这个问题在这里已经有答案了：

“找不到或加载主类”是什么意思(53个答案）
将java连接到mysql数据库（14个答案）
12小时前关门了。

import java.sql.*;

public class myjdbc{
        public static void main(String args[]){

                String url="jdbc:mysql://localhost/b04" ;
            String user="root";
        String pass="root";
        Connection conn;
        String sql="select * from students";

          try{          
        Class.forName("com.mysql.jdbc.Driver");
        System.out.println("Driver Loaded");

            conn=DriverManager.getConnection(url,user,pass);
                System.out.println("Database Connected!");

                Statement stmt=conn.createStatement();

                ResultSet res=stmt.executeQuery(sql);

              while(res.next()){
             //Retrieve by column name
                String Username = res.getString("username");
                String Password = res.getString("password");

                //Display values
                System.out.print("Username: " + Username);
                System.out.println(", Password: " + Password);
       }
     conn.close();   
    }catch (Exception e2) {System.out.println(e2);}  

  }//end of main
}//end of class

为了运行program:- java -cp mysql-connector.jar myjdbc