code: public static void rollback() throws Exception {
          Integer c = Trans.count.get();
          if (c == null)
              c = Integer.valueOf(0);
          else if (c > 0)
          	c--;
          Trans._rollback(c);
      }
docs: 回滚事务，执行它前，你必需保证你已经手工开始了一个事务
  
  @throws Exception