public synchronized Service staticFileLocation(String folder) {
        if (initialized && !isRunningFromServlet()) {
            throwBeforeRouteMappingException();
        }

        if (!staticFilesConfiguration.isStaticResourcesSet()) {
            staticFilesConfiguration.configure(folder);
        } else {
            LOG.warn("Static file location has already been set");
        }
        return this;
    }
Sets the folder in classpath serving static files. Observe: this method
must be called before all other methods.

@param folder the folder in classpath.
@return the object with folder set
src/main/java/spark/Service.java