Gary Boone, PhD » eclipse

Scala Eclipse Plugin Error: Scala Signature

gary — Thu, 25 Jun 2009 22:54:28 +0000

Trying to start a program created with the eclipse Scala plugin, I saw an error that the main class couldn’t be found. That was cryptic, as there was a main. Stranger still, this is a program I had run successfully before.

The first part of solving the problem was to look for clues in the Problems view. Opening it, I saw this:

error while loading Configgy, Scala signature Configgy has wrong version expected: 5.0 found: 4.1 in /Users/…

So the first error was due to the compilation failing and not producing a main class that eclipse could launch.

After further digging, the version error turned out to be caused by a mismatch between the Scala version (2.7.5) I used to compile the Configgy library and the Scala version (2.8) that the eclipse plugin used to compile the rest of the program. I didn’t install Scala 2.8, so how could this occur? The Scala plugin installs its own version of Scala. Because I had selected the nightly build version of the plugin, the Scala versions drifted apart during one of the eclipse updates.

You can tell which version the eclipse Scala plugin uses by looking at the jar name in the eclipse/plugins directory. Eg, scala.tools.nsc_2.75.final.jar

The solution was to revert to the stable, release version of the plugin. Do that by using this url in the eclipse Software Updates | Available Software | Manage Sites… list.

http://www.scala-lang.org/scala-eclipse-plugin

More complete directions here.

Creating Single-file Runnable Jars in Scala with eclipse

gary — Mon, 22 Jun 2009 06:47:02 +0000

How do you create a single runnable jar that contains all the files it needs to run? This is one of those questions that should be easier to answer. Here’s how to do it in eclipse and NetBeans.

First, eclipse, following the post here:
1. Create a Java class to run your Scala main:

package scala.loader;
import java.util.*;
public class ScalaEntryPoint
{
    public static void main(String[] args) {
    List argList = new ArrayList();
    argList.add("YourScalaObjectWithMain");
    for (String s : args) argList.add(s);
        scala.tools.nsc.MainGenericRunner.main(argList.toArray(new String[0]));
    }
}

Be sure to change YourScalaObjectWithMain to your class. Also, you’ll have to add the Scala tools jar to your projects classpath: Project | Properties | Java Build Path tab | Libraries tab | Add External Jars… button. Then pick the scala.tools.nsc file in your eclipse/plugins folder.

2. Create a Run Configuration to run the Java class. Check that it runs.

3. Then choose File | Export…, then Java | Runnable JAR File, then pick the Java run configuration to export and a destination. Optionally, you can save the ant XML file that does the export. Later, after some edits, you can rejar using

ant -f yourjarXMLfile.xml

If you change your libraries, be sure to repeat the complete Export process to make sure your jar scrips includes the correct jars.

Scala Eclipse Plugin: got class?

gary — Wed, 17 Jun 2009 22:35:10 +0000

The Scala Eclipse plugin is in development, so we can forgive little issues. One I keep running into is that programs sometimes can’t find themselves upon start. On the canonical hello-world program, I see:

Exception in thread “main” java.lang.NoClassDefFoundError: HelloWorld
Caused by: java.lang.ClassNotFoundException: HelloWorld

So far, the only explanation I’ve found from searching the web is that the metadata is horked, so the workspace must be nuked from orbit, that being the only way to be sure. I’ve tried it and it works, but this approach is hardly practical for production coding.