public String getColumnExportTag(FacesContext context, UIColumn column) {
        // lowerCase really? camelCase at best
        String columnTag = column.getExportTag();
        if (LangUtils.isBlank(columnTag)) {
            columnTag = getColumnFacetValue(context, column, TableExporter.ColumnType.HEADER).toString();
        }
        return EscapeUtils.forXmlTag(columnTag.toLowerCase());
    }
Gets and caches the list of UIColumns that are exportable="true", visible="true", and rendered="true".
Orders them by displayPriority so they match the UI display of the columns.

@param table the Table with columns to export
@return the List<UIColumn> that are exportable
primefaces/src/main/java/org/primefaces/component/export/TableExporter.java