Getting started: Clojure Web App with Compojure and Envlive

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Getting started: Clojure Web App with Compojure and Envlive

In this article I will show you how to create a basic Clojure web application built withCompojure 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-101λ 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-101λ lein ring server-headless                                                    ⏎2013-05-12 14:35:42.176:INFO:oejs.Server:jetty-7.6.1.v201202152013-05-12 14:35:42.228:INFO:oejs.AbstractConnector:StartedSelectChannelConnector@0.0.0.0:3000Started server on port 3000

Check if it works

λ curl localhost:3000Hello Worldλ curl localhost:3000/dummyNot Found

ROUTES WITH COMPOJURE

Compojure is a routing library built on top of Ring. Routes are defined in handler.cljusing 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 withmessage 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 ashelloworld.html.

gt;<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-headless2013-05-12 22:36:52.275:INFO:oejs.Server:jetty-7.6.1.v201202152013-05-12 22:36:52.364:INFO:oejs.AbstractConnector:StartedSelectChannelConnector@0.0.0.0:3000Started server on port 3000

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

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