利用断点续传实现下载的原理是什么

OutputStream os=null; InputStream inputStream =null; File zipFile=null; try{  long zipStart=System.currentTimeMillis();  zipFile=createFile();//动态根据业务创建文件  if(logger.isInfoEnabled()){   logger.info(String.format("压缩ZIP 花费时间 %s(s) ",  (System.currentTimeMillis()-zipStart)/1000));  }  if (zipFile.exists()) {   long downloadStart=System.currentTimeMillis();   inputStream= new BufferedInputStream(new FileInputStream(zipFile));   response.reset();   os=new BufferedOutputStream(response.getOutputStream());   String userAgent = request.getHeader("USER-AGENT");   String fileName=zipFile.getName();   if (null != userAgent && -1 != userAgent.indexOf("MSIE")) {    fileName = URLEncoder.encode(fileName, "UTF8");   } else if (null != userAgent && -1 != userAgent.indexOf("Mozilla")) {    fileName = new String(fileName.getBytes("utf-8"), "ISO-8859-1");   }   response.setHeader("Accept-Ranges", "bytes");   response.setHeader("Content-Disposition",   "attachment;filename="+ fileName);   response.setContentType(MediaType.APPLICATION_OCTET_STREAM_VALUE);   long pos = 0, fileSize=zipFile.length(), last=fileSize-1;   response.setHeader("ETag",zipFile.getName().   concat(Objects.toString(fileSize))     .concat("_").concat(Objects.toString(zipFile.lastModified())));   response.setDateHeader("Last-Modified",zipFile.lastModified());   response.setDateHeader("Expires",   System.currentTimeMillis()+1000*60*60*24);   if (null != request.getHeader("Range")) {    response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT);    try {     // 暂时只处理这2种range格式 1、RANGE: bytes=111- 2、Range: bytes=0-499     String numRang = request.getHeader("Range")   .replaceAll("bytes=", "");     String[] strRange = numRang.split("-");     if (strRange.length == 2) {      pos = Long.parseLong(strRange[0].trim());      last = Long.parseLong(strRange[1].trim());     } else {      pos = Long.parseLong(numRang.replaceAll("-", "").trim());     }    } catch (NumberFormatException e) {     logger.error(request.getHeader("Range") + " error");     pos = 0;    }   }   long rangLength = last - pos + 1;   String contentRange = new StringBuffer("bytes ").   append(String.valueOf(pos)).   append("-").append(last).append("/").   append(String.valueOf(fileSize)).toString();   response.setHeader("Content-Range", contentRange);   response.addHeader("Content-Length",Objects.toString(rangLength));   if(pos>0){    inputStream.skip(pos);   }   byte[] buffer = new byte[1024*512];//每次以512KB 0.5MB的流量下载   int length = 0,sendTotal=0;   while (sendTotal < rangLength && length!=-1) {    length = inputStream.read(buffer, 0,  ((rangLength - sendTotal) <= buffer.length &#63;      ((int) (rangLength - sendTotal)) : buffer.length));    sendTotal = sendTotal + length;    os.write(buffer, 0, length);   }   if(os!=null){    os.flush();   }   if(logger.isInfoEnabled()){    logger.info(String.format("下载 花费时间 %s(s) ",  (System.currentTimeMillis()-downloadStart)/1000));   }  } }catch (Exception e){  if(StringUtils.endsWithIgnoreCase(e.getMessage(),"Broken pipe")){   logger.error("用户取消下载");  }  logger.error(e.getMessage(),e); }finally {  if(os!=null){   try{    os.close();   }catch (Exception e){}  }  if(inputStream!=null){   try{    IOUtils.closeQuietly(inputStream);   }catch (Exception e){}  } }}