public void close(){
    try{
        if(pstm != null && !pstm.isClosed()){
            pstm.close();
        }
        if(conn != null && !conn.isClosed()){
            conn.close();
        }
    }catch(SQLException sqle){
        System.out.println("DBHelper.close：关闭资源出错。");
        sqle.printStackTrace();
    }
}