exception java.lang.NullPointerException的问题

Question

public class StaffDao {

//验证登录
    public String CheckLogin(String username, String password){
        String id = null;
        String sql="select * from staff where staff_name='"+username+"' and staff_password='"+password+"'";
        Statement stat = null;
        ResultSet rs = null;
        Connection conn = new DBHelper().getConn();
        try{

stat = conn.createStatement();//请问这行问题出在哪？

            rs = stat.executeQuery(sql);
            while (rs.next()) {
                id = rs.getString("staff_no");
            }
        }
        catch(SQLException ex){}
        return id;
    }
    //验证密码
    public boolean CheckPassword(String id, String password){
        boolean ps = false;
        String sql="select *from staff where staff_no='"+id+"' and staff_password='"+password+"'";
        Statement stat = null;
        ResultSet rs = null;
        Connection conn = new DBHelper().getConn();
        try{
            stat = conn.createStatement();
            rs = stat.executeQuery(sql);
            while (rs.next()) {
                ps=true;
            }
        }
        catch(SQLException ex){}
        return ps;
    }

Answer 1

Connection conn = new DBHelper().getConn();
很可能是你连接字符串不对，数据库配置问题等，getConn();返回了null