@Deprecated
    protected SparkApplication getApplication(FilterConfig filterConfig) throws ServletException {
        return getApplication(filterConfig.getInitParameter(APPLICATION_CLASS_PARAM));
    }
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.
src/main/java/spark/servlet/SparkFilter.java