Scala Bundle

Overview

A quote from the scala website states:

Scala is a general purpose programming language designed to express common programming patterns in a concise, elegant, and type-safe way. It smoothly integrates features of object-oriented and functional languages. It is also fully interoperable with Java.

So it seems to be able to interoperate with Java, but can it interoperate with OSGi? The answer seems to be yes. This document will demostrate a simple hello world scala bundle. Much of this is already documented on Neil Bartlett's blog at An OSGi Bundle… built in Scala. The point of this document is to quickly get a hello world scala bundle running in a self-hosting PDE workspace.

The Scala plugin

There is a Scala Eclipse plugin available for developing scala code. First install this plugin into Eclipse. The web site indicates that only 3.2 is supported but it works fine on the latest 3.3 builds. This will install the necessary tools for compiling scala code in your workspace. But in order to develop bundles with scala code you need to make the scala library available for bundles to depend on. The scala library is packaged as a bundle but it is embedded in the ch.epfl.lamp.sdt.compiler plugin. In order to make the scala library available stand alone you must copy the lib/scala-library.jar file from the ch.epfl.lamp.sdt.compiler plugin into the eclipse/plugins directory of your development target. This will allow bundles in your workspace to use the scala library without requiring the complete scala development environment.

A Scala bundle

Unfortunately PDE and Scala projects to not work together out of box. A little .project file hacking is needed in order to compile scala code in a plugin project. Use the following steps to create a simple plugin project for Scala code.

Create a Plugin project for Scala code

1. Create a Plugin project. New->Project->Plug-in Project.
2. Name the project hello.scala
3. In the Target Platform select "an OSGi framework". Click Next and finish
4. Edit the .project file of the hello.scala project. Make the <buildSpec> and <natures> sections look like the following:

<buildSpec>
  <buildCommand>
    <name>org.eclipse.jdt.core.javabuilder</name>
    <arguments>
    </arguments>
  </buildCommand>
  <buildCommand>
    <name>org.eclipse.pde.ManifestBuilder</name>
    <arguments>
    </arguments>
  </buildCommand>
  <buildCommand>
    <name>org.eclipse.pde.SchemaBuilder</name>
    <arguments>
    </arguments>
  </buildCommand>
  <buildCommand>
    <name>ch.epfl.lamp.sdt.core.scalabuilder</name>
    <arguments>
    </arguments>
  </buildCommand>
</buildSpec>
<natures>
  <nature>ch.epfl.lamp.sdt.core.scalanature</nature>
  <nature>org.eclipse.pde.PluginNature</nature>
  <nature>org.eclipse.jdt.core.javanature</nature>
</natures>

This will hook in the scala nature and scala builder for building scala code in your workspace.

Add a dependency on the scala library

Scala code gets compiled into class files which will depend on the scala library at runtime. A bundle which contains compiled scala code will require the scala library. For simplicity the easiest thing to so is to use Reqiure-Bundle to gain runtime access to the scala library. Add the following bundle manifest header to the hello.scala META-INF/MANIFEST.MF file:

Require-Bundle: scala_library

Create a scala bundle activator

1. Create a scala class in the hello.scala package. New->Other->Scala->Scala Class.
2. Name the scala class ScalaActivatory. Click Finish.
3. Make the class extend org.osgi.framework.BundleActivator and implement the start and stop method.

package hello.scala;

import org.osgi.framework._

class ScalaActivator extends BundleActivator{
  def start(context: BundleContext) {
   Console.printf("Hello from the {0} bundle.\n",
                    context.getBundle().getSymbolicName());
  }

  def stop(context: BundleContext) {
  }
}

After you have created the ScalaActivator you will have to add a Bundle-Activator header to your bundle manifest to use the ScalaActivator:

Bundle-Activator: hello.scala.ScalaActivator

Run the scala bundle on Equinox

1. Create a new OSGi Framework launch configuration. Run->Open Run Dialog->OSGi Framework->New
2. On the Bundles click Deselect All.
3. Select the hello.scala bundle from the Workspace
4. Click Add Required Bundles.
5. Click Run.

Build the scala bundle jar

So far this example has run the scala bundle from your workspace in a typical eclipse self-hosting environment. If you try to export the plugin project from your workspace then you will notice the .scala files are not compiled into .class file. This is because PDE Build only knows how to compile Java code by default. It may be possible to plug the scala compiler into some custom callbacks script in PDE Build ...