关于JDBC工具类加载驱动时出现java.lang.NoClassDefFoundError，无法初始化......的一种解决方案。

网上好像都说的是写不写  getClassLoader()，但我无论要不要这个方法都初始化不了。经过好几天折腾，发现与之无关，应该是：

 

具体代码如下(可以拷贝了试试，我用的是mysql 8.0.26)：

 db.properties:

driver= com.mysql.cj.jdbc.Driver

url = jdbc:mysql://localhost:3306/myfirst?useUnicode=true&characterEncoding=UTF-8&useSSL=true&serverTimezone=UTC

username = root

password = *****

 JdbcUtils:

package JDBC;

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

public class JdbcUtils {
    private static String driver = null;
    private static String url = null;
    private static String username = null;
    private static String password = null;
    static {
        try {
            InputStream in = JdbcUtils.class.getClassLoader().getResourceAsStream("db.properties");
            Properties properties = new Properties();
            properties.load(in);
            driver = properties.getProperty("driver");
            url = properties.getProperty("url");
            username = properties.getProperty("username");
            password = properties.getProperty("password");
            Class.forName(driver);

        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
    //获取连接
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(url,username,password);

    }
    //释放连接资源
    public static void release(Connection conn, Statement st, ResultSet rs){
        if(rs != null){
            try {
                rs.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if(st != null){
            try {
                st.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if(conn != null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }

    }
}

Testdemo2:

 

package JDBC;

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

public class Testdemo2 {
    public static void main(String[] args) {
        Connection conn = null;
        Statement st = null;
        ResultSet rs = null;
        try {
            conn = JdbcUtils.getConnection();//获取数据库连接
            st = conn.createStatement();//获取SQL的连接对象
            String sql = "select * from student";
            rs = st.executeQuery(sql);
            while (rs.next()){
                System.out.println(rs.getObject("id"));
                System.out.println(rs.getObject("name"));
                System.out.println(rs.getObject("age"));
                System.out.println(rs.getObject("email"));
            }

        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            JdbcUtils.release(conn,st,rs);
        }
    }
}