@Override
            public String getPathInfo() {
                return relativePath;
            }
Returns the instances of {@link SparkApplication} which on which {@link SparkApplication#init() init()} will be called.
Default implementation looks up the class names 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 applications containing the configuration.
@throws ServletException if anything went wrong.
src/main/java/spark/servlet/SparkFilter.java