数据库驱动加载成功，数据库连接成功。可就是出现java.lang.NullPointerException
at Gradation.main(Gradation.java:17)
这是怎么回事？
源代码如下：

import java.sql.*;
public class Gradation {
    static Connection con;
    static Statement sql;
    static ResultSet res;
    public Connection getConnection(){
     return con;
    }

public static void main(String[] args) {
// TODO 自动生成的方法存根
        Gradation c=new Gradation();
        con=c.getConnection();
        try{
         sql=con.createStatement();
         res=sql.executeQuery("select*from tb_stu");
         while(res.next()){
         String id=res.getString("id");
         String name=res.getString("name");
         String sex=res.getString("sex");
         String birthday=res.getString("birthday");
         System.out.println("编号"+id);
         System.out.println("姓名"+name);
         System.out.println("性别"+sex);
         System.out.println("生日"+birthday);
         }
        }catch(Exception e){
         e.printStackTrace();
        }
}

}