/**
     * 生成excel並下載
     */
    public void exportExcel(){
        
        File newFile = createNewFile();
        //File newFile = new File("d:/ss.xls");
        
        //新檔案寫入資料，並下載*****************************************************
        InputStream is = null;
        HSSFWorkbook workbook = null;
        HSSFSheet sheet = null;
        try {
            is = new FileInputStream(newFile);
            workbook = new HSSFWorkbook(is);
            //獲取第一個sheet
            sheet = workbook.getSheetAt(0);
        } catch (Exception e1) {
            e1.printStackTrace();
        }
        
        if(sheet != null){
            try {
                //寫資料
                FileOutputStream fos = new FileOutputStream(newFile);
                HSSFRow row = sheet.getRow(4);
                HSSFCell cell = row.getCell(1);
                System.out.println(cell.getStringCellValue());
                cell.setCellValue("ssssssssssssssssssssssssssssssssssssssssssss");
                workbook.write(fos);
                fos.flush();
                fos.close();
                
                //下載
               InputStream fis = new BufferedInputStream(new FileInputStream(newFile));
               HttpServletResponse response = this.getResponse();
               byte[] buffer = new byte[fis.available()];
               fis.read(buffer);
               fis.close();
               response.reset();
               response.setContentType("text/html;charset=UTF-8");
               OutputStream toClient = new BufferedOutputStream(response.getOutputStream());
                  response.setContentType("application/x-msdownload");
                  String newName = URLEncoder.encode("採購合同"+System.currentTimeMillis()+".xls", "UTF-8");
                  response.addHeader("Content-Disposition", "attachment;filename=\""+ newName + "\"");
                  response.addHeader("Content-Length", "" + newFile.length());
               toClient.write(buffer);
               toClient.flush();
            }
            catch(Exception e) {
                e.printStackTrace();
            }finally {
                try {
                    if (null != is) {
                        is.close();
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
        //刪除建立的新檔案
        //this.deleteFile(newFile); 
    }
    /**
    * 複製檔案
    * 
    * @param s
    * 原始檔
    * @param t
    * 複製到的新檔案
    */

    public void fileChannelCopy(File s, File t) {
        try {
            InputStream in = null;
            OutputStream out = null;
            try {
                in = new BufferedInputStream(new FileInputStream(s),1024);
                out = new BufferedOutputStream(new FileOutputStream(t),1024);
                byte[] buffer = new byte[1024];
                int len; 
                while ((len=in.read(buffer))!=-1) {
                    out.write(buffer,0,len);
                }
            } finally {
                if (null != in) {
                    in.close();
                }
                if (null != out) {
                    out.close();
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    
    /**
     * 讀取excel模板，並複製到新檔案中供寫入和下載
     * @return
     */
    public File createNewFile(){
        //讀取模板，並賦值到新檔案************************************************************
        //檔案模板路徑
        String path = this.getRequest().getRealPath(SystemConfig.FILETEMPLATE);
        String fileName="purchaseContract.xls";
        File file=new File(path+"/"+fileName);

        //儲存檔案的路徑
        String realPath = ServletActionContext.getServletContext().getRealPath(SystemConfig.UPLOAD_FILE_DIR);
        //新的檔名
        String newFileName = "採購合同"+System.currentTimeMillis() + ".xls";
        //判斷路徑是否存在
        File dir = new File(realPath);
        if(!dir.exists()){
            dir.mkdirs();
        }
        //寫入到新的excel 
        File newFile = new File(realPath, newFileName);
        try {
            newFile.createNewFile();
            //複製模板到新檔案
            fileChannelCopy(file, newFile);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return newFile;
    }

    /**
     * 下載成功後刪除
     * 
     * @param files
     */
    private void deleteFile(File... files) {
        for (File file : files) {
            if (file.exists()) {
                file.delete();
            }
        }
    }

出處：https://www.cnblogs.com/kisstear/p/5461494.html