在jsp，servlet中登錄表單時出錯？

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我有寫一些文件初級講座： -loginServlet.java：簡單的期待loginview.jsp - loginView.jsp：獲取用戶名和密碼 -doLoginServlet.java：在數據庫中查找用戶在jsp，servlet中登錄表單時出錯？

@WebServlet("/doLogin") 
public class DoLoginServlet extends HttpServlet { 
    private static final long serialVersionUID = 1L; 


/** 
* @see HttpServlet#HttpServlet() 
*/ 
public DoLoginServlet() { 
    super(); 
    // TODO Auto-generated constructor stub 
} 

/** 
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) 
*/ 
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { 
    // TODO Auto-generated method stub 
    String userName = request.getParameter("userName"); 
    String password = request.getParameter("passWord"); 
    String rememberMeStr = request.getParameter("rememberMe"); 
    boolean remember = "Y".equals(rememberMeStr); 
    UserAccount user = null; 
    boolean hasError = false; 
    String errorString = null; 
    if(userName == null||password == null||userName.length() == 0||password.length() == 0){ 
     hasError = true; 
     errorString = "Require username and password!"; 

    }else{ 
     Connection conn = MyUtils.getStoreConnection(request); 
     try { 
      user = DBUtils.findUser(conn, userName,password); 
      if(user == null){ 
       hasError = true; 
       errorString = "User Name or password invalid"; 
      } 
     } catch (SQLException e) { 
      // TODO: handle exception 
      e.printStackTrace(); 
      hasError = true; 
      errorString = e.getMessage(); 
     } 
    } 
    if(hasError){ 
     user = new UserAccount(); 
     user.setUserName(userName); 
     user.setPassword(password); 
     request.setAttribute("errorString", errorString); 
     request.setAttribute("user", user); 
     request.getRequestDispatcher("/WEB-INF/views/loginView.jsp").forward(request, response); 
    } 
    else{ 
     HttpSession session = request.getSession(); 
     MyUtils.storeLoginedUser(session, user); 
     if(remember){ 
      MyUtils.storeUserCookie(response, user); 

     } 
     else{ 
      MyUtils.deleteUserCookie(response); 
     } 
     response.sendRedirect(request.getContextPath() + "/userInfo"); 

    } 

    } 

/** 
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) 
*/ 
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { 
    // TODO Auto-generated method stub 
    doGet(request, response); 
}

} 我DBUtils.java包含finduser功能，

package nguyenhuyhoan.com.simplewebapp.utils; 

import java.sql.Connection; 
import java.sql.PreparedStatement; 
import java.sql.ResultSet; 
import java.sql.SQLException; 
import java.util.ArrayList; 
import java.util.List; 

import nguyenhuyhoan.com.simplewebapp.beans.Product; 
import nguyenhuyhoan.com.simplewebapp.beans.UserAccount; 

public class DBUtils { 
    public static UserAccount findUser(Connection conn, String userName, String passWord) throws SQLException { 
     String sql = "Select a.User_Name, a.Password, a.Gender from User_Account a " 
        + " where a.User_Name = ? and a.password= ?"; 

    PreparedStatement pstm = conn.prepareStatement(sql); 
    pstm.setString(1, userName); 
    pstm.setString(2, passWord); 
    ResultSet rs = pstm.executeQuery(); 
    if (rs.next()) { 
     String gender = rs.getString("Gender"); 
     UserAccount user = new UserAccount(); 
     user.setUserName(userName); 
     user.setPassword(passWord); 
     user.setGender(gender); 
     return user; 
    } 
    return null; 

} 

public static UserAccount findUser(Connection conn, String userName) throws SQLException { 
    String sql = "Select a.User_Name,a.PassWord,a.Gender from User_Account a" + "where a.User_Name = ?"; 
    PreparedStatement pstm = conn.prepareStatement(sql); 
    pstm.setString(1, userName); 
    ResultSet rs = pstm.executeQuery(); 
    if (rs.next()) { 
     String passWord = rs.getString("Password"); 
     String gender = rs.getString("Gender"); 
     UserAccount user = new UserAccount(); 
     user.setUserName(userName); 
     user.setPassword(passWord); 
     user.setGender(gender); 
     return user; 

    } 
    return null; 

} 

public static List<Product> queryProduct(Connection conn) throws SQLException { 
    String sql = "Select a.Code,a.Name,a.Price from Product a"; 
    PreparedStatement pstm = conn.prepareStatement(sql); 
    ResultSet rs = pstm.executeQuery(); 
    List<Product> list = new ArrayList<Product>(); 
    while (rs.next()) { 
     String code = rs.getString("Code"); 
     String name = rs.getString("Name"); 
     float price = rs.getFloat("Price"); 
     Product product = new Product(); 
     product.setCode(code); 
     product.setName(name); 
     product.setPrice(price); 
     list.add(product); 
    } 
    return list; 

} 

public static Product findProduct(Connection conn, String code) throws SQLException { 
    String sql = "Select a.Code,a.Name,a.Price from Product a where a.Code = ?"; 
    PreparedStatement pstm = conn.prepareStatement(sql); 
    pstm.setString(1, code); 
    ResultSet rs = pstm.executeQuery(); 
    while (rs.next()) { 
     String name = rs.getString("Name"); 
     float price = rs.getFloat("Price"); 
     Product product = new Product(code, name, price); 
     return product; 

    } 
    return null; 
} 

public static void updateProduct(Connection conn, Product product) throws SQLException { 
    String sql = "Update Product set Name = ?,Price=? where Code = ?"; 
    PreparedStatement pstm = conn.prepareStatement(sql); 
    pstm.setString(1, product.getName()); 
    pstm.setFloat(2, product.getPrice()); 
    pstm.setString(3, product.getCode()); 
    pstm.executeUpdate(); 
} 

public static void insertProduct(Connection conn, Product product) throws SQLException { 
    String sql = "Insert INTO Product(Code,Name,Price) value(?,?,?)"; 
    PreparedStatement pstm = conn.prepareStatement(sql); 
    pstm.setString(1, product.getCode()); 
    pstm.setString(2, product.getName()); 
    pstm.setFloat(3, product.getPrice()); 
    pstm.executeUpdate(); 
} 

public static void deleteProduct(Connection conn, String code) throws SQLException { 
    String sql = "Delete Product where code = ?"; 
    PreparedStatement pstm = conn.prepareStatement(sql); 
    pstm.setString(1, code); 
    pstm.executeUpdate(); 
}

}

但它與findUser消息錯誤函數在DBUtils.java中，但我不明白爲什麼？服務器遇到阻止它履行此請求的內部錯誤。 java.lang.NullPointerException nguyenhuyhoan.com.simplewebapp.utils.DBUtils.findUser（DBUtils.java:18）任何機構幫助我，謝謝！和myUtils.java

import java.sql.Connection; 

import javax.servlet.ServletRequest; 
import javax.servlet.http.Cookie; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.servlet.http.HttpSession; 

import nguyenhuyhoan.com.simplewebapp.beans.UserAccount; 

public class MyUtils { 
    public static final String ATT_NAME_CONECTION = "ATTRIBUTE_FOR_CONNECTION"; 
    private static final String ATT_NAME_USER_NAME = "ATTRIBUTE_FOR_STORE_USER_NAME_IN_COOKIE"; 

    public static void storeConnection(ServletRequest request,Connection conn){ 
     request.setAttribute(ATT_NAME_CONECTION, conn); 
    } 

    public static Connection getStoreConnection(ServletRequest request){ 
     Connection conn = (Connection) request.getAttribute(ATT_NAME_CONECTION); 
     return conn; 
    } 

    public static void storeLoginedUser(HttpSession session,UserAccount loginedUser){ 
     //tren JSP co the truy cap ${loginedUser} 
     session.setAttribute("loginedUser", loginedUser); 
    } 

    public static UserAccount getLoginedUser(HttpSession session){ 
     UserAccount loginedUser = (UserAccount) session.getAttribute("loginedUser"); 
     return loginedUser; 
    } 

    public static void storeUserCookie(HttpServletResponse response,UserAccount user){ 
     System.out.println("store user cookie"); 
     Cookie cookieUserName = new Cookie(ATT_NAME_USER_NAME, user.getUserName()); 
     cookieUserName.setMaxAge(24*60*60); 
     response.addCookie(cookieUserName); 
    } 
    public static String getUserNameInCookie(HttpServletRequest request){ 
     Cookie cookies[] = request.getCookies(); 
     if(cookies != null){ 
      for(Cookie cookie : cookies){ 
       if(ATT_NAME_USER_NAME.equals(cookie.getName())){ 
        return cookie.getValue(); 
       } 
      } 
     } 
     return null; 
    } 

    public static void deleteUserCookie(HttpServletResponse response){ 
     Cookie cookieUserName = new Cookie(ATT_NAME_USER_NAME, null); 

     cookieUserName.setMaxAge(0); 
     response.addCookie(cookieUserName); 
    } 
}

來源

2016-01-07 Hung Huy

的[可能的複製什麼是空指針異常，以及如何解決它？]（http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it） –

添加行號或提供18號線 –

好的謝謝你的建議 –

旁邊的NPE。你在第二findUser方法聲明不能成功執行：

String sql = "Select a.User_Name,a.PassWord,a.Gender from User_Account a" + "where a.User_Name = ?"

你forgott這裏空間：

from User_Account a "

或這裏+ " where

來源

2016-01-07 09:36:19

ok.Im修復它。但它一旦findUser方法錯誤，並且我認爲錯誤不是由SQL語句。 –

在jsp，servlet中登錄表單時出錯？

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