struts报Cannot forward after response has been committed問題

public   void  downLoadFile(ActionEvent event)  throws  DatabaseException  ... {
        try ...{
            FacesContext faces = FacesContext.getCurrentInstance();
            ExternalContext extContext = faces.getExternalContext();
            String filePath = extContext.getApplicationMap().get("filePath")
                    .toString();
            UIParameter component = (UIParameter) event.getComponent()
            .findComponent("deleteId");
            int id = Integer.parseInt(component.getValue().toString());
            Seg_Files b = DAOFactory.getSeg_FilesDAO().getSeg_FilesById(id);
            String fileName = filePath+ b.getFileName();
            //System.out.println(fileName);
            ByteArrayOutputStream baos=FileUtils.downloadFile(fileName);
            
            //獲得文件的ByteArrayOutputStream結束
            
            HttpServletResponse response = (HttpServletResponse) faces.getExternalContext().getResponse();    
            //response.setHeader("Content-disposition", "attachment; filename="+ b.getFileName());
            //response.setContentType("text/txt; charset=utf-8");
            response.setHeader("Content-disposition", "attachment; filename="+ b.getFileName());
            response.setContentType("application/txt;charset=utf-8");
            response.setContentLength(baos.size());
            ServletOutputStream sos = response.getOutputStream();
            baos.writeTo(sos);
            baos.close();
            sos.flush();
            //需要呼叫Complete，要不就報：Cannot forward after response has been committed
            faces.responseComplete();
            
        } catch (IOException ex) ...{
            System.out.println(ex);
        }
    }  
public   static  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();//must have
        bis.close();
        
        return baos;
    }