<h:commandLink actionListener="#{prodUCtBean.downloadAction}" styleClass="highLightLink">
   <h:outputText value="download"/>
   <f:param name="productId" value="#{productBean.id}"/>
</h:commandLink>

   public void downloadAction(ActionEvent event) {
    try {
      String fileName="D://temp//images//products//" + this.id + ".xls";
      logger.debug("file name=" + fileName);
      ByteArrayOutputStream baos=this.serviceLocator.getFileService().downloadFile(fileName); //調用Service方法，獲得文件的ByteArrayOutputStream
      HttpServletResponse response=FacesUtils.getServletResponse();
      response.setHeader("Content-disposition", "attachment; filename=" + id+ ".xls" ); //不是內嵌顯示(inline)，而是作為附件下載
      response.setContentLength(baos.size());
      ServletOutputStream sos=response.getOutputStream();
      baos.writeTo(sos);
      baos.close();
      sos.flush();
    } catch (IOException ex) {      
      logger.debug(ex);
    }   
  }

 public ByteArrayOutputStream downloadFile(String fileName) throws IOException {
    FileInputStream fis=new FileInputStream(fileName);
    BufferedInputStream bis=new BufferedInputStream(fis);
    ByteArrayOutputStream baos=new ByteArrayOutputStream();
    BufferedOutputStream bos=new BufferedOutputStream(baos);
    
    int i;
    while((i=bis.read())!=-1) {
      bos.write(i);
    }
    bos.flush();//提交文件流，很關鍵
    bis.close();
    return baos;
  }