如何使用ByteArrayOutputStream下载文件

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//文件名称
String filepath = ServletActionContext.getServletContext()
        .getRealPath(farmerQrCode.getQrCodeUrl());
        File file = new File(filepath);
        String fileName = new Date().getTime()+".png";
//设置请求信息
        HttpServletResponse response = ServletActionContext.getResponse();
        response.setContentType(response.getContentType());
        response.setHeader("Content-disposition",
                "attachment; filename="+fileName);
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        int len = 0;
        FileInputStream inputStream = new FileInputStream(file);
        byte [] buffer  = new byte[3];
        while((len = inputStream.read(buffer)) != -1)
        {
            baos.write(buffer, 0,  len);
        }
        byte[] bytes = baos.toByteArray();
        response.setHeader("Content-Length", String.valueOf(bytes.length));
        BufferedOutputStream bos = null;
        bos = new BufferedOutputStream(response.getOutputStream());
        bos.write(bytes);
        bos.close();
        baos.close();

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//此处将HSSFWorkbook wb处理好，然后最后要导出文件时加上此代码。
ByteArrayOutputStream baos = new ByteArrayOutputStream();
        response.setContentType(response.getContentType());
        response.setHeader("Content-disposition",
                "attachment; filename=monthPayment.xls");
        wb.write(baos);
        byte[] bytes = baos.toByteArray();
        response.setHeader("Content-Length", String.valueOf(bytes.length));
        BufferedOutputStream bos = null;
        bos = new BufferedOutputStream(response.getOutputStream());
        bos.write(bytes);
        bos.close();
        baos.close();

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byte[] buf = new byte[(int) fileSize];
        InputStream in = ossObject.getObjectContent();
        try {
            for (int n = 0; n != -1; ) {
                n = in.read(buf, 0, buf.length);
            }
        } catch (IOException e) {
            log.error(e.getMessage());
        } finally {
            try {
                in.close();
            } catch (IOException e) {
                log.error(e.getMessage());
            }
        }

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private static byte[] inputToByte(InputStream inStream, int fileSize) throws IOException {
        ByteArrayOutputStream swapStream = new ByteArrayOutputStream();
        byte[] buff = new byte[fileSize];
        int rc;
        while ((rc = inStream.read(buff, 0, fileSize)) > 0) {
            swapStream.write(buff, 0, rc);
        }
        return swapStream.toByteArray();
    }