mysql-[jbdc]打开connection空指针异常

小白才开始学JBDC 这是网上照着写的代码……
出现了

Exception in thread "main" java.lang.NullPointerException
at FirstExample.main(FirstExample.java:23)

空指针异常
但是不知道怎么改，求解。谢谢了

import java.sql.*;
public class FirstExample {
//JDBC driver name and databaase URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/EMP";

//Database credentials
static final String USER = "root";
static final String PASS = "2573718";

public static void main(String args[]) throws ClassNotFoundException{    Connection conn = null;    Statement stmt = null;    try{        //注册 JDBC driver        Class.forName("com.mysql.jdbc.Driver");        //打开 a connection        System.out.println("Connecting to database..");        stmt = conn.createStatement();        String sql;        sql= "SELECT s_no  FROM student";        ResultSet rs = stmt.executeQuery(sql);        //插入数据        while(rs.next())        {            int no = rs.getInt("s_no");            String name = rs.getString("s_name");            System.out.println("NO"+no);            System.out.println("Name"+name);        }        rs.close();        stmt.close();        conn.close();    }    catch(SQLException se){        se.printStackTrace();    }    finally{        try{            if(stmt!=null)                stmt.close();            }        catch(SQLException se2){        }        try{            if(conn!=null)                conn.close();        }        catch(SQLException se){            se.printStackTrace();        }    }}

}