From https://ppettitau.wordpress.com/2013/02/21/spring-3-1-server-side-validation-for-ajax-request-using-valid-and-requestbody/

Ajax Validate Spring MVC

@ExceptionHandler(MethodArgumentNotValidException.class)
@ResponseBody
public ValidationError handleValidationException(MethodArgumentNotValidExceptionexception) {
    Set<ValidationError> errors = new HashSet<ValidationError>();
    for (ObjectError er : exception.getBindingResult().getAllErrors()) {
        errors.add(new ValidationError(er.getObjectName(), er.getDefaultMessage()));
    }

    return new ValidationErrorResponse(errs);

}

js

// Function that Makes Ajax call
function doesSomeStuff() {
    $.ajax({
        type: "POST",
        url: "some/url/in/your/app",
       // ... etc ...
        error: function(data) {
            if (data.errorMessages != null) {
                handleAjaxError(data);
                return;
            }
            // ... etc ...
        }
    });
}

// function that handles ajax error
function handleAjaxError(data) {
        var $errorMessageArea = $("#errorMessageArea");
        for(var i = 0; i < data.errorMessages.length; i++) {
            // append the current error message to the element on the web page
            $errorMessageArea.append($("<li />", { text: data.errorMessages[i] }));
        }
    }
}