POI未写入excel[重复]

我使用POI来编写excel报告。这是我的代码:

        HSSFWorkbook wb = new HSSFWorkbook();
        HSSFSheet sheet = wb.createSheet("Export To Excel");     

        response.setContentType("application/vnd.ms-excel");
        response.setHeader("Content-Disposition", "attachment; filename=avt.xls");

        HSSFRow row = sheet.createRow(0);
        HSSFCell cell = row.createCell(0);
        cell.setCellValue("AAAAAAAAAAAAAA");
        cell = row.createCell(1);
        cell.setCellValue("BBBBB");
//        cell.setCellStyle(cellStyle);




        FileOutputStream out =  new FileOutputStream("avt.xls");    
        wb.write(out);     
        out.close();     System.out.println("Excel written successfully..");  

当我点击按钮时,excel弹出窗口会打开并显示警告消息,但没有任何内容写入excel。我在编译器中也没有看到任何错误。怎么了>?