@Bean
 @ConditionalOnMissingBean
 public GsonHttpMessageConverter gsonHttpMessageConverter(Gson gson) {
   GsonHttpMessageConverter converter = new GsonHttpMessageConverter();
   converter.setGson(gson);
   return converter;
 }
 @Bean
 public GsonHttpMessageConverter customGsonMessageConverter(Gson gson) {
   GsonHttpMessageConverter converter = new GsonHttpMessageConverter();
   converter.setGson(gson);
   return converter;
 }