중요한 부분

@GetMapping("/items/{itemId}/edit")
public String updateItemForm(@PathVariable("itemId") Long itemId, Model model) {
	Book item = (Book) itemService.findItem(itemId);

	BookForm form = new BookForm();
	form.setId(item.getId());
	form.setName(item.getName());
	form.setPrice(item.getPrice());
	form.setStockQuantity(item.getStockQuantity());
	form.setAuthor(item.getAuthor());
	form.setIsbn(item.getIsbn());

	model.addAttribute("form", form);

	return "items/updateItemForm";
}
@PostMapping("/items/{itemId}/edit")
public String updateItem(@PathVariable("itemId") Long itemId, @ModelAttribute("form") BookForm form) {
//		Book book = (Book) itemService.findItem(form.getId());

	Book book = new Book();

	book.setId(form.getId());
	book.setName(form.getName());
	book.setPrice(form.getPrice());
	book.setStockQuantity(form.getStockQuantity());
	book.setAuthor(form.getAuthor());
	book.setIsbn(form.getIsbn());

	itemService.save(book);

	return "redirect:/items";
}