Exemplo de configuração de messages (validação) no Spring Boot

EndpointExceptionHandler.java

package com.maplink.axiodis.rest.api.endpoint;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.MessageSource;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

import java.util.Locale;

@ControllerAdvice
public class EndpointExceptionHandler{   


    @Autowired
    private MessageSource messageSource;

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity handleMethodArgumentNotValid(MethodArgumentNotValidException ex, Locale locale){
        ex.getBindingResult()
                .getAllErrors().forEach(error -> System.out.println(messageSource.getMessage(error, locale)));


        return ResponseEntity.badRequest().body(ex.getBindingResult().getAllErrors());
    }

}

MessagesConfig.java

import java.util.Locale;

/**
 * Messages Configuration regarding information, warnings, errors and others messages types
 */
@Configuration
public class MessagesConfig {

    @Bean
    public MessageSource messageSource(){
        ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();
        messageSource.setBasename("classpath:messages/messages");
        messageSource.setDefaultEncoding("UTF-8");
        messageSource.setFallbackToSystemLocale(false);

        return messageSource;
    }

    @Bean
    public Validator validatorFactoryBean(MessageSource messageSource){
        LocalValidatorFactoryBean validatorFactory = new LocalValidatorFactoryBean();
        validatorFactory.setValidationMessageSource(messageSource);

        return validatorFactory;
    }

    @Bean
    public LocaleResolver localeResolver() {
        AcceptHeaderLocaleResolver localeResolver = new AcceptHeaderLocaleResolver();
        localeResolver.setDefaultLocale(Locale.US);

        return localeResolver;
    }
}