lein

Leiningen, commonly invoked as lein, is a build automation and project management tool for Clojure. It orchestrates compilation, testing, dependency resolution, packaging, and running applications. By centralizing configuration and tooling, lein simplifies project setup and repeatable workflows. It relies on Maven-compatible repositories such as Maven Central and Clojars to fetch libraries.

Projects are described in a project.clj file, written in Clojure data structures. The file typically includes the project name and version, description, license, source and test paths, dependencies, and plugins. The defproject form defines the project, while dependencies specify library coordinates and versions, enabling transitive resolution.

Leiningen provides a set of built-in tasks for common operations: deps fetches dependencies; compile compiles source files; test runs tests; run launches a main class or namespace; jar packages a JAR; uberjar builds a standalone executable JAR; clean removes build artifacts. The repl task starts an interactive programming session. Tasks can be composed with do to run multiple steps in sequence, for example lein do clean, deps, compile, test.

Leiningen also supports scaffolding and plugins. The lein new command creates new projects from templates (such as app or plugin). The plugin system allows extending functionality, for example integrating with web servers, packaging types, or code quality tools. Leiningen uses a local cache and resolves dependencies from remote repositories, storing artifacts in the local Maven repository.

As an established tool, Leiningen remains widely used in Clojure projects despite the emergence of alternatives such as the Clojure CLI tools and the deps.edn workflow. It emphasizes a mature plugin ecosystem, reproducible builds, and a familiar project-centric workflow that many teams continue to rely on.