Errors 객체를 ResponseEntity body에 담아서 JSON 형태로 리턴하기

스프링의 @Valid를 사용하면 유효성 검증에 실패한 내용이 Errors에 담긴다.

@PostMapping("api/car")
public ResponseEntity<?> saveCar(@RequestBody @Valid CarSaveRequestDto requestDto, Errors errors) {
    log.info("\n\n=== saveCar start ===\n* requestDto: " + requestDto + "\n");
    if (errors.hasErrors()) {
        return ResponseEntity.badRequest().body(errors);
    }

    CarSaveResponseDto responseDto = carService.save(requestDto);

    log.info("\n\n=== saveCar end ===\n* savedCarId: " + responseDto.getId() + "\n");
    return ResponseEntity.created(null).body(responseDto);
}

하지만 이 Errors 객체를 ResponseEntity의 body에 담아서 리턴하면 에러가 발생한다.

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class org.springframework.validation.DefaultMessageCodesResolver]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.springframework.validation.DefaultMessageCodesResolver and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: org.springframework.validation.BeanPropertyBindingResult["messageCodesResolver"])

    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014)
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:909)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:652)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
    ...

Errors 객체는 JSON으로 바로 변환할 수 없기 때문에 에러가 발생한다.
그 이유는 Errors 객체는 자바 빈 스펙을 준수하고 있는 객체가 아니기 때문이다.

따라서 JsonSerializer<Errors>를 상속받는 별도의 클래스를 만들고 serialize 메서드를 Override 해야한다.
이때 @JsonComponent를 꼭 붙여줘야 한다.

코드

@JsonComponent
public class ErrorsSerializer extends JsonSerializer<Errors> {

    @Override
    public void serialize(Errors errors, JsonGenerator generator, SerializerProvider provider) throws IOException {
        generator.writeStartArray();
        errors.getFieldErrors().stream().forEach(e -> {
            try {
                generator.writeStartObject();
                generator.writeStringField("field", e.getField());
                generator.writeStringField("objectName", e.getObjectName());
                generator.writeStringField("code", e.getCode());
                generator.writeStringField("defaultMessage", e.getDefaultMessage());
                Object rejectedValue = e.getRejectedValue();
                if (rejectedValue != null) {
                    generator.writeStringField("rejectedValue", e.getRejectedValue().toString());
                }
                generator.writeEndObject();
            } catch (IOException e1) {
                e1.printStackTrace();
            }
        });

        errors.getGlobalErrors().stream().forEach(e -> {
            try {
                generator.writeStartObject();
                generator.writeStringField("objectName", e.getObjectName());
                generator.writeStringField("code", e.getCode());
                generator.writeStringField("defaultMessage", e.getDefaultMessage());
                generator.writeEndObject();
            } catch (IOException e1) {
                e1.printStackTrace();
            }
        });
        generator.writeEndArray();
    }

}

결과 로그

MockHttpServletResponse:
           Status = 400
    Error message = null
          Headers = [Content-Type:"application/json"]
     Content type = application/json
             Body = [{"field":"model","objectName":"carSaveRequestDto","code":"NotEmpty","defaultMessage":"must not be empty"},{"field":"carNumber","objectName":"carSaveRequestDto","code":"NotEmpty","defaultMessage":"must not be empty"},{"field":"vin","objectName":"carSaveRequestDto","code":"NotEmpty","defaultMessage":"must not be empty"},{"field":"category","objectName":"carSaveRequestDto","code":"NotNull","defaultMessage":"must not be null"},{"field":"color","objectName":"carSaveRequestDto","code":"NotEmpty","defaultMessage":"must not be empty"},{"field":"productionYear","objectName":"carSaveRequestDto","code":"NotEmpty","defaultMessage":"must not be empty"}]
    Forwarded URL = null
   Redirected URL = null
          Cookies = []