code: @Deprecated
      protected SparkApplication getApplication(FilterConfig filterConfig) throws ServletException {
          return getApplication(filterConfig.getInitParameter(APPLICATION_CLASS_PARAM));
      }
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 filterConfig the filter configuration for retrieving parameters passed to this filter.
  @return the spark application containing the configuration.
  @throws ServletException if anything went wrong.
  @deprecated Use {@link #getApplications(FilterConfig)} instead.