docs: 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;
          }
      }