code: public NutTxDao begin(int transLevel) throws DaoException {
          if (this.conn != null)
              throw new DaoException("NutTxDao has been begined!!");
          id = R.UU32();
          if (debug)
              log.debugf("begin level=%d id=%s", transLevel, id);
          try {
              this.conn = dataSource.getConnection();
              this.conn.setTransactionIsolation(transLevel);
              if (this.conn.getAutoCommit() == true) {
                  this.conn.setAutoCommit(false);
                  _autoCommit = true;
              }
              setSavepoint(id);
          }
          catch (SQLException e) {
              throw new DaoException(e);
          }
          return this;
      }
docs: 开启事务
  
  @param transLevel
  事务级别
  @see java.sql.Connection
  @return 原对象
  @throws DaoException
  如果已经开启过事务