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

        if (!externalStaticResourcesSet) {
            try {
                ExternalResource resource = new ExternalResource(folder);
                if (!resource.getFile().isDirectory()) {
                    LOG.error("External Static resource location must be a folder");
                    return;
                }

                if (staticResourceHandlers == null) {
                    staticResourceHandlers = new ArrayList<>();
                }
                staticResourceHandlers.add(new ExternalResourceHandler(folder, "index.html"));
                LOG.info("External StaticResourceHandler configured with folder = " + folder);
            } catch (IOException e) {
                LOG.error("Error when creating external StaticResourceHandler", e);
            }

            externalStaticResourcesSet = true;
        }
    }
Configures location for static resources

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