code: private void encodeHeader(final FacesContext context, final ResponseWriter responseWriter, final Sheet sheet)
                  throws IOException {
          // header
          final UIComponent header = sheet.getFacet("header");
          if (header != null) {
              responseWriter.startElement("div", null);
              responseWriter.writeAttribute("class", "ui-datatable-header ui-widget-header ui-corner-top", null);
              header.encodeAll(context);
              responseWriter.endElement("div");
          }
      }
docs: Encode the Sheet header
  
  @param context
  @param responseWriter
  @param sheet
  @throws IOException