code: protected SparkApplication getApplication(String applicationClassName) throws ServletException {
          try {
              Class<?> applicationClass = Class.forName(applicationClassName);
              return (SparkApplication) applicationClass.newInstance();
          } catch (Exception exc) {
              throw new ServletException(exc);
          }
      }
docs: Returns an instance of {@link SparkApplication} which on which {@link SparkApplication#init() init()} will be called.
  Default implementation looks up the class name in the filterConfig using the key {@value #APPLICATION_CLASS_PARAM}.
  Subclasses can override this method to use different techniques to obtain an instance (i.e. dependency injection).
  
  @param applicationClassName the spark application class name passed to this filter.
  @return the spark application containing the configuration.
  @throws ServletException if anything went wrong.