My First Scala Servlet (with Eclipse)

In this article, I will show you how to tweak Eclipse so that you will be able to code “classical” webapps in Scala.

Note: I know about Lift, I just want to see how Scala can integrate in my existing infrastructure step-by-step.

Setting up your Eclipse project

Eclipse has a plugin(found at the Scala IDE site) that let you help develop with Scala. The plugin is by no mean perfect (autocompletion does not works in Ganymede), but it gets the job done, letting you create Scala projects that automatically compile Scala code with scalac.

The Web Tools Platform, another Eclipse plugin, let you create web application projects that can be deployed to application servers.

Most projects created with one or another Eclipse plugin are compatible with one another. For example, you can create a new Dynamic Web Project, then add a JPA extension and presto, you can now use the Java Persistence API in you project. Likewise, you can add a Vaadin extension to your Web Project and you can use Vaadin in it. These extensions are called “Facets” in Eclipse. Each project can have many facets: Java, JPA, Vaadin, etc. depending on the plugins you installed.

Unfortunately, the Scala plugin does not use the Facet mechanism. Thus, Dynamic Web Projects cannot be enhanced with the Scala facet. Creating a web project that compiles your Scala code requires a little tweaking. First, you should create a simple Dynamic Web Project. Then, open the .project file. If you do not see the file, go to Customize View in the view and uncheck .*resources in the Filter tab (the first one). It should look something like this:

<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
    <name>scala-servlet-example</name>
    <comment></comment>
    <projects>
    </projects>
    <buildSpec>
        <buildCommand>
            <name>org.eclipse.wst.jsdt.core.javascriptValidator</name>
            <arguments>
            </arguments>
        </buildCommand>
        <buildCommand>
            <name>org.eclipse.jdt.core.javabuilder</name>
            <arguments>
            </arguments>
        </buildCommand>
        <buildCommand>
            <name>org.eclipse.wst.common.project.facet.core.builder</name>
            <arguments>
            </arguments>
        </buildCommand>
        <buildCommand>
            <name>org.eclipse.wst.validation.validationbuilder</name>
            <arguments>
            </arguments>
        </buildCommand>
    </buildSpec>
    <natures>
        <nature>org.eclipse.jem.workbench.JavaEMFNature</nature>
        <nature>org.eclipse.wst.common.modulecore.ModuleCoreNature</nature>
        <nature>org.eclipse.wst.common.project.facet.core.nature</nature>
        <nature>org.eclipse.jdt.core.javanature</nature>
        <nature>org.eclipse.wst.jsdt.core.jsNature</nature>
    </natures>
</projectDescription>

Replace the org.eclipse.jdt.core.javabuilder with org.scala-ide.sdt.core.scalabuilder. Note: you can remove the Java builder with the IDE but you cannot add another one so hacking the configuration file is a necessary evil. Now add the Scala nature to your project: <nature>org.scala-ide.sdt.core.scalanature</nature>.

The .project file should now look like this:

<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
    <name>scala-servlet-example</name>
    <comment></comment>
    <projects>
    </projects>
    <buildSpec>
        <buildCommand>
            <name>org.eclipse.wst.jsdt.core.javascriptValidator</name>
            <arguments>
            </arguments>
        </buildCommand>
        <buildCommand>
            <name>org.scala-ide.sdt.core.scalabuilder</name>
            <arguments>
            </arguments>
        </buildCommand>
        <buildCommand>
            <name>org.eclipse.wst.common.project.facet.core.builder</name>
            <arguments>
            </arguments>
        </buildCommand>
        <buildCommand>
            <name>org.eclipse.wst.validation.validationbuilder</name>
            <arguments>
            </arguments>
        </buildCommand>
    </buildSpec>
    <natures>
        <nature>org.scala-ide.sdt.core.scalanature</nature>
        <nature>org.eclipse.jem.workbench.JavaEMFNature</nature>
        <nature>org.eclipse.wst.common.modulecore.ModuleCoreNature</nature>
        <nature>org.eclipse.wst.common.project.facet.core.nature</nature>
        <nature>org.eclipse.jdt.core.javanature</nature>
        <nature>org.eclipse.wst.jsdt.core.jsNature</nature>
    </natures>
</projectDescription>

 If you made the right change, you should see a S instead of a J on your project’s icon (and it should keep the globe!).

The last thing to do should be to add the Scala library to your path. Depending on your application server configuration, either add the Scala library to your build path (Build Path -> Configure Build Path -> Add Library -> Scala Library) or manually add the needed library to your WEB-INF/lib folder. You’re all set to create your first Scala servlet!

Creating your servlet

In order to create your servlet “the easy way”, just do New -> Other -> Scala Class. Choose a package (it is a good practice to respect the Java guidelines regarding the class location according to its package name). Choose the HttpServlet as the superclass. Name it however you please and click OK.

It seems to compile but you and I know there will be some problem later since you don’t override the servlet’s doGet() method. Do it:

override def doGet(request:HttpServletRequest, response:HttpServletResponse) {

}

Don’t forget the imports. Let’s use the Scala way:

import javax.servlet.http.{HttpServlet, HttpServletRequest, HttpServletResponse}

Last but not least, let’s code some things for your servlet to do. Since I’m feeling very innovative, I will print the good old “Hello world!”:

override def doGet(request: HttpServletRequest, response: HttpServletResponse) {

  response setContentType ("text/html")

  val out = response getWriter

  out println """<html>
  <head>
      <title>Scala Servlet
  <body>
      <p>Hello world!"""
  }
}

Don’t forget to add the servlet to your web.xml and presto, you should see some familiar example, in a familiar IDE, but with some unorthodox language.

Conclusion

Once Eclipse correctly set up, it’s pretty straightforward to develop your webapp in Scala. I do hope the next version of the Scala IDE plugin will definitely improve Scala’s integration into Eclipse and WTP further.

Here are the sources of the example, in Eclipse format.

To go further:

• The Scala Programming Language
• The Scala IDE site
• Scala for Java refugees

From http://blog.frankel.ch/my-first-scala-servlet