Ideal MySQL在servlet里连接报错问题

Question

单独在java程序里是可以和数据库交互的，怎么到了servlet里就报错了？

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.*;

public class Login extends HttpServlet {
    private static final String JDBC_DRIVER = "com.mysql.cj.jdbc.Driver";
    private static final String DB_URL = "jdbc:mysql://localhost:3306/demo";

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        if(username.isEmpty() || password.isEmpty()){
            System.out.println("用户名或密码不能为空，请重新输入！");
            resp.sendRedirect("/index.jsp");
        }else{//连接数据库对比
            Connection conn = null;
            Statement stmt = null;
            try{
                // 注册 JDBC 驱动
                Class.forName(JDBC_DRIVER);

                // 打开链接
                System.out.println("连接数据库...");
                conn = DriverManager.getConnection(DB_URL,"root","root");
                // 执行查询

                //System.out.println(" 实例化Statement对象...");
                stmt = conn.createStatement();
                String sql;
                sql = "SELECT password FROM user where username = '"+username+"' ";
                ResultSet rs = stmt.executeQuery(sql);

                // 展开结果集数据库
                System.out.println(rs);
                System.out.println(password);
                if(rs.equals(password)){
                    resp.sendRedirect("/success.jsp");
                }
                // 完成后关闭
                rs.close();
                stmt.close();
                conn.close();
            }catch(SQLException se){
                // 处理 JDBC 错误
                resp.sendRedirect("/failed.jsp");
                se.printStackTrace();
            }catch(Exception e){
                // 处理 Class.forName 错误
                resp.sendRedirect("/failed.jsp");
                e.printStackTrace();
            }finally{
                // 关闭资源
                try{
                    if(stmt!=null) stmt.close();
                }catch(SQLException se2){
                }// 什么都不做
                try{
                    if(conn!=null) conn.close();
                }catch(SQLException se){
                    resp.sendRedirect("/failed.jsp");
                    se.printStackTrace();
                }
            }
        }
    }
@Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

报错如下：
java.lang.ClassNotFoundException: com.mysql.cj.jdbc.Driver
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1407)
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1215)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:332)
    at com.carlos.jdbc.Login.doGet(Login.java:29)
    at com.carlos.jdbc.Login.doPost(Login.java:79)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:681)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)

Answer 1

java.lang.ClassNotFoundException 说明mysql的包没找到，你看看对应版本，这是mysql8的包