<form action="postGetParameter" method="post">
    <input type="text" name="userId">
    <input type="password" name="password">
    <input type="submit" value="sumbit">
</form>

<input type="text" id="userId">
<input type="text" id="classId">
<input type="button" value="sumbit" id="submit">

let userIdInput = document.querySelector("#userId");
let classIdInput = document.querySelector("#classId");
let button = document.querySelector("#sumbit");
button.onclick = function(){
    $.ajax({
        type: 'post',
        url: 'path',
        data: JSON.stringify({
            userId: userIdInput.value,
            classId: classIdInput.value
        }),
        success: function(body){
            console.log(body);
        }
    });
}

class User {
    //You must need to note the types.
    //If you wrote a String, but you send a int, it will throw a exception.
    public String userId;
    public String classId;
}

@WebServlet("/path")
public class HelloServlet extends HttpServlet {

    private ObjectMapper objectMapper = new ObjectMapper();

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       //You must to comment this method otherwise your codes will goes wrong
       //super.doGet(req, resp);

       //And you must add the setContentType method if you want to print in Chinese.
       resp.setContentType("text/html; charset=utf8");
       //You can use this method to print something to the console of the broswer
      resp.getWriter().write("Hello Servlet");


       //HANDLE THE STRING
       String userId = req.getParameter("userId");
       String classId = req.getParameter("classId");
       resp.getWriter().write("userId: " + user.userId + "classId: " + user.classId);

        //HANDLE THE JSON STRING
        //The first parameter, you can give a String or a InputStream Object
        //The second parameter, present what object you want t make and then you will get a class.
        User user = objectMapper.readValue(req.getInputStream(), User.class);
        resp.getWriter().write("userId: " + user.userId + "classId: " + user.classId);
    }
}