utilities.gradle

apply plugin: UtilitiesPlugin

class UtilitiesPlugin implements Plugin<Project> {
    def void apply(Project project) {
        project.convention.plugins.utilities = new UtilitiesPluginDef()
    }
}

class UtilitiesPluginDef {
    public String determinePackageName(SourceDirectorySet sourceDirectorySet, File javaFile) {
        final javaFileAbsolutePath = javaFile.absolutePath;
        for ( File sourceDirectory : sourceDirectorySet.srcDirs ) {
            final String sourceDirectoryAbsolutePath = sourceDirectory.absolutePath;
            if ( javaFileAbsolutePath.startsWith( sourceDirectoryAbsolutePath ) ) {
                final String javaFileRelativePath = javaFileAbsolutePath.substring(
                        sourceDirectoryAbsolutePath.length() + 1,
                        javaFileAbsolutePath.lastIndexOf( File.separator )
                );
                return javaFileRelativePath.replace( File.separator, "." );
            }
        }
        throw new RuntimeException( "ugh" );
    }
}