Spring MVC validation status 400

I have just started learning Spring and stuck with form validation (There is dao/service and all the default set)

I try to make a validation task to prevent inserting into the field more than 3 symbols

@Size (min = 1, max = 3)

After that I want to insert the form data to DB and output it on my homepage, when the data satisfies my validation and consists of 1 to 3 symbols, then everything is ok, but when these conditions are not met and I try to insert 4 or more symbols, I receive HTTP STATUS 400. No DB inserts and no view outputs. So I understand that there is validation, however I don't understand why does it show me HTTP STATUS 400

@Controller
public class BookController {

    @Autowired
    private BookService bookService;

    @RequestMapping(value = "addBook", method = RequestMethod.GET)
    public String addUser(Model model) {

        model.addAttribute("user", new ValidationField());
        model.addAttribute("book", new Book());

        return "addBook";
    }

    @RequestMapping(value = "addBook", method = RequestMethod.POST)
    public String addBook( @ModelAttribute("user") @Valid ValidationField validationField, Book book, BindingResult result) {
        /*this.user(book, result);*/

        if (result.hasErrors()) {
            return "addBook";
        }
        this.bookService.addBook(book);

        return "redirect:/";
    }
}

Validation Class

public class ValidationField {

    @Size(min = 1, max = 3)
    private String name;

    @Size(min = 1, max = 3)
    private String genre;
}

and View addBook.jsp

<%@taglib prefix="form" uri="http://ift.tt/IED0jK" %>
<%@taglib prefix="t" tagdir="/WEB-INF/tags" %>
<%@taglib prefix="c" uri="http://ift.tt/QfKAz6" %>

<t:tamplate>
  <form:form method="post" action="addBook" commandName="book" modelAttribute="user">
      <form:errors path="*" cssClass="alert alert-danger" element="div" />
        <table>
            <tr>
                <td><form:input path="name"/></td>
                <td><form:errors path="name"/></td>
            </tr>
            <tr>
                <td><form:input path="genre" /></td>
                <td><form:errors path="genre"/></td>
            </tr>
            <tr>
                <td colspan="2"> <input type="submit" value="add book"> </td>
            </tr>
        </table>
    </form:form>
</t:tamplate>