Make Checkstyle only validate Java files in Eclipse

The Checkstyle plugin for Eclipse no longer includes a confiuration option to only check Java files. For projects using Wicket that generates validation errors on the .html files that are placed alongside the Java sources. To configure Checkstyle for Java sources only open the project properties dialog and for Checkstyle:

1. Uncheck the Use simple configuration option
2. Create a file set that includes this expression to select Java files: src/main/java/.*\.java$