default Map<String, FilterMeta> initFilterBy(FacesContext context) {
        Map<String, FilterMeta> filterBy = new LinkedHashMap<>();

        // build columns filterBy
        forEachColumn(c -> {
            FilterMeta meta = FilterMeta.of(context, getVar(), c, isFilterNormalize());
            if (meta != null) {
                filterBy.put(meta.getColumnKey(), meta);
            }

            return true;
        });

        // merge internal filterBy with user filterBy
        Object userFilterBy = getFilterBy();
        if (userFilterBy != null) {
            updateFilterByWithUserFilterBy(context, filterBy, userFilterBy);
        }

        // build global filterBy
        updateFilterByWithGlobalFilter(context, filterBy);

        setFilterByAsMap(filterBy);

        return filterBy;
    }
ID of the global filter component
primefaces/src/main/java/org/primefaces/component/api/UITable.java