package com.springmvc.controller;
...
@Controller
@RequestMapping("/books")
public class BookController {
    ...

➊

    @GetMapping("/update")
    public String getUpdateBookForm(@ModelAttribute("updateBook") Book book, @RequestParam("id") String bookId, Model model) {
        Book bookById = bookService.getBookById(bookId);
        model.addAttribute("book", bookById);
        return "updateForm";
    }

➋

    @PostMapping("/update")
    public String submitUpdateBookForm(@ModelAttribute("updateBook") Book book) {
        MultipartFile bookImage = book.getBookImage();
        String rootDirectory = "c:/upload/";
        if (bookImage != null && !bookImage.isEmpty()) {
            try {
                String fname = bookImage.getOriginalFilename(); 
                bookImage.transferTo(new File("c:/upload/" + fname));
                book.setFileName(fname);
            } catch (Exception e) {
                throw new RuntimeException("Book Image saving failed", e);
            }
        }
        bookService.setUpdateBook(book);
        return "redirect:/books";
    }

}