下面的代碼不會在相應的表中插入一行。我已經(jīng)測試了數(shù)據(jù)庫連接并在數(shù)據(jù)庫中運行了查詢，但是當我通過.jsp形式添加輸入時，值仍然沒有插入。public class UserDao {public String registerUser(User user){    String username = user.getUsername();    String email = user.getEmail();    String password = user.getPassword();    Connection con;    con = DBConnection.createConnection();    PreparedStatement preparedStatement;    try{        con.setAutoCommit(false);        String query = ("INSERT INTO user (username, email, password, user_id) VALUES(?, ?, ?, ?)");        preparedStatement = con.prepareStatement(query);        preparedStatement.setString(1, username);        preparedStatement.setString(2, email);        preparedStatement.setString(3, password);        preparedStatement.setString(4,null);        int i = preparedStatement.executeUpdate();        con.commit();        preparedStatement.close();        con.close();        if(i !=0 ){            return "SUCCESS";        }    }catch(SQLException e){      throw new RuntimeException(e);    }    return "Something is wrong!";}}作為參考，這是我的 servlet 類和.jsp文件的樣子：public class UserRegistrationServlet extends HttpServlet {public UserRegistrationServlet(){}/** * Handles the HTTP <code>POST</code> method. * * @param request servlet request * @param response servlet response * @throws ServletException if a servlet-specific error occurs * @throws IOException if an I/O error occurs */@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response)        throws ServletException, IOException {    String userName = request.getParameter("username");    String email = request.getParameter("email");    String password = request.getParameter("password");    User user = new User();不確定我的代碼中的錯誤在哪里，所以任何建議都會非常有幫助。謝謝