rs.getInt(i)控制台输入有值，为什么赋值给变量就报java.lang.NullPointerException错？

JDBC连接数据库查找成绩信息，报java.lang.NullPointerException
试着输出结果集rs.getInt()，全都有数据

//dao层
public ArrayList<Score> getScore(String sql){
            ArrayList<Score> score = new ArrayList<Score>();
            try {
                conn =new DBconn().getConn();
                ps = conn.prepareStatement(sql);
                rs = ps.executeQuery();

                while(rs.next()) {

                    System.out.println("StuId"+rs.getInt(3));

                    Score sco=new Score();
                    sco.setScore_id(rs.getInt(1));

                    int course_id=rs.getInt(2);  System.out.println("CouId："+course_id);
                    sco.setCourse_name(this.getName("SELECT course_name FROM course WHERE course_id="+course_id+""));

                    int s_id=rs.getInt(3); 
                    sco.setS_name(this.getName("SELECT s_name FROM student WHERE s_id="+s_id+""));

                    int t_id=rs.getInt(4);
                    sco.setT_name(this.getName("SELECT t_name FROM teacher WHERE t_id="+t_id+""));

                    sco.setScore(rs.getInt(5));
                    sco.setScore(85);

                    int class_id=rs.getInt(6);
                    sco.setClass_name(this.getName("SELECT class_name FROM class WHERE class_id="+class_id+""));

                    score.add(sco); 
                    System.out.println("score"+score);
                }
            } 
            catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            finally{
                this.close();
            }
            return score;
        }

//servlet层
al=dbh.getScore("SELECT * FROM score");
                System.out.println("all:"+al);
                rowCount = dbh.getRowCount("SELECT COUNT(*) FROM score");
                request.setAttribute("class_name", "");
                request.setAttribute("t_name","");
                request.setAttribute("course_name", "");

看看报错信息，哪一行空指针了