Getting started: Clojure Web App with Compojure and Envlive

2013-05-15 ·

In this article I will show you how to create a basic Clojure web application built with Compojure and Enlive. Our HTML templates will be Pure.

You can find the code for this tutorial at Github.

Create a project

Generate a new Compojure project using Leiningen. For more details about Leiningen, check an older Clojure App 101 article.

λ lein new compojure clojure-web-101
λ tree
.
├── README.md
├── project.clj
├── resources
├── src
│   └── clojure_web_101
│       └── handler.clj
└── test
    └── clojure_web_101
        └── test
            └── handler.clj

project.clj is a Leiningen configuration. It stores project metadata: its dependencies, description, version et al.

(defproject clojure-web-101 "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "http://example.com/FIXME"
  :dependencies [[org.clojure/clojure "1.5.1"]
                 [compojure "1.1.5"]]
  :plugins [[lein-ring "0.8.5"]]
  :ring {:handler clojure-web-101.handler/app}
  :profiles
  {:dev {:dependencies [[ring-mock "0.1.5"]]}})

In resources directory we put assets such as images, JavaScript and CSS files along with HTML templates. src and test usually have similar structure, the first directory stores the actual application code while the second one the tests that goes with it.

Run the application

λ cd clojure-web-101
λ lein ring server-headless                                                    ⏎
2013-05-12 14:35:42.176:INFO:oejs.Server:jetty-7.6.1.v20120215
2013-05-12 14:35:42.228:INFO:oejs.AbstractConnector:Started
SelectChannelConnector@0.0.0.0:3000
Started server on port 3000

Check if it works

λ curl localhost:3000
Hello World
λ curl localhost:3000/dummy
Not Found

Routes with Compojure

Compojure is a routing library built on top of Ring. Routes are defined in handler.clj using defroutes macro. This macro creates a Ring handler for routes defined inside it. Each route consists of a method name (GET, POST, ...), a path, a list of arguments (which may be empty) and a response.

(defroutes app-routes
  (GET "/" [] "Hello World")
  (route/resources "/")
  (route/not-found "Not Found"))

In our example we have 3 routes. route/resources and route/not-found are special routes, the 1st serves resources from the classpath and the 2nd returns 404 for undefined route paths.

Templates with Enlive

In the presentation layer we will use pure HTML files - no cutom templating language, not even single 'staches. Data will be put into that HTML using transformations defined with Enlive. This way the design will be kept separated from the code that implements the behaviour for the display.

Add enlive as a project dependency in project.clj.

:dependencies [[org.clojure/clojure "1.5.1"]
               [compojure "1.1.5"]
               [enlive "1.1.1"]]

Create a template function inside src/clojure_web_101/templates.clj

(ns clojure-web-101.templates
  (:require [net.cgrand.enlive-html :refer [deftemplate content]]))

(deftemplate tpl-helloworld "public/helloworld.html"
  [value]
  [:#message] (content value))

Enlive provides a deftemplate macro which allows to specify a list of elements along with their transformations for a given HTML file. In our example we set the content of a tag with message identifier to a value passed to the template definition as a parameter.

Last piece of the puzzle is the actual HTML file. Create it under resources/public as helloworld.html.

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Clojure Web 101</title>
  </head>
  <body>
    <h1 id="message">Title goes here</h1>
  </body>
</html>

Finally, run it as follows

lein ring server-headless
2013-05-12 22:36:52.275:INFO:oejs.Server:jetty-7.6.1.v20120215
2013-05-12 22:36:52.364:INFO:oejs.AbstractConnector:Started
SelectChannelConnector@0.0.0.0:3000
Started server on port 3000

Go to localhost:3000 to see the application in action.

Summary

We just created a simple and flexible Clojure web application, built with Compojure (routing) and Enlive (templating). It is highly performant out of the box. We also nicely separated the presentation layer from the display logic. Our view templates are pure HTML files without custom tags or decorated by directives.

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