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For those of you new to the JVM who have never touched Ant or Maven in anger: don't panic. Leiningen is designed with you in mind. This tutorial will help you get started and explain Leiningen's take on project building and JVM-land dependency management.

Creating a Project

We'll assume you've got Leiningen installed as per the README. Generating a new project is easy:

$ lein new myproject

Created new project in: myproject

$ cd myproject
$ tree
|-- project.clj
|-- src
|   `-- myproject
|       `-- core.clj
`-- test
    `-- myproject
        `-- core_test.clj

Here we've got your project's README, a src/ directory containing implementation code, a test/ directory, and a project.clj file which describes your project to Leiningen. The src/myproject/core.clj file corresponds to the myproject.core namespace.

Note that we use that instead of just myproject since single-segment namespaces are discouraged in Clojure. Also the file test/myproject/core_test.clj corresponds with the myproject.core-test namespace--you need to remember to replace dashes in namespace names with underscores in file names on disk since the JVM has trouble loading files with dashes in the name.


You can package your project up now, even though at this stage it's fairly useless:

$ lein jar

Created ~/src/myproject/myproject-1.0.0-SNAPSHOT.jar

Libraries for the JVM are packaged up as .jar files, which are basically just .zip files with a little extra JVM-specific metadata. They usually contain .class files (JVM bytecode) and .clj source files, but they can also contain other things like config files. Leiningen downloads them from remote Maven repositories for you.


$ cat project.clj

(defproject myproject "1.0.0-SNAPSHOT"
  :description "FIXME: write"
  :dependencies [[org.clojure/clojure "1.1.0"]
                 [org.clojure/clojure-contrib "1.1.0"]])

Fill in the :description with a short paragraph so that your project will show up in search results once you upload to Clojars (as described below). At some point you'll need to flesh out the README too, but for now let's skip ahead to setting :dependencies. Note that Clojure is just another dependency here. Unlike most languages, it's easy to swap out any version of Clojure. If you're using Clojure Contrib, make sure that version matches the Clojure version.

If you've got a simple pure-clojure project, you will be fine with the default of depending only on Clojure and Contrib, but otherwise you'll need to list other dependencies.


Clojars is the Clojure community's centralized jar repository, and it's where you'll find Clojure dependencies for your project. Each dependency even lists out the snippet you'll need to put in your project.clj to use it. Let's take a look at what it would take to add a library named Robert Hooke:

It's available on Clojars with the Leiningen dependency notation shown as below:

[robert/hooke "1.0.1"]
  • "robert" is called the "group-id"
  • "hooke" is called the "artifact-id"
  • "1.0.1" is the version of the jar file you require

For projects on Clojars, often the group-id is the same as the artifact-id, in which case you may leave it out of the Leiningen dependency notation. For Java libraries often a domain name is used as the group-id. The group and artifact names and version at the top of the defproject form in project.clj follows the same rules.

Java libraries can be found by searching Jarvana, though you'll need to translate the Maven XML notation into Leiningen's. Lucene is a typical example:


This becomes:

[org.apache.lucene/lucene-core "3.0.2"]

Sometimes versions will end in "-SNAPSHOT". This means that it is not an official release but a development build. Relying on snapshot dependencies is discouraged but is sometimes necessary if you need bug fixes etc. that have not made their way into a release yet. Adding a snapshot dependency to your project will cause Leiningen to actively go seek out the latest version of the dependency once a day when you run lein deps, (whereas normal release versions are cached in the local repository) so if you have a lot of snapshots it will slow things down.

Speaking of the local repository, all the dependencies you pull in using Leiningen or Maven get cached in $HOME/.m2/repository since Leiningen uses the Maven API under the covers. You can install the current project in the local repository with this command:

$ lein install

Wrote pom.xml
[INFO] Installing myproject-1.0.0-SNAPSHOT.jar to ~/.m2/repository/myproject/myproject/1.0.0-SNAPSHOT/myproject-1.0.0-SNAPSHOT.jar

Generally Leiningen will fetch your dependencies when they're needed, but if you have just added a new dependency and you want to force it to fetch it, you can do that too:

$ lein deps

Copying 4 files to ~/src/myproject/lib
Copied :dependencies into ~/src/myproject/lib.

Dependencies are downloaded from Clojars, the central Maven (Java) repository, the official Clojure build server, and any other repositories that you add to your project.clj file. See :repositories in sample.project.clj.

If you've confirmed that your project will work with a number of different versions of a given dependency, you can provide a range instead of a single version:

[org.clojure/clojure "[1.1,1.2]"] ; <= will match 1.1.0 through 1.2.0.

See Maven's version range specification for details.

Dev Dependencies

Sometimes you want to pull in dependencies that are really only for your convenience while developing; they aren't strictly required for the project to function. Leiningen calls these :dev-dependencies. They're listed in project.clj alongside regular dependencies and downloaded when you run lein deps, but they are not brought along when another project depends on your project. Using swank-clojure for Emacs support would be a typical example; you may not want it included at runtime, but it's useful while you're hacking on the project.

Writing the Code

This is the part Leiningen can't really help you with; you're on your own here. Well--not quite. Leiningen can help you with running your tests.

$ lein test

Testing myproject.core-test
FAIL in (replace-me) (core_test.clj:6)
No tests have been written.
expected: false
  actual: false
Ran 1 tests containing 1 assertions.
1 failures, 0 errors.

Of course, we haven't written any tests yet, so we've just got the skeleton failing tests that Leiningen gave us with lein new. But once we fill it in the test suite will become more useful. Sometimes if you've got a large test suite you'll want to run just one or two namespaces at a time:

$ lein test myproject.parser-test

Testing myproject.parser-test
Ran 2 tests containing 10 assertions.
0 failures, 0 errors.

Because it must start a new process, lein test is not a good solution for a development cycle that involves running the tests often. For that you would either need to look into better editor integration (see clojure-test-mode for Emacs) or keep a repl open and call run-tests from there as you work.


If you're lucky you'll be able to get away without doing any AOT (ahead-of-time) compilation. But there are some Java interop features that require it, so if you need to use them you should add an :aot option into your project.clj file. It should be a seq of namespaces you want AOT-compiled. Again, the sample.project.clj has example usage.

Like dependencies, this should happen for you automatically, but if you need to force it you can:

$ lein compile

Compiling myproject.core

For your code to compile, it must be run. This means that you shouldn't have any code with side-effects in the top-level. If you have code that should run on startup, place it in a -main function as explained below under "Uberjar".

What to do with it

Generally speaking, there are three different goals that are typical of Leiningen projects:

  • An application you can distribute to end-users
  • A library
  • A server-side application

For the first, you will want to build an uberjar. For libraries, you will want to have them published to a repository like Clojars. For server-side applications it varies as described below.


The uberjar task is used to create a standalone, executable jar. For this to work you'll need to specify a namespace as your :main in project.clj. By this point our project.clj file should look like this:

(defproject myproject "1.0.0-SNAPSHOT"
  :description "This project is MINE."
  :dependencies [[org.clojure/clojure "1.1.0"]
                 [org.clojure/clojure-contrib "1.1.0"]
                 [org.apache.lucene/lucene-core "3.0.2"]
                 [robert/hooke "1.0.1"]]
  :main myproject.core)

The namespace you specify will need to contain a -main function that will get called when your standalone jar is run. This namespace should have a (:gen-class) declaration in the ns form at the top. The -main function will get passed the command-line arguments. Let's try something simple in src/myproject/core.clj:

(ns myproject.core

(defn -main [& args]
  (println "Welcome to my project! These are your args:" args))

Now we're ready to generate your uberjar:

$ lein uberjar
Cleaning up
Copying 4 files to /home/phil/src/leiningen/myproject/lib
Created ~/src/myproject/myproject-1.0.0.jar
Including myproject-1.0.0-SNAPSHOT.jar
Including clojure-contrib-1.1.0.jar
Including hooke-1.0.1.jar
Including clojure-1.1.0.jar
Including lucene-core-3.0.2.jar
Created myproject-1.0.0-standalone.jar

This creates a single jar file that contains the contents of all your dependencies. Users can run it with a simple java invocation, or on some systems just by double-clicking the jar file.

$ java -jar myproject-1.0.0-standalone.jar Hello world.
Welcome to my project! These are your args: (Hello world.)

You can run a regular (non-uber) jar with the java command-line tool, but that requires constructing the classpath yourself, so it's not a good solution for end-users.


If your project is a library and you would like others to be able to use it as a dependency in their projects, you will need to get it into a public repository. While it's possible to maintain your own repository or get it into Maven central, the easiest way is to publish it at Clojars. Once you have created an account there, publishing is easy:

$ lein jar && lein pom
$ scp pom.xml myproject-1.0.0.jar

Once that succeeds it will be available as a package on which other projects may depend. You will need to have permission to publish to the project's group-id under Clojars, though if that group-id doesn't exist yet then Clojars will automatically create it and give you permissions.

Sometimes you'll need to publish libraries that you don't directly maintain, either because the original maintainer hasn't published it or because you need some bugfixes that haven't been applied upstream yet. In this case you don't want to publish it under its original group-id, since this will prevent the true maintainer from using that group-id once they publish it. You should use "org.clojars.$USERNAME" as the group-id instead.

Server-side Projects

There are many ways to get your project deployed as a server-side application. Simple programs can be packaged up as tarballs with accompanied shell scripts using the lein-release plugin and then deployed using chef, pallet, or other mechanisms. Web applications may be deployed using the lein-war plugin. You can even create Hadoop projects. These kinds of deployments are so varied that they are better-handled using plugins rather than tasks that are built-in to Leiningen itself.

That's It!

If you prefer a visual introduction, try the Full Disclojure screencast on project management. Now go start coding your next project!

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