有一个关于jpa休眠（crud操作）的问题，实际上我可以做创建，读取和删除操作，但当我遇到更新操作时，我不知道我应该做什么。这是我的代码。更新操作有问题。你能帮帮我吗？

@Autowired
ProductRepository productRepository;

//**************WELCOME***************
@GetMapping("/welcome")
public String welcomePage() {
    return "welcome";
}

// **************İNSERT****************
@GetMapping(path = { "/productinsert"})
public String insertProduct(Model model) {
    Product product = new Product(" ", 0.0); // Gösterim Katmanına ilk boş bir sayfa gitmeli o yüzden değerler boş
    model.addAttribute("product", product);
    return "/productinsert";
}

@PostMapping("/productinsert")
public String insertProduct(Product product, BindingResult bindingResult, Model model) {
    productRepository.save(product);
    if (bindingResult.hasErrors()) {
        return "/productinsert";
    } else {
        model.addAttribute("mesaj", "Ürününüz Başarıyla Kaydedilmiştir.");
        return "/productsuccess";
    }
}

//************UPDATE**************
@GetMapping("/productedit")
public String editProduct() {
    return "/productedit";
}
***//PROBLEM İS HERE//***
@RequestMapping(value = "/productedit", method = RequestMethod.POST)
public String editProductt(@RequestParam long productId, Model model) {
    Optional<Product> proOptional =  productRepository.findById(productId);
    Product product = proOptional.get();
    model.addAttribute("product", product);
    return "/productinsert";
}

}
问题在于
出现意外错误（类型=错误请求，状态=400）。方法参数类型long的所需请求参数“productId”不存在org.springframework.web.bind.MissingServletRequestParameterException：方法参数类型long所需的请求参数“productId”不存在