Immutant is suite of Clojure libraries that is part of the JBossAS/Wildfly ecosystem. Great, fantastic! Clojure is a lisp that runs on the JVM, the CLR, and compiles to Javascript. Lets get started!

Some links we’ll need:

Getting Leiningen

Assuming ~/bin/ is a folder in your $PATH

DEST=~/bin/lein
curl https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein > $DEST
chmod +x $DEST

Getting Clojure

You get Clojure (oddly) through lein. Just run it, and it’ll download everything it needs.

lein

That was easy!

Setting up an Immutant Project

This is a lightly expanded version of this page.

Use lein to initialize a new project.

lein new app learning
cd learning

The folder learning/ should have a file project.clj which you should edit to look like this:

(defproject learning "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "http://example.com/FIXME"
  :license {:name "MPL"
            :url "http://choosealicense.com/licenses/mpl-2.0/"}
  :dependencies [[org.clojure/clojure "1.6.0"]
                 [org.immutant/immutant "2.x.incremental.186"]]
  :repositories [["Immutant incremental builds"
                  "http://downloads.immutant.org/incremental/"]]
  :main ^:skip-aot learning.core
  :target-path "target/%s"
  :profiles {:uberjar {:aot :all}})

Note how the dependencies and repositories now contain references to immutant. You should change 186 to the latest incremental build at this page.

The project.clj file is similar to a package.json in Node.js or Cargo.toml in Rust.

Your First Route

This is a lightly expanded version of this page.

In order to have a route (and thus a real web server) you need to add a require for immutant to your src/learning/core.clj. Then, define the app function like so and use run to start it:

(ns learning.core
  (:use compojure.core)
  (:require [compojure.route :as route]
             [immutant.web :as immutant])
  (:gen-class))

(defn app [request]
  "A basic route for learning."
  {:status 200
   :body "Hello world!"})

(defn -main
  "Start the server."
  [& args]
  (immutant/run app {:host "localhost" :port 8080 :path "/"}))

To start the server with lein run in the folder.

Now browse to http://localhost:8080 and you should see something like:

Hello world!

More Routes

You’re probably thinking “Well that’s great, an app with one route, how useless.” You’d be quite right, unless you’re implementing Yo or something.

If you’re reading along the Immutant web tutorial you may have noticed in “Advanced Usage” they use multiple run functions to serve multiple routes.

To quote:

…actually creates two Undertow web server instances: one serving requests for the hello and howdy handlers on port 8080, and one serving ola responses on port 8081.

Not the most desirable option. Looking back up under “Common Usage”:

First, you’ll need a Ring handler. If you generated your app using a template from Compojure, Luminus, Caribou or some other Ring-based library, yours will be associated with the :handler key of your :ring map in your project.clj file.

That sounds certainly more desirable. Let’s use Compojure.

Change your project.clj:

:dependencies [[org.clojure/clojure "1.6.0"]
               [org.immutant/immutant "2.x.incremental.186"]
               [compojure "1.1.8"]]

And then extend your source file.

(ns learning.core
  (:use compojure.core)
  (:require [compojure.route :as route]
             [immutant.web :as immutant])
  (:gen-class))


(defn handler
  "Comment"
  []
  "<h1>Hello World</h1>")

(defroutes app
  "The router."
  (GET "/" [] (handler))
  (route/not-found
       "<h1>Page not found</h1>"))

(defn -main
  "Start the server"
  [& args]
  (immutant/run app {:host "localhost" :port 8080 :path "/"}))

Now you should have multiple routes.