package com.springmvc.controller;
...
import org.springframework.web.bind.annotation.PostMapping;

@Controller
@RequestMapping("/books")
public class BookController {
    ...
    @GetMapping("/add")
    public String requestAddBookForm(@ModelAttribute("NewBook") Book book) {
        return "addBook";
    }

    @PostMapping("/add") ➊

➋

    public String submitAddNewBook(@ModelAttribute("NewBook") Book book) {
        bookService.setNewBook(book); ➌
        return "redirect:/books"; ➍
    }

}