public boolean checkUsername(String username) throws Exception{
             boolean flag = false;
            String sql = "select * from user where username=?" ;
            QueryRunner runner = new QueryRunner(WebUtil.getDataSource());
            User user = runner.query(sql,new BeanHandler(User.class ),username);
             //在数据库中没有改用户名
             if(user == null){
                  flag = true;
            }
             return flag;
      }