p:commandButton does not open window with update - ajax

Question

I generate an exportList in my Bean:
public void exportExcel() throws WriteException {
try {
FacesContext context = FacesContext.getCurrentInstance();
HttpServletResponse response = (HttpServletResponse) context.getExternalContext().getResponse();
response.setContentType("application/vnd.ms-excel");
response.setHeader("Content-Disposition", "attachment; filename=\"hours.xls\";");
OutputStream out = response.getOutputStream();
WorkbookSettings ws = new WorkbookSettings();
ws.setLocale(new Locale("de", "DE"));
WritableWorkbook workbook = Workbook.createWorkbook(out, ws);
WritableSheet sheet = workbook.createSheet("Sheet1", 0);
sheet.addCell(new Label(0, 0, "ID", bold));
int row = 1;
for (Hour hour : this.listHours) {
sheet.addCell(new Label(0, row, String.valueOf(hour.getId())));
row++;
}
SheetFormatter.setOptimalColumnWidth(sheet);
workbook.write();
workbook.close();
response.flushBuffer();
context.responseComplete();
context.addMessage(null, new FacesMessage("Liste Exportiert"));
}
catch (Exception e) {
}
}
In my page I call method in p:commandButton
<p:commandButton value="#{msg.export}" update="growl"
immediate="true" action="#{hoursView.exportExcel()}" />
My page will not open the excel-List... If if add the attribute ajax="false" it works but then update will not execute...
For information my Bean is SessionScoped if this makes some differences

Answer 1

Your first mistake is that you're trying to download a file using ajax. That just isn't possible. Ajax is executed by JavaScript code which has due to security reasons no facilities to force a "Save as" dialogue and/or write the retrieved response to the local disk file system. That would otherwise open doors to various nasty security breach possibilities.
So, using ajax="false" is absolutely necessary.
Your second mistake is that you're trying to mix different responses into a single response. That just isn't possible. You can only return either a file download, or an ajax update, not both. To retrieve two different responses, you basically need to let the client send two different requests. You could approach this as follows:
Let client send an ajax request to backing bean.
Let server create and save Excel file in server's temporary storage system and generate an unique URL so that it could be accessed by a servlet.
Let server send an ajax response containing the message and the URL.
Let client display the message and invoke a new GET request on the URL. You could use window.location=url; to let JavaScript invoke a new GET request on the URL.