docs: public Db tx(TransactionLevel transactionLevel, VoidFunc1<Db> func) throws SQLException {
  		final Connection conn = getConnection();
  
  		// 检查是否支持事务
  		checkTransactionSupported(conn);
  
  		// 设置事务级别
  		if (null != transactionLevel) {
  			final int level = transactionLevel.getLevel();
  			if (conn.getTransactionIsolation() < level) {
  				// 用户定义的事务级别如果比默认级别更严格，则按照严格的级别进行
  				conn.setTransactionIsolation(level);
  			}
  		}
  
  		// 开始事务
  		boolean autoCommit = conn.getAutoCommit();
  		if (autoCommit) {
  			conn.setAutoCommit(false);
  		}
  
  		// 执行事务
  		try {
  			func.call(this);
  			// 提交
  			conn.commit();
  		} catch (Throwable e) {
  			quietRollback(conn);
  			throw (e instanceof SQLException) ? (SQLException) e : new SQLException(e);
  		} finally {
  			// 还原事务状态
  			quietSetAutoCommit(conn, autoCommit);
  			// 关闭连接或将连接归还连接池
  			closeConnection(conn);
  		}
  
  		return this;
  	}