How-To - IntelliJ Idea for Clojure/Compojure on Google App Engine

StrangeLoop conference really spiked my interest in learning Clojure. I wanted to find a way to do web development using Clojure. So this entry is primarily a how-to on setup of IntelliJ Idea to do Clojure using Compojure web framework, Leiningen build tool and Google App Engine deployment environment.

Step 1: Download Leiningen
Download Leiningen script file and follow instruction to put it in PATH.

Step 2: Install "La Clojure" and "Leiningen" plugin for IntelliJ Idea.
To do this go to Preferences -> Plugins. Go to "Available" Tab


Step 3: Setup Leiningen plugin properties
To setup Leiningen plugin properties in IntelliJ click on Preferences -> Leiningen and give the path to the executable.

Step 4: Create project template
Now with the installation complete, we are ready to create a new project. For now the project creation is done outside IntelliJ Idea using Lein on command line. This is the only step done outside the IntelliJ Idea. Choose the folder structure where your IntelliJ workspace is.

     $ lein new gae-cmpjr
     Created new project in: gae-cmpjr

You can see that Lein created project folder structure. In the project root it created a project.clj file used by Leiningen. Under src folder, it also created default namespace by the name of the project and created a core.clj file under it.

Step 5: Create project IntelliJ
Let's create IntelliJ Project, even though project structure exists pick the same folder path that you selected in step 4.

• File -> Project New... and select project from scratch
• Select both GAE and Clojure. Provide the path to the appengine sdk

Now basic setup for the project is done. You can browse the project files using IntelliJ. (Biggest benefit of La Clojure is it auto-inserts and matches brackets for very "brackety language" :-))

Step 6: Setup Leiningen to run from IntelliJ
To setup Leiningen from IntelliJ, click on Leiningen tool window. (Window -> Tool Windows -> Leiningen)

Step 7: Setup web development libraries
Setup the project to include Compojure and Ring libraries for development.
project.clj

(defproject gae-cmpjr "1.0.0-SNAPSHOT"
  :description "Example for Compojure on GAE using IntelliJ"
  :namespaces [gae-cmpjr.core] 
  :dependencies [[compojure "0.4.0-SNAPSHOT"]
                 [ring/ring-servlet "0.2.1"]]
  :dev-dependencies [[leiningen/lein-swank "1.2.0-SNAPSHOT"]]
  :compile-path "web/WEB-INF/classes"
  :library-path "web/WEB-INF/lib")

core.clj

(ns gae-cmpjr.core
  (:gen-class :extends javax.servlet.http.HttpServlet)
  (:use compojure.core
        ring.util.servlet)
  (:require [compojure.route :as route]))

(defroutes hello
  (GET "/" [] "<h1>Hello World Wide Web!</h1>")
  (route/not-found "Page not found"))

(defservice hello)

Step 8:  GAE setup

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
            http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
           version="2.5">
    <display-name>Compojure on GAE using IntelliJ</display-name>

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>gae_cmpjr.core</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
</web-app>

appengine-web.xml

<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
  <application>compgae</application>
  <version>v0-2</version>
</appengine-web-app>

Step 9: Compile and Build war

From the Leiningen Tool window in Idea run following commands.

lein clean
lein deps 
lein compile 

Step 10: Run locally
To locally run click on already create "Run Configuration"

Step 11: To upload application to appengine
Tools -> Upload Appengine application

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