code: protected void encodeSortVar(final FacesContext context, final Sheet sheet, final WidgetBuilder wb)
                  throws IOException {
          final JavascriptVarBuilder vb = new JavascriptVarBuilder(null, false);
  
          for (final SheetColumn column : sheet.getColumns()) {
              if (!column.isRendered()) {
                  continue;
              }
  
              if (column.getValueExpression("sortBy") == null) {
                  vb.appendArrayValue("false", false);
              }
              else {
                  vb.appendArrayValue("true", false);
              }
          }
          wb.nativeAttr("sortable", vb.closeVar().toString());
      }
docs: Encodes a javascript sort var that informs the col header event of the column's sorting options. The var is an array of boolean indicating whether or not
  the column is sortable.
  
  @param context
  @param sheet
  @param wb
  @throws IOException