Wednesday, June 02, 2010

Getting started with AppEngine + Scala

This time, after long time-out I decided to continue blogging in english. Sorry for my english in advance, but I will try to do my best ;-)I spent some time playing with Google App Engine and Scala integration so I think it could be helpful for somebody else. Since they are both innovative technologies I could not resist the temptation to use unusual build tool with self-explanatory name simple-build-tool. In short, this is Maven-like build tool which actually works with Maven repositories (using Apache Ivy). But instead of boilerplate xml you get Scala code.

First of all you need to install simple-build-tool.
Let’s create our project dir:

$ mkdir gae_scala

And run simple-build-tool:

$ cd gae_scala
$ sbt
Project does not exist, create new project? (y/N/s) y
Name: gae_scala
Organization: com.example
Version [1.0]: 
Scala version [2.7.7]: 
sbt version [0.7.3]: 
Getting Scala 2.7.7 ...
:: retrieving :: org.scala-tools.sbt#boot-scala
 confs: [default]
 2 artifacts copied, 0 already retrieved (9911kB/94ms)
Getting org.scala-tools.sbt sbt_2.7.7 0.7.3 ...
:: retrieving :: org.scala-tools.sbt#boot-app
 confs: [default]
 15 artifacts copied, 0 already retrieved (4023kB/153ms)
[success] Successfully initialized directory structure.
[info] Building project gae_scala 1.0 against Scala 2.7.7
[info]    using sbt.DefaultProject with sbt 0.7.3 and Scala 2.7.7

Now we have initial project skeleton. You can run help command and read about existsing commands.
There is existing google app engine plugin for sbt. Let’s build and install it. Run in some temporary directory:

$ git clone git://
$ cd sbt-appengine-plugin
$ sbt
$ publish-local
[info] == publish-local ==
[info] :: publishing :: net.stbbs.yasushi#sbt-appengine-plugin
[info]  published sbt-appengine-plugin to /home/brick/.ivy2/local/net.stbbs.yasushi/sbt-appengine-plugin/2.1-SNAPSHOT/jars/sbt-appengine-plugin.jar
[info]  published sbt-appengine-plugin to /home/brick/.ivy2/local/net.stbbs.yasushi/sbt-appengine-plugin/2.1-SNAPSHOT/poms/sbt-appengine-plugin.pom
[info]  published ivy to /home/brick/.ivy2/local/net.stbbs.yasushi/sbt-appengine-plugin/2.1-SNAPSHOT/ivys/ivy.xml
[info] == publish-local ==
[success] Successful.
[info] Total time: 9 s, completed 02.06.2010 21:38:55

We installed gae plugin for sbt, getting back to gae_scala:

$ mkdir project/build
$ mkdir project/plugins

Create project/plugins/plugins.scala with your favourite editor:

import sbt._

class Plugins(info: ProjectInfo) extends PluginDefinition(info) {
  val appEngine = "net.stbbs.yasushi" % "sbt-appengine-plugin" % "2.1-SNAPSHOT"

Above we defined dependency on App Engine plugin (the same as maven plugin dependency).
Create project/build/project.scala custom build configuration with appengine support:

import sbt._

class AppengineScalaProject(info: ProjectInfo)
   extends AppengineProject(info) with DataNucleus

DataNucleus trait adds domain classes enhacer to the build process. It is required only when used with JDO or JPA.
At the moment we have sbt project with configured appengine plugin. Specify App Engine SDK home:

$ export APPENGINE_SDK_HOME=~/installed/appengine-java-sdk

Run sbt shell and update command:

[info] Recompiling project definition...
[info]    Source analysis: 1 new/modified, 0 indirectly invalidated, 0 removed.
[info] Building project gae_scala 1.0 against Scala 2.7.7
[info]    using AppengineTestProject with sbt 0.7.3 and Scala 2.7.7
> update
[info] == update ==
[info] :: retrieving :: com.example#gae_scala_2.7.7 [sync]
[info]  confs: [compile, runtime, test, provided, system, optional, sources, javadoc]
[info]  1 artifacts copied, 0 already retrieved (102kB/37ms)
[info] == update ==
[success] Successful.
[info] Total time: 1 s, completed 02.06.2010 22:17:07

Now we need to create web.xml and appengine-web.xml descriptors for out application:

$ mkdir -p src/main/webapp/WEB-INF

Create src/main/webapp/WEB-INF/web.xml file:

<?xml version="1.0" encoding="utf-8"?>
<web-app xmlns:xsi=""
xsi:schemaLocation="" version="2.5">

Create src/main/webapp/WEB-INF/appengine-web.xml file:

<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="">

At last, we need to create our HelloWorld servlet:

$ mkdir -p src/main/scala/com/example


package com.example

import javax.servlet.http._

class HelloWorld extends HttpServlet {
  override def doGet(req: HttpServletRequest, resp: HttpServletResponse) = {
    resp.getWriter().println("Hello World!")

Now we are ready to run:

$ sbt
> dev-appserver-start
[info] == compile ==
[info]   Source analysis: 1 new/modified, 0 indirectly invalidated, 0 removed.
[info] Compiling main sources...
[info] Compilation successful.
[info]   Post-analysis: 1 classes.
[info] == compile ==
[info] == copy-resources ==
[info] == copy-resources ==
[info] == prepare-webapp ==
[info] == prepare-webapp ==
[info] == dev-appserver-start ==
[info] == dev-appserver-start ==
[success] Successful.
[info] Total time: 1 s, completed 02.06.2010 22:44:55

Open http://localhost:8080/hello in web browser and enjoy result…
Here are several useful actions:

  • dev-appserver-start - starts development server
  • dev-appserver-stop - stops development server
  • enhance - executes ORM enhancement
  • update-webapp - uploads application to Google App Engine server
  • action - prints all available actions

Now you are ready to code something. Have a fun!

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