BasicAttachment bAttachment = attachmentService.getById(attachmentId);
        if (bAttachment == null || StringUtils.isBlank(bAttachment.getUrl())) {
            throw new Exception("未找到下载文件!");
        }
        File downloadFile = new File(bAttachment.getUrl());
        if(!downloadFile.exists()) {
            throw new Exception("未找到下载文件!");
        }
        long fileLength = downloadFile.length();// 记录文件大小
        long pastLength = 0;// 记录已下载文件大小
        int rangeSwitch = 0;// 0：从头开始的全文下载；1：从某字节开始的下载（bytes=27000-）；2：从某字节开始到某字节结束的下载（bytes=27000-39000）
        long contentLength = 0;// 客户端请求的字节总量
        String rangeBytes = "";// 记录客户端传来的形如“bytes=27000-”或者“bytes=27000-39000”的内容
        RandomAccessFile raf = null;// 负责读取数据
        OutputStream os = null;// 写出数据
        OutputStream out = null;// 缓冲
        int bsize = 1024;// 缓冲区大小
        byte b[] = new byte[bsize];// 暂存容器

        String range = request.getHeader("Range");
        int responseStatus = 206;
        if (range != null && range.trim().length() > 0 && !"null".equals(range)) {// 客户端请求的下载的文件块的开始字节
            responseStatus = javax.servlet.http.HttpServletResponse.SC_PARTIAL_CONTENT;
            log.info("request.getHeader('Range')=" + range);
            rangeBytes = range.replaceAll("bytes=", "");
            if (rangeBytes.endsWith("-")) {
                rangeSwitch = 1;
                rangeBytes = rangeBytes.substring(0, rangeBytes.indexOf('-'));
                pastLength = Long.parseLong(rangeBytes.trim());
                contentLength = fileLength - pastLength;
            } else {
                rangeSwitch = 2;
                String temp0 = rangeBytes.substring(0, rangeBytes.indexOf('-'));
                String temp2 = rangeBytes.substring(rangeBytes.indexOf('-') + 1);
                pastLength = Long.parseLong(temp0.trim());
                contentLength = Long.parseLong(temp2);
            }
        } else {
            contentLength = fileLength;// 客户端要求全文下载
        }


        // 清除首部的空白行
        response.reset();
        // 告诉客户端允许断点续传多线程连接下载,响应的格式是:Accept-Ranges: bytes
        response.setHeader("Accept-Ranges", "bytes");
        // 如果是第一次下,还没有断点续传,状态是默认的 200,无需显式设置;响应的格式是:HTTP/1.1

        if (rangeSwitch != 0) {
            response.setStatus(responseStatus);
            // 不是从最开始下载，断点下载响应号为206
            // 响应的格式是:
            // Content-Range: bytes [文件块的开始字节]-[文件的总大小 - 1]/[文件的总大小]
            switch (rangeSwitch) {
                case 1: {
                    String contentRange = new StringBuffer("bytes ")
                            .append(new Long(pastLength).toString()).append("-")
                            .append(new Long(fileLength - 1).toString())
                            .append("/").append(new Long(fileLength).toString())
                            .toString();
                    response.setHeader("Content-Range", contentRange);
                    break;
                }
                case 2: {
//                    String contentRange = range.replace("=", " ") + "/"
//                            + new Long(fileLength).toString();
//                    response.setHeader("Content-Range", contentRange);
                    String contentRange = new StringBuffer("bytes ")
                            .append(new Long(pastLength).toString()).append("-")
                            .append(new Long(fileLength - 1).toString())
                            .append("/").append(new Long(fileLength).toString())
                            .toString();
                    response.setHeader("Content-Range", contentRange);
                    break;
                }
                default: {
                    break;
                }
            }
        } else {
            String contentRange = new StringBuffer("bytes ").append("0-")
                    .append(fileLength - 1).append("/").append(fileLength)
                    .toString();
            response.setHeader("Content-Range", contentRange);
        }

        try {
            response.setContentType("video/mp4");
            response.setHeader("Content-Length", String.valueOf(contentLength));
            os = response.getOutputStream();
            out = new BufferedOutputStream(os);
            raf = new RandomAccessFile(downloadFile, "r");
            try {
                long outLength = 0;// 实际输出字节数
                switch (rangeSwitch) {
                    case 0: {
                    }
                    case 1: {
                        raf.seek(pastLength);
                        int n = 0;
                        while ((n = raf.read(b)) != -1) {
                            out.write(b, 0, n);
                            outLength += n;
                        }
                        break;
                    }
                    case 2: {
                        raf.seek(pastLength);
                        int n = 0;
                        long readLength = 0;// 记录已读字节数
                        while (readLength <= contentLength - bsize) {// 大部分字节在这里读取
                            n = raf.read(b);
                            readLength += n;
                            out.write(b, 0, n);
                            outLength += n;
                        }
                        if (readLength <= contentLength) {// 余下的不足 1024 个字节在这里读取
                            n = raf.read(b, 0, (int) (contentLength - readLength));
                            out.write(b, 0, n);
                            outLength += n;
                        }
                        break;
                    }
                    default: {
                        break;
                    }
                }
                log.info("Content-Length为：" + contentLength + "；实际输出字节数：" + outLength);
                out.flush();
            } catch (IOException ie) {
                // ignore
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (out != null) {
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (raf != null) {
                try {
                    raf.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }