code: public void close() {
          if (debug)
              log.debugf("close id=%s", id);
          try {
              if (conn == null)
                  return;
              if (_autoCommit)
                  conn.setAutoCommit(true);
              conn.close();
              conn = null;
          }
          catch (Throwable e) {
          }
      }
docs: 关闭事务及连接