Getting Leiningen feature-parity in Boot

An introduction to the Boot build tool and how to get started on your Clojure project

David Humphreys
Software Engineer

For a basic Clojure project it can be daunting to begin with Boot (even for the experienced Clojurist). This post will go through the basics of setting up the build.boot file to keep the main tasks in Leiningen.

This post assumes a lot of prior knowledge in Leiningen and at least some familiarity with Boot.

We start from a basic Lein project; project.clj with src & test folders. I started with lein new my-project to get the structure.

Feature-parity

Of course, not everything will be exactly the same, there will be some differences between the two build systems. The main features that I will show are:

• REPL
• test
• jar
• uberjar

REPL

By far, the most important task is to launch a REPL to work effectively.

To get your work done, both src & test directories need to be available.

(set-env!
:source-paths #{"src" "test"}
:dependencies '[[org.clojure/clojure "1.7.0"]])

Many developers have a namespace loaded on REPL startup to help with interactive development: to reload source files; to startup systems; and to run tests. We want to have this available to use when starting a REPL in boot.

I usually call this namespace user which lives in a supplementary dev folder. This folder is added in to the REPL only.

If there is a boot hook available, the simplest way to change the task is to use task-options! to set defaults. For more complicated replacement we need to use replace-task!. Below is an example

(task-options!
repl {:eval (println "Howdy!")
;; my user ns in dev folder
:init-ns 'user
;; skip the Boot automatic ns injection, also called user
:skip-init true})

[r repl] (fn [& xs]
(merge-env! :source-paths #{"dev"})
;; just to verify the change, not required
(println "Our paths are now" (get-env :source-paths))
;; and continue on with our original repl task.
(apply r xs)))

So things got complicated real fast! Stick with it, it should make sense soon.

Test

In Leiningen when we run lein test it just works. There isn't much to worry about. In Boot, it will be a bit more involved.

We must add some more boot code to our script.

;; add in to the existing :dependencies
(set-env! :dependencies '[[adzerk/boot-test "1.1.0" :scope "test"]])

(require '[adzerk.boot-test :refer :all])

Now we have boot test available as a task. If you use Leiningen's test selectors to filter tests then more work is needed to run these tests.

Creating a JAR

In Leiningen, the jar command will do many things: compile classes (if there is a :main or :aot declared); create the pom file; then package clj from src and the classes into a JAR.

If we just run boot jar without any configuration we won't get much, just an empty project.jar in the target directory!

Understanding the pipeline

We must understand how to pipeline tasks in Boot. This is where Boot starts to digress from Leiningen.

We wish to make our own task that will do all of the Leiningen sub-tasks in one go.

(deftask build
"Build the JAR file"
[]
(comp
(aot)
(pom)
(jar)))

We have now created a build task to compose all of the tasks together. It's quite simple really. But in order for this to work we will need to set some options for pom. Also we'd like to compile some files. And set the main class.

;; again, both should be added into the existing declarations.

(set-env! :project 'my-project
:version "0.1.0-SNAPSHOT")

:version (get-env :version)}
aot {:namespace '#{my-project.core}}
jar {:main 'my-project.core})

If we inspect the JAR after running boot build

$boot build Howdy # <- we still see our REPL running! Writing pom.xml and pom.properties... Writing my-project-0.1.0-SNAPSHOT.jar...$ unzip -l target/my-project-0.1.0-SNAPSHOT.jar
Archive  target/my-project-0.1.0-SNAPSHOT.jar
Length     Date   Time    Name
--------    ----   ----    ----
0  01-05-16 10:54   META-INF/
0  01-05-16 10:54   META-INF/maven/
0  01-05-16 10:54   META-INF/maven/my-project/
0  01-05-16 10:54   META-INF/maven/my-project/my-project/
146  01-05-16 10:54   META-INF/maven/my-project/my-project/pom.properties
1063  01-05-16 10:54   META-INF/maven/my-project/my-project/pom.xml
25  01-05-16 10:54   META-INF/MANIFEST.MF
--------                   -------
1234                   7 files

D'oh! We don't get anything.

Resources & sources

You must set :resource-paths #{"src"} in order to include the clj files in the JAR. Leiningen has helpfully included our src directory as part of the resources to copy into the JAR. Boot has done the opposite, we must explicitly include our sources if we want to publish them.

Building an uberjar

In a similar fashion to building a jar we can create our own pipeline task.

(deftask build-uber
"Build the uberjar file"
[]
(comp
(aot)
(pom)
(uber)
(jar)))

Unfortunately this just makes a jar named project.jar. We can alter the jar name by setting task options on our new task.

(deftask build-uber
"Build the uberjar file"
[]
(comp
(aot)
(pom)
(uber)
(jar :file (format "%s-%s-standalone.jar" (get-env :project) (get-env :version)))))

It's a bit awkward but it works.

Conclusion

Leiningen has been around for years and it works really well for creating projects. There are some nice features built-in to get working quickly without needing to know a lot of things.

Boot has some really nice features, most of which are not displayed in this demonstration. The fact that you can simply pipeline all of the subtasks in the build process means that you can replace any part you don't like. It is not for the feint of heart, there is a lot to set up before you can get hacking effectively.

The final files

A fully working setup is available on Github to experiment with.

The build.boot file:

;; An example setup showing how to move a simple library from Leiningen to Boot.

(set-env!

:source-paths #{"src" "test"}
;; If resource-paths is not set then the clj files will not appear in
;; the JAR or uberjar
:resource-paths #{"src"}

;; these values must be set to use the pom task.
:project 'my-project
:version "0.1.0-SNAPSHOT"

;; beware the initial quote on the vector!
:dependencies '[[org.clojure/clojure "1.7.0"]

;; we must include this boot task in order to run
;; boot test. It is scoped for the test life-cycle
;; only

;; or :refer [test] if you prefer to specify.

;; We set the default values for task options. We may override them
;; from the command line or if we call tasks.
pom {;; needed to write the pom.xml file.
:project (get-env :project)
:version (get-env :version)

"http://www.eclipse.org/legal/epl-v10.html"}

;; And url.
:url "https://juxt.pro/"}

;; beware the initial quote here too.
;; you could use :all true instead
aot {:namespace '#{my-project.core}}
jar {:main 'my-project.core}

;; we have our own dev/user.clj file that we wish to load.  We
;; skip-init so that we don't clash with Boot's user ns.
repl {:init-ns 'user
:skip-init true})

;; We want to change the behavior of the repl task to include our own
;; namespace.
[r repl] (fn [& xs]
;; we only want to have "dev" included for the REPL task
(merge-env! :source-paths #{"dev"})
(apply r xs)))

"Build the JAR file"
[] ;; we have no options for this task.

(comp
(aot)
(pom)
(jar)))

"Build the uberjar file"
[]
(comp
(aot)
(pom)
(uber)
;; In this case, we want the jar to be named in a way that mirrors
;; the Leiningen way.
(jar
:file (format "%s-%s-standalone.jar"
(get-env :project)
(get-env :version)))))

versus project.clj definition:

(defproject my-project "0.1.0-SNAPSHOT"
:description "An example project that demonstrates using Boot as a build tool."
:url "https://juxt.pro/"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.7.0"]]
:main my-project.core
:profiles {:dev {:source-paths ["dev"]}}
:repl-options {:init-ns user})
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