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maven-scala-plugin

Continuing from my last post: With the Scala script written, I wanted to integrate it into our Maven build as an optional step. But this didn’t go so well.

When running mvn scala:script with a link to my script file in the configured pom.xml, I got a Fatal Error. I also couldn’t get a simple helloWorld script to run, as a separate file or scripted right into the pom.xml file. This is the error I got:


[INFO] ------------------------------------------------------------------------
[ERROR] FATAL ERROR
[INFO] ------------------------------------------------------------------------
[INFO] scala/ScalaObject
scala.ScalaObject
[INFO] ------------------------------------------------------------------------
[INFO] Trace
java.lang.NoClassDefFoundError: scala/ScalaObject
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)
at java.lang.ClassLoader.defineClass(ClassLoader.java:616)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
at org_scala_tools_maven.ScalaScriptMojo.runScript(ScalaScriptMojo.java:185)
at org_scala_tools_maven.ScalaScriptMojo.doExecute(ScalaScriptMojo.java:154)
at org_scala_tools_maven.ScalaMojoSupport.execute(ScalaMojoSupport.java:340)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:490)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:694)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:556)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:535)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:387)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:348)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:180)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:328)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:138)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:362)
at org.apache.maven.cli.compat.CompatibleMain.main(CompatibleMain.java:60)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)
Caused by: java.lang.ClassNotFoundException: scala.ScalaObject
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
... 33 more
[INFO] ------------------------------------------------------------------------

The maven-scala-plugin couldn’t even find the ScalaObject class, which is analagous to Java Object. Of course I googled the heck out of this error and found a few people suffering the same problem, but no one posted a solution. Then I found maven-scala-plugin’s repo on GitHub and discovered that this issue has been reported but not yet fixed. So I decided to download the source code and take a look.

Here’s what I found, in ScalaScriptMojo.java:

    private URLClassLoader createScriptClassloader(File scriptDir,
            Set<String> classpath) throws MalformedURLException {
        List<URL> urls = new ArrayList<URL>();

        // add the script directory to the classpath
        urls.add(scriptDir.toURI().toURL());

        for (String string : classpath) {
            urls.add(new URL("file://" + string));
        }

        URLClassLoader loader = new URLClassLoader(urls.toArray(new URL[urls
                .size()]), getClass().getClassLoader());
        return loader;
    }

That seemed like a pretty hinky way to me to send files to the URLClassLoader. The code that generates the Set of classpath Strings uses File.getCanonicalPath() to create the Strings. Thus they are platform-dependent Strings, and on a Windows box you can’t use them to create a valid File URL by prepending “file://” to them because the path separators are “\”. I assume that the plugin contributors are on Linux machines and couldn’t replicate the problem to fix it.

Like all the best bug fixes, this one was a one-liner:

for (String string : classpath) {
    urls.add(new File(string).toURI().toURL());
}

Now the URLs will be correct no matter what platform you’re on.

I forked the maven-scala-plugin repository on GitHub and checked in my fix. I was waiting to post this entry until my Pull Request was accepted by the repo owner, but I guess he is too busy working on other efforts. Until the request is accepted, please check out my forked repo.

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