@Bean
 public EmbeddedServletContainerCustomizer containerCustomizer() {
  return new EmbeddedServletContainerCustomizer(){
   @Override
   public void customize(ConfigurableEmbeddedServletContainer container) {
    container.addErrorPages(new ErrorPage(HttpStatus.NOT_FOUND, "/404.html"));
   }
  };
 }

 @GetMapping("/404")
 @ResponseBody
 public Object customNotFound(){
  return "not found";
 }

@Bean
 public EmbeddedServletContainerCustomizer containerCustomizer() {
  return new EmbeddedServletContainerCustomizer(){
   @Override
   public void customize(ConfigurableEmbeddedServletContainer container) {
    container.addErrorPages(new ErrorPage(HttpStatus.NOT_FOUND, "/404.html"));
   }
  };
 }