Leiningen "No :main namespace specified" in CI

By Kaveh Alemi·Latchkey

Leiningen needs a :main namespace to know which entry point to run or bake into an uberjar. When lein run or lein uberjar runs and :main is absent, there is nothing to launch.

What this error means

lein run fails with "No :main namespace specified. See lein help run." or the uberjar builds without a runnable entry point.

No :main namespace specified in project.clj.
See `lein help run`.

Common causes

project.clj declares no :main

The :main key is missing, so lein run has no default namespace to execute.

The main namespace lacks a -main and gen-class

For an uberjar, the entry namespace must define -main and (:gen-class) so a runnable class is produced.

How to fix it

Declare the main namespace

1. Add :main to project.clj pointing at your entry namespace.
2. Ensure that namespace defines -main and (:gen-class).
3. Run lein run to confirm the entry point launches.

(defproject myapp "0.1.0"
  :main ^:skip-aot myapp.core
  :dependencies [[org.clojure/clojure "1.11.1"]])

Define the entry point

Give the main namespace a -main function and gen-class so it can run and be AOT compiled for an uberjar.

(ns myapp.core (:gen-class))
(defn -main [& args] (println "started"))

How to prevent it

• Set :main in project.clj for runnable projects.
• Define -main with (:gen-class) in the entry namespace.
• Test lein run locally before relying on it in CI.

Frequently asked questions

Related guides

References

• Apache Maven guides
• Gradle user manual
• GitHub Actions documentation