public synchronized Service externalStaticFileLocation(String externalFolder) {
        if (initialized && !isRunningFromServlet()) {
            throwBeforeRouteMappingException();
        }

        if (!staticFilesConfiguration.isExternalStaticResourcesSet()) {
            staticFilesConfiguration.configureExternal(externalFolder);
        } else {
            LOG.warn("External static file location has already been set");
        }
        return this;
    }
Sets the external folder serving static files. <b>Observe: this method
must be called before all other methods.</b>

@param externalFolder the external folder serving static files.
@return the object with external folder set
src/main/java/spark/Service.java