jdbc 连接mysql 首次成功，之后连接就报错

 Connection conn = null;        Statement stat = null;        ResultSet rs = null;        String url = "jdbc:mysql://*********:3306/*******?useUnicode=true&characterEncoding=GBK";        String sql = null;        String tableName = "***";        List<ClockBO> clock = new ArrayList<ClockBO>();            sql = "select * from "                    + tableName                    + " where timevalue between '"                    + startTime                    + "' and '" + endTime + "' and doorName='" + doorName.substring(2) + "'";            System.out.println(sql);            Class.forName("com.mysql.jdbc.Driver");            conn = DriverManager.getConnection(url, "root", "****");            stat = conn.createStatement();            rs = stat.executeQuery(sql);            while (rs.next()) {                ClockBO bo = new ClockBO();                ClockPKBO pkbo = new ClockPKBO();                pkbo.setCardNo1(rs.getString(1));                pkbo.setCardNo2(rs.getString(2));                pkbo.setTimeValue(ft.format(rs.getTimestamp(6)));                bo.setPersonNo(rs.getString(3));                bo.setPersonName(rs.getString(4));                bo.setDepartment(rs.getString(5));                bo.setCtlIp(rs.getString(7));                bo.setDoorNo(rs.getString(8));                bo.setDoorName(rs.getString(9));                bo.setStateId(rs.getString(10));                bo.setRptState(rs.getString(11));                bo.setStateStr(rs.getString(12));                bo.setClockPK(pkbo);                clock.add(bo);                System.out.println(rs.getString(4));                System.out.println(bo.toString());            }        } catch (Exception e) {            e.printStackTrace();        }finally{            try{            if(null!=rs){                rs.close();            }            if(null!=stat){                stat.close();            }            if(null!=conn){                conn.commit();            conn.close();            }            }catch(SQLException e){                e.printStackTrace();            }        }

为什么会出现这种情况呢？