HttpURLConnection 文件下载转发细节

细节:outputStream对象必须在inputStream对象之前创建,否则会引起bug(即用户第一次点击发起下载请求会一直等待, 必须再次点击才能下载)
使用的是springMVC,下载文件的action如下

@RequestMapping("/report_generalPdf")
    public void downloadFile(@RequestParam("filePath") String filePath, HttpServletRequest request, HttpServletResponse response) {
        if (filePath != null) {
            response.addHeader("Content-Disposition",
                    "attachment;fileName=" + "result.pdf");// 设置文件名
            byte[] buffer = new byte[1024];
            FileInputStream fis = null;
            BufferedInputStream bis = null;
            try {
                // 测试用的内网资源
                URL url = new URL("http://192.168.xxx.xx/uploadFiles/report/xxx/xxx/xxx_report.pdf");
                // 连接类的父类，抽象类
                URLConnection urlConnection = url.openConnection();
                // http的连接类
                HttpURLConnection httpURLConnection = (HttpURLConnection) urlConnection;
                // 设定请求的方法，默认是GET
                httpURLConnection.setRequestMethod("GET");
                // 设置字符编码
                httpURLConnection.setRequestProperty("Charset", "UTF-8");
                // 打开到此 URL 引用的资源的通信链接（如果尚未建立这样的连接）。
                httpURLConnection.connect();
                httpURLConnection.setConnectTimeout(100);
                httpURLConnection.setReadTimeout(100);
                // 文件大小
//              int fileLength = httpURLConnection.getContentLength();

                // 文件名
//              String filePathUrl = httpURLConnection.getURL().getFile();
//              String fileFullName = filePathUrl.substring(filePathUrl.lastIndexOf(File.separatorChar) + 1);

//              System.out.println("file length---->" + fileLength);
                OutputStream os = response.getOutputStream();
                int i = bis.read(buffer);
                bis = new BufferedInputStream(httpURLConnection.getInputStream());
                while (i != -1) {
                    os.write(buffer, 0, i);
                    i = bis.read(buffer);
                }


            } catch (Exception e) {
                // TODO: handle exception
                e.printStackTrace();
            } finally {
                if (bis != null) {
                    try {
                        bis.close();
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                if (fis != null) {
                    try {
                        fis.close();
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }

        }
    }