运行服务器(Tomcat)但是到JDBC这一步，一直报错java.lang.NoClassDefFoundError: Could not initialize class DruidDemo

原创已于 2022-03-25 15:51:58 修改 · 2.4k 阅读

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#tomcat #java #服务器

于 2022-02-26 14:29:36 首次发布

博客讲述了在使用Tomcat运行含有Druid和MySQL的JDBC项目时遇到的NoClassDefFoundError错误。问题在于运行测试没有问题，但在HTML页面通过Tomcat启动时出现错误。解决方案是将Druid和MySQL的jar包复制到Tomcat的lib目录下，重启服务器即可解决问题。博客还详细介绍了项目的各个部分，包括HTML页面、实体类、DAO、JDBC工具类和Servlet的实现。

先说题目的解决方案

将德鲁伊的jar包和mysql的驱动包，复制到tomcat下的lib文件夹内，重启即可

问题

运行测试JDBC的代码，运行结果是没有任何问题的，但是只要用tomcat跑代码，在html页面上输入账号密码，就会报错

网页上的报错提示

IDEA后台的报错提示

详细的解决步骤

将这两个jar包，复制到tomcat的lib文件夹内

觉得有帮助的话，帮忙点赞哦😁

======================================================================

详细代码

案例：用户登陆

1导入对应jar包

(上面的正常把jar包复制到tomcat的lib文件夹内，这里正常的把jar包导入IDEA中)

将德鲁伊的jar包和mysql的jar包复制到web文件夹下的WEB-INF文件夹中

2在src文件夹下，把德鲁伊的配置文件导入进去

driverClassName=com.mysql.cj.jdbc.Driver
#这里是day14数据库的名称
url=jdbc:mysql://localhost:3306/day14
username=root
#xxx是数据库的密码
password=xxx
#初始化连接数量
initialSize=5
#最大连接数
maxActive=10
#最大等待时间
maxWait=3000

2使用Druid数据库连接池技术，操作mysql，day14数据库中user表

3编写login.html登陆页面：username&password两个输入框

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>登陆</title>
</head>
<body>
<form action="/test1/loginServlet" method="post">
    用户名：<input type="text" name="username">
    密码：<input type="password" name="password">
    <input type="submit" value="登陆">
</form>
</body>
</html>

4创建包domain，创建类User(用户的实体类)

package domain;

public class User {
    private int id;
    private String username;
    private String password;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

5创建工具包util，创建类JDBCUtils，JDBC工具类，使用Durid连接池

package util;

import com.alibaba.druid.pool.DruidDataSourceFactory;

import javax.sql.DataSource;
import java.io.InputStream;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Properties;

//Druid
public class JDBCUtils {
    private static DataSource dataSource;
    static {
        try {
            Properties properties = new Properties();
            InputStream is = JDBCUtils.class.getClassLoader().getResourceAsStream("druid.properties");
            properties.load(is);
            //获取连接池对象
            dataSource = DruidDataSourceFactory.createDataSource(properties);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    //获取连接的方法
    public static Connection getConnection() throws SQLException {
        return dataSource.getConnection();
    }
    //获取连接池的方法
    public static DataSource getDataSource(){
        return dataSource;
    }
    //释放资源：查询DQL
    public static void close(ResultSet rs, Statement stmt,Connection conn){
        if (rs != null){
            try {
                rs.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if (stmt != null){
            try {
                stmt.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if (conn != null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
    }
}

6创建包dao，创建类UserDao，提供login方法

package dao;

import domain.User;
import util.JDBCUtils;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class UserDao {
    public User login(User loginUser){
        Connection connection = null;
        PreparedStatement preparedStatement = null;
        ResultSet resultSet = null;
        User user = new User();
        try {
            //获取连接
            connection = JDBCUtils.getConnection();
            String sql = "select * from user where username = ? and password = ?";
            //获取prepareStatement对象
            preparedStatement = connection.prepareStatement(sql);
            //给？赋值
            preparedStatement.setString(1,loginUser.getUsername());
            preparedStatement.setString(2,loginUser.getPassword());
            resultSet = preparedStatement.executeQuery();
            while (resultSet.next()){
                user.setId(resultSet.getInt("id"));
                user.setUsername(resultSet.getString("username"));
                user.setPassword(resultSet.getString("password"));
            }
            return user;
        } catch (SQLException throwables) {
            throwables.printStackTrace();
            return null;
        }finally {
            JDBCUtils.close(resultSet,preparedStatement,connection);
        }
    }
}

7对上面的代码先进行测试，当测试没问题，再继续创建svelte等操作

package test;

import dao.UserDao;
import domain.User;
import org.junit.Test;

public class UserDaoTest {
    @Test
    public void testLogin(){
        User loginUser = new User();
        loginUser.setUsername("superbaby");
        loginUser.setPassword("1231");
        UserDao dao = new UserDao();
        User login = dao.login(loginUser);
        System.out.println(login);
    }
}

8创建servlet包，创建类Login Servlet类，完成登陆的具体逻辑

package servlet;

import dao.UserDao;
import domain.User;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/loginServlet")
public class LoginServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        //获取请求参数
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        //封装user对象
        User loginuser = new User();
        loginuser.setUsername(username);
        loginuser.setPassword(password);
        //调用UserDao的login方法
        UserDao dao = new UserDao();
        User user = dao.login(loginuser);
        //判断user
        if (user.getId() == 0){
            //登陆失败：转发到/failServlet
            req.getRequestDispatcher("/failServlet").forward(req,resp);
        }else {
            //登陆成功：存储数据，再转发到/successServlet
            //存储数据
            req.setAttribute("user",user);
            //转发
            req.getRequestDispatcher("/successServlet").forward(req,resp);
        }
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doGet(req,resp);
    }
}

就是在这里，我只要一测试，就出bug，最后将德鲁伊的jar包和mysql的jar包，复制到tomcat的lib文件夹内才能正常执行了

9登陆失败

package servlet;

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

@WebServlet("/failServlet")
public class FailServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //给页面写一句话
        //设置页面的编码
        response.setContentType("text/html;charset=utf-8");
        //输出
        response.getWriter().write("登陆失败，用户名或密码错误");
    }

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

10登陆成功

package servlet;

import domain.User;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/successServlet")
public class SuccessServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取request域中共享的user对象
        User user = (User) request.getAttribute("user");
        if (user != null){
            //给页面写一句话
            //设置页面的编码
            response.setContentType("text/html;charset=utf-8");
            //输出
            response.getWriter().write("登陆成功！"+user.getUsername()+"欢迎您");
        }
    }

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

要是觉得不错多多点赞哦😁