Get the root directory that contains files added through
        C{SparkContext.addFile()}.

def getRootDirectory(cls):
        """
        Get the root directory that contains files added through
        C{SparkContext.addFile()}.
        """
        if cls._is_running_on_worker:
            return cls._root_directory
        else:
            # This will have to change if we support multiple SparkContexts:
            return cls._sc._jvm.org.apache.spark.SparkFiles.getRootDirectory()