Seata AT模式前后镜像是如何生成详解

    protected T executeAutoCommitFalse(Object[] args) throws Exception {
        // 获取前镜像
        TableRecords beforeImage = beforeImage();
        // 执行业务SQL
        T result = statementCallback.execute(statementProxy.getTargetStatement(), args);
        // 获取后镜像
        TableRecords afterImage = afterImage(beforeImage);
        // 存储前后镜像
        prepareUndoLog(beforeImage, afterImage);
        return result;
    }

@Override
    protected TableRecords beforeImage() throws SQLException {
        SQLDeleteRecognizer visitor = (SQLDeleteRecognizer) sqlRecognizer;
        // 根据表名解析出元数据
        TableMeta tmeta = getTableMeta(visitor.getTableName());
        ArrayList<List<Object>> paramAppenderList = new ArrayList<>();
        // 生成查询beforeImage的SQL语句
        // SELECT [列名,] FROM [表名] (别名) (WHERE) (ORDER BY) (LIMIT) FOR UPDATE
        String selectSQL = buildBeforeImageSQL(visitor, tmeta, paramAppenderList);
        // 执行SQL查询beforeImage
        return buildTableRecords(tmeta, selectSQL, paramAppenderList);
    }

    @Override
    protected TableRecords beforeImage() throws SQLException {
        return TableRecords.empty(getTableMeta());
    }

@Override
    protected TableRecords afterImage(TableRecords beforeImage) throws SQLException {
        // 获取元数据
        TableMeta tmeta = getTableMeta();
        // 没有前镜像，也不存在后镜像，说明没有数据被修改
        if (beforeImage == null || beforeImage.size() == 0) {
            return TableRecords.empty(getTableMeta());
        }
        // 生成查询后镜像SQL
        // SELECT [列名,] FROM [表名] (别名) WHERE 主键 in (前镜像的主键值)
        // 这里面有一个配置项[client.undo.onlyCareUpdateColumns]，是否只关心被修改的列名，默认是true
        String selectSQL = buildAfterImageSQL(tmeta, beforeImage);
        ResultSet rs = null;
        try (PreparedStatement pst = statementProxy.getConnection().prepareStatement(selectSQL)) {
            SqlGenerateUtils.setParamForPk(beforeImage.pkRows(), getTableMeta().getPrimaryKeyOnlyName(), pst);
            // 执行查询后镜像
            rs = pst.executeQuery();
            // 包装查询结果
            return TableRecords.buildRecords(tmeta, rs);
        } finally {
            IOUtil.close(rs);
        }
    }