在这种情况下，为什么会出现错误“ Bean名称'user'的BindingResult和普通目标对象均不可用”？

Question

Why do I get this error? Before connecting the validation and adding forms of the form: label / input - everything worked for me

 @RequestMapping(method = POST, value = "/reg")
    public String registration(@ModelAttribute("userReg") @Valid User user, BindingResult bindingResult,
                               HttpSession session) {
        if(bindingResult.hasErrors()) return "redirect:/reg";
        if (userDAO.findByLogin(user.getLogin()) == null) {
            userDAO.persist(user);
            session.setAttribute("login", user.getLogin());
            return "welcomepage";
        }
        return "Registration";
    }

Exceptions:

 org.apache.jasper.JasperException: An exception occurred processing JSP page /Registration.jsp at line 18

    15: <form:form action="/reg" method="post" modelAttribute="user">
    16:         <table>
    17:             <tr>
    18:                 <td><form:label path="login">Login:</form:label></td>
    19:                 <td> <form:input path="login"/> </td>
    20:                 <td> <form:errors path="login"/></td>
    21:             </tr>

java.lang.IllegalStateException: Neither BindingResult nor plain target object for bean name 'user' available as request attribute

Continuing the form from the error:

 ...<p>
            <tr>
                <td> <form:label  path="password">Password:</form:label></td>
                <td> <form:input  path="password"/> </td>
                <td> <form:errors path="password"/></td>
            </tr>
        <p>
    <input type="submit" value="Registration">
</form:form>

Answer 1

This error happens because you don't pass the object 'user' to Model in your controller. Returning ModelAndView is a option but you can put the object in the Model object and return your page address.

@GetMapping("/user") 
public View retrieveUser(ModelMap model, HttpServletRequest request) {
  model.addAttribute("user", userService.getUser());
  String contextPath = request.getContextPath();

return new RedirectView(contextPath); }

Answer 2

The problem was that the page did not have the required attribute. And the error crashed because the name field was not found in the "user" that was not filled. To fix the error i added object ModelAndView() with "user" atribute.

@RequestMapping(method = GET, value = "/reg")
public ModelAndView registration() {
    return new ModelAndView("Registration","user",new User());
}