Add support for external rules

[matthewfarwell]

Certain rules are very specific to a project. A standard way should be added to allow a user to specify a jar which would contain:

scalastyle_definition.xml
The checker classes.

Then, within the scalastyle configuration, you'd allow the user to specify these classes. This would require a dynamic classloader of course.

[RichardBradley]

How might this be implemented? Do you have a design in mind?

I thought I might get away with defining a rule class in my project/*.scala files, when using the Scalastyle SBT plugin. However, it seems that while those classes are available during the configuration of the Scalastyle Task, they are not available during its run. Do you think that sounds like a workable approach? My project is currently single-module, and it would be nice to avoid creating and deploying a separate JAR file just to get a single class onto Scalastyle's classpath: http://stackoverflow.com/questions/22372717

[matthewfarwell]

There is an ongoing discussion on how to do this on Scalastyle users - Is it possible to have my custom rule run by scalastyle ?

You're right about the basic way of doing it - include it on the classpath of the build. The problem is the message that is output - we need to find the message, which doesn't happen with resource bundles. The easiest way to do this is using typesafe config, which can automatically handle multiple config files on the classpath. See #90 for more information on this.