What is the difference between ConstraintViolationException and MethodArgumentNotValidException

Question

When I am validating bean using @valid annotation in javax.validations, for some objects I am getting ConstraintViolationException and for some I am getting MethodArgumentNotValidException.

I understand that, if I validate anything in @ResponseBody at controller , it throws MethodArgumentNotValidException.

But for some custom validations(eg. @MyCustomValidation) at class level it is throwing ConstraintViolationException even if it is being validated in @ResponseValidation.

And for some other custom validation for different REST endpoint, it throws MethodArgumentNotValidException.

I am finding it bit difficult to understand as it's behavior.

@PostMapping(path = "/someEndPoint")    
@Validated(OnASave.class)
public ResponseEntity<ClassA> saveObjA(@Valid @RequestBody ClassA objA)

Result - throws MethodArgumentNotValidException

@PostMapping(path = "/someOtherEndPoint")   
@Validated(OnBSave.class)
public ResponseEntity<ClassB> saveObjB(@Valid @RequestBody ClassB objB)

Result - throws ConstraintViolationException

Both ClassA and ClassB has custom validations.

Answer 1

for a simple understanding, if validation happens at controller/service layer by using @Valid annotation, it generates MethodArgumentNotValidException, you can add handler for this and return the response accordingly, this class is part of spring framework and validation is performed by spring framework see sample below

@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity<Response> handleMethodArgumentNotValidException(MethodArgumentNotValidException ex) {

    logger.info("Invalid arguments found : " + ex.getMessage());
    // Get the error messages for invalid fields
    List<FieldError> errors = ex.getBindingResult()
            .getFieldErrors()
            .stream()
            .map(fieldError -> new FieldError(fieldError.getField(), fieldError.getDefaultMessage()))
            .collect(Collectors.toList());

    String message = messageSource.getMessage("invalid.data.message", null, LocaleContextHolder.getLocale());
    Response response = new Response(false, message)
            .setErrors(errors);
    ResponseEntity<Response> responseEntity = ResponseEntity.status(HttpStatus.BAD_REQUEST).body(response);
    return responseEntity;
}

and if you do not validate using @Valid annotation, and exception is raised by hibernate at jpa layer it generates ConstraintViolationException, this exception is part of Javax bean validation framework, and raised at the time of performing persistence operation (before the actual sql execution) see sample below

@ExceptionHandler(ConstraintViolationException.class)
    public ResponseEntity<Response> handleConstraintViolationException(ConstraintViolationException ex) {
        List<FieldError> errors = ex.getConstraintViolations()
                .stream()
                .map(constraintViolation -> {
                    return new FieldError(constraintViolation.getRootBeanClass().getName() + " " + constraintViolation.getPropertyPath(), constraintViolation.getMessage());
                })
                .collect(Collectors.toList());

        String message = messageSource.getMessage("invalid.data.message", null, LocaleContextHolder.getLocale());
        Response response = new Response(false, message)
                .setErrors(errors);
        ResponseEntity<Response> responseEntity = ResponseEntity.status(HttpStatus.BAD_REQUEST).body(response);
        return responseEntity;
    }

Answer 2

When you use @Valid, you are applying validation which is defined by you on your model class fields, while there are different types of validations, you can choose like @NotNull, @Max, @Min and so on, you will get the matching type.

In general, all of these are parallel to MethodArgumentNotValidException which will be thrown in all cases.

From official document

Exception to be thrown when validation on an argument annotated with @Valid fails.

ConstraintViolationException is thrown by hibernate entity manager when some constrain violated, so this means you violated some fields in some entity you are using.