In ReservationController.java I have the following method, which gets a reservation object from new-reservation.jsp

    @PostMapping("/addBookToReservation")
    public String addBookToReservation(Model model,
                                      @Valid @ModelAttribute("reservation") Reservation reservation,
                                      BindingResult result,
                                      RedirectAttributes redirectAttributes) {
        
        if(result.hasErrors()) {
            return "reservation/new-reservation";
        }
    
        redirectAttributes.addFlashAttribute("reservation", reservation);
        
        return "redirect:/book/add-book";
    }

and sends it to BookController.java, where another method adds another attribute to the model

    @GetMapping("/book/add-book")
    public String showAddBookForm(Model model) {

        model.addAttribute("book", new Book());
        Reservation reservation = (Reservation) model.getAttribute("reservation");
        System.out.println(reservation);       //prints the object I passed it!

        return "/book/add-book";
    }

and returns the following add-book.jsp

<form:form action="/addBook" modelAttribute="book" method="post">                    
   <div>
        <div>
            <form:label path="title">Title</form:label>
            <form:input type="text" id="title" path="title" />
            <form:errors path="title" />
        </div>
        ...
    </div>
    <div>
        <input type="submit" value="Add book">
    </div>
</form:form>

Now, when I handle the form's action addBook

    @PostMapping("/addBook")
    public String addBook(@Valid @ModelAttribute Book book,
                            BindingResult result,
                            Model model) {
        
        if (result.hasErrors()) {
            return "book/add-book";
        }
        
        Reservation reservation = (Reservation) model.getAttribute("reservation");
        System.out.println(reservation); // reservation is null!!
        
        
        return "somewhere/else";
    } 

and I try to retrieve the reservation object from the model I get a null. How can I pass my reservation object through the JSPs I've showed you before?

EDIT-SOLVED:

I was able to solve my problem by adding @SessionAttributes("reservation") on top of the ReservationController.java class definition and removing it once finished (following the instructions in the answer linked).

CodePudding user response：

try adding @SessionAttributes for the model attribute on top of the controller and remove the session attribute when done

@SessionAttributes("reservation")