Project automation for Clojure
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
**Table of Contents** *generated with [DocToc](*
- [Tutorial](#tutorial)
- [What This Tutorial Covers](#what-this-tutorial-covers)
- [Getting Help](#getting-help)
- [Leiningen Projects](#leiningen-projects)
- [Creating a Project](#creating-a-project)
- [Directory Layout](#directory-layout)
- [Filename-to-Namespace Mapping Convention](#filename-to-namespace-mapping-convention)
- [project.clj](#projectclj)
- [Dependencies](#dependencies)
- [Overview](#overview)
- [Artifact IDs, Groups, and Versions](#artifact-ids-groups-and-versions)
- [Snapshot Versions](#snapshot-versions)
- [Repositories](#repositories)
- [Checkout Dependencies](#checkout-dependencies)
- [Search](#search)
- [Setting JVM Options](#setting-jvm-options)
- [Running Code](#running-code)
- [Tests](#tests)
- [Profiles](#profiles)
- [What to do with it](#what-to-do-with-it)
- [Uberjar](#uberjar)
- [Framework (Uber)jars](#framework-uberjars)
- [Server-side Projects](#server-side-projects)
- [Publishing Libraries](#publishing-libraries)
- [That's It!](#thats-it)
<!-- END doctoc generated TOC please keep comment here to allow auto update -->
# Tutorial
Leiningen is for automating Clojure projects without setting your hair
on fire. If you experience your hair catching on fire or any other
frustrations while following this tutorial, please
[let us know](
It offers various project-related tasks and can:
* create new projects
* fetch dependencies for your project
* run tests
* run a fully-configured REPL
* compile Java sources (if any)
* run the project (if the project isn't a library)
* generate a maven-style "pom" file for the project for interop
* compile and package projects for deployment
* publish libraries to repositories such as [Clojars](
* run custom automation tasks written in Clojure (leiningen plug-ins)
If you come from the Java world, Leiningen could be thought of as
"Maven meets Ant without the pain". For Ruby and Python folks,
Leiningen combines RubyGems/Bundler/Rake and pip/Fabric in a single
## What This Tutorial Covers
This tutorial will briefly cover project structure, dependency
management, running tests, the REPL, and topics related to deployment.
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 automation and JVM-land dependency management.
## Getting Help
Also keep in mind that Leiningen ships with fairly comprehensive help;
`lein help` gives a list of tasks while `lein help $TASK` provides
details. Further documentation such as the readme, sample
configuration, and even this tutorial are also provided.
## Leiningen Projects
Leiningen works with *projects*. A project is a directory containing a
group of Clojure (and possibly Java) source files, along with a bit of
metadata about them. The metadata is stored in a file named
`project.clj` in the project's root directory, which is how you tell
Leiningen about things like
* Project name
* Project description
* What libraries the project depends on
* What Clojure version to use
* Where to find source files
* What's the main namespace of the app
and more.
Most Leiningen tasks only make sense in the context of a project. Some
(for example, `repl` or `help`) can also be called from any directory.
Next let's take a look at how projects are created.
## Creating a Project
We'll assume you've got Leiningen installed as per the
Generating a new project is easy:
$ lein new app my-stuff
Generating a project called my-stuff based on the 'app' template.
$ # see how it looks like using the "tree" command
$ tree -F -a --dirsfirst my-stuff/
├── doc/
├── resources/
├── src/
   └── my_stuff/
   └── core.clj
├── test/
   └── my_stuff/
   └── core_test.clj
├── .gitignore
├── .hgignore
├── project.clj
In this example we're using the `app` template, which is intended for
an application project rather than a library. Omitting the `app`
argument will use the `default` template, which is suitable for
### Directory Layout
Here we've got your project's README, a `src/` directory containing the
code, a `test/` directory, and a `project.clj` file which describes your
project to Leiningen. The `src/my_stuff/core.clj` file corresponds to
the `my-stuff.core` namespace.
### Filename-to-Namespace Mapping Convention
Note that we use `my-stuff.core` instead of just `my-stuff` since
[single-segment namespaces are discouraged in Clojure]( as using those would imply classes are being assigned
to the default (no-name) package.
Also note that if a Clojure namespaces segment contains a dash (`-`), the
corresponding path/filename will contain an underscore (`_`) instead. This is due to the fact that
[Java disallows dashes in identifiers](,
in particular in package and class names. A Clojure "dash-adorned" namespace identifier is thus mapped
to a Java-compatible "underscore-adorned" package identifier. This change is reflected in pathnames
as these must match the package and class names.
The intricacies of namespaces are a common source of confusion for newcomers, and while they are
mostly outside the scope of this tutorial you can
read up on them elsewhere, for example [here]( and [here](
## project.clj
Your `project.clj` file will start off looking something like this:
(defproject my-stuff "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:url ""
:license {:name "Eclipse Public License"
:url ""}
:dependencies [[org.clojure/clojure "1.8.0"]]
:main ^:skip-aot my-stuff.core
:target-path "target/%s"
:profiles {:uberjar {:aot :all}})
If you don't fill in the `:description` with a short sentence, your
project will be harder to find in search results, so start there. Be
sure to fix the `:url` as well. At some point you'll need to flesh out
the `` file 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.
## Dependencies
### Overview
Clojure is a hosted language and Clojure libraries are distributed the same
way as in other JVM languages: as [jar]( files.
Jar files 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, JavaScript files or text files with static data.
Published JVM libraries have *identifiers* (artifact group, artifact id) and
*versions* based on [Maven naming conventions](
### Artifact IDs, Groups, and Versions
You can [search Clojars]( using
its web interface or via `lein search $TERM`. On the Clojars page for
`clj-http` at the time of this writing it shows this:
[clj-http "2.0.0"]
It also shows the Maven and Gradle syntax for dependencies. You can copy the
Leiningen version directly into the `:dependencies` vector in
`project.clj`. So for instance, if you change the `:dependencies`
line in the example `project.clj` above to
:dependencies [[org.clojure/clojure "1.8.0"]
[clj-http "2.0.0"]]
Leiningen will automatically download the `clj-http` jar file and make sure
it is on your classpath. If you want to explicitly tell `lein` to
download new dependencies, you can do so with `lein deps`, but it will
happen on-demand if you don't.
Within the vector, "clj-http" is referred to as the "artifact id".
"2.0.0" is the version. Some libraries will also have "group ids",
which are displayed like this:
[com.cedarsoft.utils.legacy/hibernate "1.3.7"]
The group id is the part before the slash. Especially for Java
libraries, it's often a reversed domain name. Clojure libraries often
use the same group-id and artifact-id (as with clj-http), in which case
you can omit the group-id. If there is a library that's part of a
larger group (such as `ring-jetty-adapter` being part of the `ring`
project), the group-id is often the same across all the sub-projects.
### Snapshot Versions
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. However,
snapshot versions are not guaranteed to stick around, so it's
important that non-development releases never depend upon snapshot versions that
you don't control. Adding a snapshot dependency to your project will
cause Leiningen to actively go seek out the latest version of the
dependency daily (whereas normal release versions are cached in the local
repository) so if you have a lot of snapshots it will slow things
Note that some libraries make their group-id and artifact-id
correspond with the namespace they provide inside the jar, but this is
just a convention. There is no guarantee they will match up at all, so
consult the library's documentation before writing your `:require`
and `:import` clauses.
### Repositories
Dependencies are stored in *artifact repositories*. If you are
familiar with Perl's CPAN, Python's Cheeseshop (aka PyPi), Ruby's, or Node.js's NPM, it's the same thing. Leiningen reuses
existing JVM repository infrastructure. There are several popular
open source repositories. Leiningen by default will use two of them:
[]( and
[Maven Central](
[Clojars]( is the Clojure community's centralized
Maven repository, while [Central]( is for the
wider JVM community.
You can add third-party repositories by setting the `:repositories` key
in project.clj. See the
for examples on how to do so. This sample uses additional repositories such as the Sonatype
repository which gives access to the latest SNAPSHOT development version of a library (Clojure or Java).
It also contains other relevant settings regarding repositories such as update frequency.
### Checkout Dependencies
Sometimes it is necessary to develop two or more projects in parallel,
the main project and its dependencies, but it is very inconvenient to
run `lein install` and restart your repl all the time to get your
changes picked up. Leiningen provides a solution called *checkout
dependencies* (or just *checkouts*). To use it, create a directory
called `checkouts` in the project root, like so:
├── checkouts/ <--- here
├── doc/
├── resources/
├── src/
   └── my_stuff/
   └── core.clj
├── test/
   └── my_stuff/
   └── core_test.clj
├── .gitignore
├── .hgignore
├── project.clj
Then, under the checkouts directory, create symlinks to the root directories of projects you need.
The names of the symlinks don't matter: Leiningen just follows all of them to find
`project.clj` files to use. Traditionally, they have the same name as the directory they point to.
├── checkouts/
   ├── commons -> [link to /code/company/commons]
   └── suchwow -> [link to /code/oss/suchwow]
Libraries located under the `checkouts` directory take precedence over
libraries pulled from repositories, but this is not a replacement for listing
the project in your main project's `:dependencies`; it simply supplements
that for convenience. The project in `:dependencies` must be able to be
resolved, either from a remote repo or via `lein install` locally. That is,
given the above directory hierarchy, `project.clj` should contain something
:dependencies [[org.clojure/clojure "1.9.0"]
[suchwow "0.3.9"]
[com.megacorp/commons "1.3.5"]
Note here that the Maven groupid `com.megacorp` has no effect on the way checkouts work.
The `suchwow` and `commons` links look the same in `checkouts`, and the groupid
hierarchy doesn't need to appear in the way `commons` is actually laid out on disk.
After you've updated `:dependencies`, `lein` will still need to be able
to find the library in some repository like clojars or your `~/.m2`
directory. If `lein` complains with a message like "Could not find artifact suchwow:jar:0.3.9",
it's possible that `project.clj` and `suchwow/project.clj` use different version numbers.
It's also possible that you're working on the main project and `suchwow` at the same time,
have bumped the version number in both project files, but still have the old version in your
local Maven repository. Run `lein install` in the `suchwow` directory. That is: the `suchwow`
version number must be the same in *three* places:
in suchwow's `project.clj`, in the main project's `project.clj`, *and in some repository the main project uses*.
If you change the
dependencies of a checkout project you will still have to run `lein
install` and restart your repl; it's just that source changes will be
picked up immediately.
Checkouts are an opt-in feature; not everyone who is working on the
project will have the same set of checkouts, so your project should
work without checkouts before you push or merge.
Make sure not to override the `base` profile while using checkouts. In
practice that usually means using `lein with-profile +foo run` rather
than `lein with-profile foo run`.
### Search
Leiningen supports searching remote Maven repositories for matching
jars with the command `lein search $TERM`. Currently only searching
Central and Clojars is supported.
### Maven Read Timeout
The underlying [Maven Wagon]( transport
reads the `maven.wagon.rto` system property to determine the timeout used
when downloading artifacts from a repository. The `lein` script sets that property to be 10000.
If that timeout isn't long enough (for example, when using a slow corporate mirror),
it can be overridden via `LEIN_JVM_OPTS`:
export LEIN_JVM_OPTS="-Dmaven.wagon.rto=1800000"
## Setting JVM Options
To pass extra arguments to the JVM, set the `:jvm-opts` vector. This will override
any default JVM opts set by Leiningen.
:jvm-opts ["-Xmx1g"]
If you want to pass [compiler options](
to the Clojure compiler, you also do this here.
:jvm-opts ["-Dclojure.compiler.disable-locals-clearing=true"
"-Dclojure.compiler.elide-meta=[:doc :file :line :added]"
; notice the array is not quoted like it would be if you passed it directly on the command line.
You can also pass options to Leiningen in the `JVM_OPTS` environment variable. If you
want to provide the Leiningen JVM with custom options, set them in `LEIN_JVM_OPTS`.
## Running Code
Enough setup; let's see some code running. Start with a REPL
(read-eval-print loop):
$ cd my-stuff
$ lein repl
nREPL server started on port 55568 on host - nrepl://
REPL-y 0.5.1, nREPL 0.8.3
Clojure 1.10.1
OpenJDK 64-Bit Server VM 1.8.0_222-b10
Docs: (doc function-name-here)
(find-doc "part-of-name-here")
Source: (source function-name-here)
Javadoc: (javadoc java-object-or-class-here)
Exit: Control+D or (exit) or (quit)
Results: Stored in vars *1, *2, *3, an exception in *e
The REPL is an interactive prompt where you can enter arbitrary code
to run in the context of your project. Since we've added `clj-http` to
`:dependencies` earlier, we are able to load it here along with code from the
`my-stuff.core` namespace in your project's own `src/` directory:
my-stuff.core=> (require 'my-stuff.core)
my-stuff.core=> (my-stuff.core/-main)
Hello, World!
my-stuff.core=> (require '[clj-http.client :as http])
my-stuff.core=> (def response (http/get ""))
my-stuff.core=> (keys response)
(:status :headers :body :request-time :trace-redirects :orig-content-encoding)
The call to `-main` shows both println output ("Hello, World!") and
the return value (nil) together.
Built-in documentation is available via `doc`, and you can examine the
source of functions with `source`:
my-stuff.core=> (source -main)
(defn -main
"I don't do a whole lot ... yet."
[& args]
(println "Hello, World!"))
my-stuff.core=> ; use control+d to exit
If you already have code in a `-main` function ready to go and don't
need to enter code interactively, the `run` task is simpler:
$ lein run
Hello, World!
Providing a `-m` argument will tell Leiningen to look for
the `-main` function in another namespace. Setting a default `:main` in
`project.clj` lets you omit `-m`.
For long-running `lein run` processes, you may wish to save memory
with the higher-order trampoline task, which allows the Leiningen JVM
process to exit before launching your project's JVM.
$ lein trampoline run -m my-stuff.server 5000
If you have any Java to be compiled in `:java-source-paths` or Clojure
namespaces listed in `:aot`, they will always be compiled before
Leiningen runs any other code, via any `run`, `repl`,
etc. invocations.
## Tests
We haven't written any tests yet, but we can run the failing tests
included from the project template:
$ lein test
lein test my-stuff.core-test
lein test :only my-stuff.core-test/a-test
FAIL in (a-test) (core_test.clj:7)
FIXME, I fail.
expected: (= 0 1)
actual: (not (= 0 1))
Ran 1 tests containing 1 assertions.
1 failures, 0 errors.
Tests failed.
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 my-stuff.core-test` will do that. You
also might want to break up your tests using test selectors; see `lein
help test` for more details.
Running `lein test` from the command-line is suitable for regression
testing, but the slow startup time of the JVM makes it a poor fit for
testing styles that require tighter feedback loops. In these cases,
either keep a repl open for running the appropriate call to
or look into editor integration such as
Keep in mind that while keeping a running process around is convenient,
it's easy for that process to get into a state that doesn't reflect
the files on disk: functions that are loaded and then deleted from the
file will remain in memory, making it easy to miss problems arising
from missing functions (often referred to as "getting
slimed"). Because of this it's advised to do a `lein test` run with a
fresh instance periodically in any case, perhaps before you commit.
## Profiles
Profiles are used to add various things into your project map in
different contexts. For instance, during `lein test` runs, the
contents of the `:test` profile, if present, will be merged into your
project map. You can use this to enable configuration that should only
be applied during test runs, either by adding directories containing
config files to your classpath via `:resource-paths` or by other
means. See `lein help profiles` for more details.
Unless you tell it otherwise, Leiningen will merge the default set of
profiles into the project map. This includes user-wide settings from
your `:user` profile, the `:dev` profile from `project.clj` if
present, and the built-in `:base` profile which contains dev tools
like nREPL and optimizations which help startup time at the expense of
runtime performance. Never benchmark with the default profiles. (See
the FAQ entry for "tiered compilation")
## 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 server-side application
* A library for other Clojure projects to consume
For the first, you typically build an uberjar. For libraries, you will
want to have them published to a repository like Clojars or a private
repository. For server-side applications it varies as described below.
Generating a project with `lein new app myapp` will start you out with
a few extra defaults suitable for non-library projects, or you can
browse the
[available templates on Clojars](
for things like specific web technologies or other project types.
### Uberjar
The simplest thing to do is to distribute an
[uberjar]( This is a single
standalone executable jar file most suitable for giving to
nontechnical users. For this to work you'll need to specify a
namespace as your `:main` in `project.clj` and ensure it's also AOT (Ahead Of Time)
compiled by adding it to `:aot`. By this point, our `project.clj` file
should look like this:
(defproject my-stuff "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:url ""
:license {:name "Eclipse Public License"
:url ""}
:dependencies [[org.clojure/clojure "1.8.0"]
[clj-http "2.0.0"]]
:profiles {:dev {:dependencies [[ring/ring-devel "1.4.0"]]}}
:main my-stuff.core
:aot [my-stuff.core])
We have also added a development dependency, `ring-devel`. `ring-devel` will not
be available in uberjars, and will not be considered a dependency if you publish
this project to a repository.
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 easy in `src/my_stuff/core.clj`:
(ns my-stuff.core
(defn -main [& args]
(println "Welcome to my project! These are your args:" args))
Now we're ready to generate your uberjar:
$ lein uberjar
Compiling my-stuff.core
Created /home/phil/my-stuff/target/uberjar+uberjar/my-stuff-0.1.0-SNAPSHOT.jar
Created /home/phil/my-stuff/target/uberjar/my-stuff-0.1.0-SNAPSHOT-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 my-stuff-0.1.0-SNAPSHOT-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.
Of course if your users already have Leiningen installed, you can
instruct them to use `lein run` as described above.
### Framework (Uber)jars
Many Java frameworks expect deployment of a jar file or derived archive
sub-format containing a subset of the application's necessary
dependencies. The framework expects to provide the missing dependencies
itself at run-time. Dependencies which are provided by a framework in
this fashion may be specified in the `:provided` profile. Such
dependencies will be available during compilation, testing, etc., but
won't be included by default by the `uberjar` task or plugin tasks
intended to produce stable deployment artifacts.
For example, Hadoop job jars may be just regular (uber)jar files
containing all dependencies except the Hadoop libraries themselves:
(project example.hadoop "0.1.0"
:profiles {:provided
[[org.apache.hadoop/hadoop-core "1.2.1"]]}}
:main example.hadoop)
$ lein uberjar
Compiling example.hadoop
Created /home/xmpl/src/example.hadoop/example.hadoop-0.1.0.jar
Created /home/xmpl/src/example.hadoop/example.hadoop-0.1.0-standalone.jar
$ hadoop jar example.hadoop-0.1.0-standalone.jar
12/08/24 08:28:30 INFO util.Util: resolving application jar from found main method on: example.hadoop
12/08/24 08:28:30 INFO flow.MultiMapReducePlanner: using application jar: /home/xmpl/src/example.hadoop/./example.hadoop-0.1.0-standalone.jar
Plugins are required to generate framework deployment jar derivatives
(such as [war files]( which include additional metadata, but the
`:provided` profile provides a general mechanism for handling the
framework dependencies.
### Server-side Projects
There are many ways to get your project deployed as a server-side
application. Aside from the obvious uberjar approach, simple programs can be
packaged up as [tarballs]( with
accompanied shell scripts using the [lein-tar plugin](
and then deployed using [pallet](, [chef](,
or other mechanisms.
Web applications may be deployed as uberjars using embedded Jetty with
`ring-jetty-adapter` or as [war (web application archive) files](
created by the [lein-ring plugin]( For
things beyond uberjars, server-side deployments are so varied that they
are better-handled using plugins rather than tasks that are built-in
to Leiningen itself.
It's possible to involve Leiningen during production, but there are
many subtle gotchas to that approach; it's strongly recommended to use
an uberjar if you can. If you need to launch with the `run` task, you
should use `lein trampoline run` in order to save memory, otherwise
Leiningen's own JVM will stay up and consume unnecessary memory.
In addition it's very important to ensure you take steps to freeze all
the dependencies before deploying, otherwise it could be easy to end
up with
[unrepeatable deployments](
Consider including `~/.m2/repository` in your unit of deployment
(tarball, .deb file, etc) along with your project code. It's
recommended to use Leiningen to create a deployable artifact in a
continuous integration setting. For example, you could have a
[Jenkins]( CI server run your project's full
test suite, and if it passes, upload a tarball to S3. Then deployment
is just a matter of pulling down and extracting the known-good tarball
on your production servers. Simply launching Leiningen from a checkout
on the server will work for the most basic deployments, but as soon as
you get a number of servers you run the risk of running with a
heterogeneous cluster since you're not guaranteed that each machine
will be running with the exact same codebase.
Also remember that the default profiles are included unless you
specify otherwise, which is not suitable for production. Using `lein
trampoline with-profile production run -m myapp.main` is
recommended. By default the production profile is empty, but if your
deployment includes the `~/.m2/repository` directory from the CI run
that generated the tarball, then you should add its path as
`:local-repo` along with `:offline? true` to the `:production`
profile. Staying offline prevents the deployed project from diverging
at all from the version that was tested in the CI environment.
Given these pitfalls, it's best to use an uberjar if possible.
### Publishing Libraries
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 private
or get it into [Central](, the easiest way is
to publish it at [Clojars]( Once you have
[created an account]( there, publishing
is straightforward. You'll need to have a [verified group
name](, but you get
some for free just for having a Clojars account. You'll need to change
the name of your project to include the group name. Edit the first
line of your `project.clj` to look like:
(defproject "0.1.0-SNAPSHOT"
:description "FIXME: write description"
Clojars doesn't use passwords, so you'll need to [generate a deploy
token]( Once
you have that, you are ready to deploy:
$ lein deploy clojars
No credentials found for clojars
See `lein help deploying` for how to configure credentials to avoid prompts.
Username: me
Created ~/src/my-stuff/target/my-stuff-0.1.0-SNAPSHOT.jar
Wrote ~/src/my-stuff/pom.xml
Retrieving org/clojars/my-clojars-username/my-stuff/0.1.0-SNAPSHOT/maven-metadata.xml
Sending org/clojars/my-clojars-username/my-stuff/0.1.0-SNAPSHOT/my-stuff-0.1.0-20190525.161117-2.jar (9k)
Sending org/clojars/my-clojars-username/my-stuff/0.1.0-SNAPSHOT/my-stuff-0.1.0-20190525.161117-2.pom (2k)
Retrieving org/clojars/my-clojars-username/my-stuff/maven-metadata.xml
Sending org/clojars/my-clojars-username/my-stuff/0.1.0-SNAPSHOT/maven-metadata.xml (1k)
Sending org/clojars/my-clojars-username/my-stuff/maven-metadata.xml (1k)
Once that succeeds it will be available as a package on which other
projects may depend. For instructions on storing your credentials so
they don't have to be re-entered every time, see `lein help
deploying`. When deploying a release that's not a snapshot, Leiningen
will attempt to sign it using [GPG]( to prove your
authorship of the release. See the
[deploy guide](
for details of how to set that up. The deploy guide includes
instructions for deploying to other repositories as well.
## That's It!
Now go start coding your next project!