Learning Clojure with Google App Engine and Emacs

2010 September 18 programming

Learning Clojure with Google App Engine and Emacs

This is an overview to my attempts at getting Clojure with Emacs. Inspired by this post, I set out to learn three things simultaneously: emacs, clojure and Google App Engine.

For comparison, my current web prototyping combination is TextMate, Rails, and Heroku. Prior to that it was a mix of python, php or perl, with vim.

There are already number of articles on getting GAE and Clojure to work. Consider this me making notes for myself because I keep forgetting the Emacs shortcuts!

Emacs

I settled on Aquamacs to start out with emacs on the Mac. It seems to be more forgiving if I use normal Mac keyboard shortcuts while I learn emacs keys. Also makes it easy to switch between it and TextMate.

The other feature I love is full screen mode: ⌘-Shift-Return

To get command line access, you need to go to Tools->Install Command Line Tools. This gives you an “aquamacs” shortcut in the shell. I aliased it to “aq“.

For some help on learning emacs, consider:

You may also want to setup Clojure to run with Emacs. This article assumes you are using lein.

Google App Engine (GAE)

The GAE distribution is available here: GAE Download.

I setup my shell to include a $GAE_BASE directory for where I unpacked it, and added $GAE_BASE/bin to my path.

Creating a Clojure Project

The first post I read assumed that you were starting with a manual layout of your project. I’m going to use lein, a build manager for clojure, similar to rake for ruby. Behind the scenes lein uses Java’s maven as a package management system (similar to gems).

To make things a bit more interesting, I plan to write a very basic CMS to explore the various APIs. I’m keeping everything on the githubs.

The steps to getting setup look something like:

$ lein new gaecms
$ mkdir -p war/WEB-INF

Then make some changes to your project.clj file:

(defproject gae-cms "1.0.0-SNAPSHOT"
  :description "A basic CMS built on Google App Engine (GAE)"
  :dependencies [[org.clojure/clojure "1.2.0"]
                 [org.clojure/clojure-contrib "1.2.0"]
                 [compojure "0.4.1"]
                 [ring/ring-servlet "0.2.1"]
                 [appengine "0.4-SNAPSHOT"]]
:dev-dependencies [[leiningen/lein-swank "1.2.0-SNAPSHOT"]]
:compile-path "war/WEB-INF/classes"
:library-path "war/WEB-INF/lib")

To save some typing, you can clone: git://github.com/gmwils/gaecms.git

To run the interpreter locally, try the following:

$ lein deps
$ lein swank

To test out that the interpreted mode is working, open up src/gae_cms/core.clj in AQ.

Then connect to the server:

M-x slime-connect

From within the editor, you can now run either of the following:

C-x C-e    % evaluate current line
C-c C-k    % compile the current file

You also have an interactive clojure shell for typing commands into. Try adding the following into your source code and then pressing C-x C-e to evaluate:

(System/getProperty "java.class.path")

You should see your current CLASSPATH printed out.

Setup for GAE

In $HOME/war/WEB-INF/web.xml map the URLs that you want your custom servlet to handle. In this case, send everything into the servlet:

<?xml version="1.0" encoding="ISO-8859-1"?>
<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>CMS on GAE</display-name>
  <servlet>
    <servlet-name>cms</servlet-name>
    <servlet-class>gae-cms.core</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>cms</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>
</web-app>

In $HOME/war/WEB-INF/appengine-web.xml add the following to setup Google App Engine to refer to your servlet above.

<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
  <application>cms-clj</application>  <!-- GAE app id for your app -->
  <version>v0-1</version>  <!-- Arbritrary Version Id -->
</appengine-web-app>

Next, update src/gaecms/core.clj with:

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

(defroutes cms-public
  (GET "/" []
    "<html><title>GAE CMS</title><body><h1>Hello World!</h1></body>")  )
(defroutes cms
  cms-public
  (route/not-found "Page not found")) ; 404 error page

(defservice cms)

To test this locally, try:

$ lein deps
$ lein compile
$ $GAESDK/bin/dev_appserver.sh war

Deploying to Google

First go to the GAE console and create an application. The following should then work:

$ lein deps
$ lein compile
$ $GAESDK/bin/appcfg.sh update war

I’m still playing with getting it working with Google App Engine. The above is enough to get hello world working.

For more details, I suggest you follow the Compojure on GAE blog. Specifically, the article on Deploying to App Engineis a good overview.

Next steps include getting interactive development working with GAE, and actually handling dynamic content.