AppConfig.java

public class AppConfig  extends WebMvcConfigurerAdapter{
  @Bean
  public MessageSource messageSource() {
      ReloadableResourceBundleMessageSource bean = new ReloadableResourceBundleMessageSource();
      bean.setBasename("classpath:messages");
      bean.setDefaultEncoding("UTF-8");
      return bean;
  }

  @Bean
  public LocalValidatorFactoryBean validator() {
      LocalValidatorFactoryBean bean = new LocalValidatorFactoryBean();
      bean.setValidationMessageSource(messageSource());
      return bean;
  }

  @Override
  public Validator getValidator() {
      return validator();
  }
}

Category.java

@Size(min = 5, max = 12, message = "{category.name.size}")
private String name;

messages.properties

category.name.size = "The category must be {min} to {max} characters in length.