a Java program:

java -cp classes myclojureapp.Main

In which case, you need a way to create the Main class from Clojure. That’s what gen-class is for. So by adding a gen-class directive, when you compile your Clojure code, it will create a Java Main class.

Now, there’s another way to start a Clojure program, one that is Clojure specific and only works for Clojure apps, not for Java apps. That’s to use a Clojure based launcher like lein or clj. In those cases, you don’t need to use java, though you still need a JVM installed. When you use those launchers instead of the standard java launcher you don’t need a Java Main class. All you need is a Clojure namespace with a -main function in it. Which also means you do not need to have a gen-class.

clj -m myclojureapp

Or

; Given a project.clj file with :main myclojureapp defined
lein run

Will both similarly launch your Clojure program without needed a Java Main class, and thus you don’t need a gen-class.

If you intend to launch your app both with java and with lein or clj, you will need to add the gen-class. It won’t conflict with lein and clj.

5 Likes
5

There is another option you missed to run directly from java that doesn’t require AOT or clojure tools.

java -cp classes clojure.main --main ns-name

Where ns-name has a -main function, the name for the main function of your program doesn’t have to be called be -main though, you can do:

java -cp classes clojure.main ns-name

Where ns-name presumably has a function application/call entry point.

3 Likes
6

One more addition to the excellent answers above: gen-class can also be used to implement a named class that inherits a Java class (proxy can also do this, but it only gave you an anonymous class).

There are some more in-depth discussion if you like to dig further:

1 Like
7

This topic was automatically closed 182 days after the last reply. New replies are no longer allowed.