找了1个多小时才发现的错误。。

如果只是跟着课本敲，那就肯定报错了

好久没碰有点生疏了

之前敲过一个类似的加载properties的文件的方法，直接把课本的这个改成这个就可以解决500的错误了

注释的地方是行不通的错误代码！！！

注释下边这个没问题，记得路径改成绝对路径！！！

package com.sun.web.utils;

import java.io.FileInputStream;
import java.io.IOException;
import java.sql.*;
import java.util.Properties;

public final class jdbcUtil {

    private static String driver;
    private static String url;
    private static String user;
    private static String password;
    private static Properties pr = new Properties();

    public jdbcUtil() {
    }

    static {
        try {
            pr.load(new FileInputStream("E:\\IDEA_SAVE\\javaweb_ laboratory_course\\src\\db.properties"));
//            pr.load(jdbcUtil.class.getClassLoader().getResourceAsStream("E:\\IDEA_SAVE\\javaweb_ laboratory_course\\src\\db.properties"));
        } catch (IOException e) {
            e.printStackTrace();
        }
        driver = pr.getProperty("driver");
        url = pr.getProperty("url");
        user = pr.getProperty("user");
        password = pr.getProperty("password");
        try {
            Class.forName(driver);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    public static Connection getConn() throws Exception {
        return DriverManager.getConnection(url, user, password);
    }

    public static void free(Connection connection, PreparedStatement preparedStatement, ResultSet resultSet) throws Exception {
        if (resultSet != null) resultSet.close();
        if (preparedStatement != null) preparedStatement.close();
        if (connection != null) connection.close();
    }


}

针对添加信息不成功的问题，原因在于sql语句的问题，因为是固定的，id不能改动，所以修改id的时候，sql语句就执行不了了。

    public void update(User user) throws Exception {
        Connection conn = jdbcUtil.getConn();
        String sql = "update user set username=?,sex=? where userid=?";
        PreparedStatement preparedStatement = conn.prepareStatement(sql);
        preparedStatement.setString(1, user.getUsername());
        preparedStatement.setString(2, user.getSex());
        preparedStatement.setString(3, user.getUserid());
        int n=preparedStatement.executeUpdate();
        System.out.println(n);
        jdbcUtil.free(conn, preparedStatement, null);
    }

记得加上解决数据库乱码的问题

request.setCharacterEncoding("utf-8");

    request.setCharacterEncoding("utf-8");
    String bh = request.getParameter("userid3");
    String xm = request.getParameter("username3");
    String xb = request.getParameter("sex3");

    System.out.println(bh+xm+xm);
    User u = new User(bh, xm, xb);
    UserDao dao = new UserDao();
    dao.update(u);
    List<User> users = dao.QueryAll();
    request.setAttribute("user_list", users);