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我开发了一个简单的 CRUD 应用程序。使用javabean并在UserDao文件中获取空指针异常但无法解决。谁能告诉我是什么问题？？

这是代码，

User.jsp <%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
        <title>Add New User</title>
    </head>

    <body>
        <form method="POST" action='UserHandler' name="frmAddUser"><input type="hidden" name="action" value="insert" />
        <p><b><center>Add New Record<center></b></p>

            <table>
                <tr>
                    <td>Name</td>
                    <td><input type="text" name="name" /></td>
                </tr>

                <tr>
                    <td>Roll Number</td>
                    <td><input type="text" name="rollno" /></td>
                </tr>

                <tr>
                    <td>Class</td>
                    <td><input type="text" name="clas" /></td>
                </tr>

                <tr>
                    <td>Mobile Number</td>
                    <td><input type="text" name="mono" /></td>
                </tr>

                <tr>
                    <td></td>
                    <td><input type="submit" value="Submit" /></td>
                </tr>
            </table>
        </form>

        <p><a href="UserHandler?action=listUser">View-All-Records</a></p>
    </body>
</html>

用户处理程序.java

package handler;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

import dao.UserDao;
import bean.UserBean;

public class UserHandler extends HttpServlet    {
    private static String INSERT = "/user.jsp";
    private static String Edit = "/edit.jsp";
    private static String UserRecord = "/listUser.jsp";
    private UserDao dao;

    public UserHandler()    {
        this.dao = new UserDao();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String redirect = "";
        String roll = request.getParameter("rollno");
        String mo = request.getParameter("mono");
        String action = request.getParameter("action");

        if ((roll == null) && (action.equalsIgnoreCase("insert")))  {
            int rollno = Integer.parseInt(roll);
            long mono = Integer.parseInt(mo);
            UserBean user = new UserBean();
            user.setName(request.getParameter("name"));
            user.setRollno(rollno);
            user.setClas(request.getParameter("clas"));
            user.setMono(mono);
            this.dao.addUser(user);
            redirect = UserRecord;
            request.setAttribute("student", this.dao.getAllUsers());
            System.out.println("Record Added Successfully");
        }
        else if (action.equalsIgnoreCase("delete")) {
            String roll1 = request.getParameter("rollno");
            int rl = Integer.parseInt(roll1);
            this.dao.removeUser(rl);
            redirect = UserRecord;
            request.setAttribute("student", this.dao.getAllUsers());
            System.out.println("Record Deleted Successfully");
        }
        else if (action.equalsIgnoreCase("editform")) {
            redirect = Edit;
        }
        else if (action.equalsIgnoreCase("edit")) {
            String roll2 = request.getParameter("rollno");
            String monu = request.getParameter("mobileno");
            int uid = Integer.parseInt(roll2);
            long mono = Integer.parseInt(monu);

            UserBean user = new UserBean();
            user.setName(request.getParameter("name"));
            user.setRollno(uid);
            user.setClas(request.getParameter("clas"));
            user.setMono(mono);
            this.dao.editUser(user);
            request.setAttribute("user", user);
            redirect = UserRecord;
            System.out.println("Record updated Successfully");
        }
        else if (action.equalsIgnoreCase("listUser")) {
            redirect = UserRecord;
            request.setAttribute("student", this.dao.getAllUsers());
        }
        else {
            redirect = INSERT;
        }

        RequestDispatcher rd = request.getRequestDispatcher(redirect);
        rd.forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

用户道.java

package dao;

import java.sql.*;
import java.util.*;

import dbconnection.ConnectionProvider;
import bean.UserBean;

public class UserDao    {
    private Connection conn;

    public UserDao()    {
        this.conn = ConnectionProvider.getConnection();
    }

    public void addUser(UserBean userBean) {
        try {
            String sql = "INSERT INTO student (name, rollno, class, mobileno) VALUES (?, ?, ?, ? )";

            PreparedStatement ps = this.conn.prepareStatement(sql);
            ps.setString(1, userBean.getName());
            ps.setInt(2, userBean.getRollno());
            ps.setString(3, userBean.getClas());
            ps.setLong(4, userBean.getMono());
            ps.executeUpdate();
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void removeUser(int rollno) {
        try {
            String sql = "DELETE FROM student WHERE rollno=?";

            PreparedStatement ps = this.conn.prepareStatement(sql);
            ps.setInt(1, rollno);
            ps.executeUpdate();
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void editUser(UserBean userBean) {
        try {
            String sql = "UPDATE student SET name=?, class=?, mobileno=? WHERE rollno=?";

            PreparedStatement ps = this.conn.prepareStatement(sql);
            ps.setString(1, userBean.getName());
            ps.setString(2, userBean.getClas());
            ps.setLong(3, userBean.getMono());
            ps.executeUpdate();
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public List<UserBean> getAllUsers() {
        List student = new ArrayList();
        try {

/*          if(this.conn.isClosed())    {
                System.out.println("Connection is closed");
            }
            else    {
*/          String sql = "SELECT * FROM student WHERE name IS NOT NULL ORDER BY name";
            PreparedStatement ps = this.conn.prepareStatement(sql);
            ResultSet rs = ps.executeQuery();

                while (rs.next()) {
                    UserBean userBean = new UserBean();
                    userBean.setName(rs.getString("name"));
                    userBean.setRollno(rs.getInt("rollno"));
                    userBean.setClas(rs.getString("clas"));
                    userBean.setMono(rs.getLong("mono"));
                    student.add(userBean);
                }
//          }
        }
        catch (SQLException e) {
            e.printStackTrace();
        }

        return student;
    }

    public UserBean getUserById(int rollno) {
        UserBean userBean = new UserBean();
        try {
            String sql = "SELECT * FROM student WHERE rollno=?";

            PreparedStatement ps = this.conn.prepareStatement(sql);
            ps.setInt(2, rollno);
            ResultSet rs = ps.executeQuery();

            if (rs.next()) {
                userBean.setName(rs.getString("name"));
                userBean.setRollno(rs.getInt("rollno"));
                userBean.setClas(rs.getString("clas"));
                userBean.setMono(rs.getLong("mono"));
            }
        }
        catch (SQLException e) {
            e.printStackTrace();
        }
    return userBean;
    }
}

这是错误

HTTP Status 500 - 

--------------------------------------------------------------------------------

type Exception report

message 

description The server encountered an internal error that prevented it from fulfilling this request.

exception 

java.lang.NullPointerException
    dao.UserDao.getAllUsers(UserDao.java:69)
    handler.UserHandler.doGet(UserHandler.java:68)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)


note The full stack trace of the root cause is available in the Apache Tomcat/7.0.39 logs.


--------------------------------------------------------------------------------

Apache Tomcat/7.0.39
4

1 回答 1

0

创建连接是否成功

try {
 String driver = "com.mysql.jdbc.Driver";
 String url = "jdbc:mysql://localhost:port/db";
 String user = "xx";
 String password = "xxt";
 Class.forName(driver);
 Connection con = DriverManager.getConnection(url, user, password);
 System.out.println("Connection"+con);
 } 
 catch(Exception e) {
  System.out.println("Exception when creating connection "+e.getMessage());
 }
于 2013-07-10T10:56:57.603 回答