docs: 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;
          }
      }