public synchronized void configure(String folder) {
        Assert.notNull(folder, "'folder' must not be null");

        if (!staticResourcesSet) {

            if (staticResourceHandlers == null) {
                staticResourceHandlers = new ArrayList<>();
            }

            staticResourceHandlers.add(new ClassPathResourceHandler(folder, "index.html"));
            LOG.info("StaticResourceHandler configured with folder = " + folder);
            staticResourcesSet = true;
        }
    }
Configures location for static resources

@param folder the location
src/main/java/spark/staticfiles/StaticFilesConfiguration.java