upload(file) {
 //從后臺(tái)獲取已經(jīng)上傳的文件分片數(shù)
 getIdx(md5)
  .then(function(res) {
   let retry = 3;
   uploadPart(retry, file, res.data);
  })
  .catch(); 
}

uploadPart(retry, file, idx) {
 //設(shè)置分片大?。▎挝籅yte）
 let bufferLength = 1024 * 1024 * 5;
 //計(jì)算開(kāi)始的切割點(diǎn),idx是上傳成功的分片數(shù)，未上傳過(guò)文件則開(kāi)始點(diǎn)為0
 let start = idx * bufferLength;
 //全部上傳完畢或重試次數(shù)用完則退出
 if(start>=file.size || retry<=0) return;
 //計(jì)算分割的位置
 let end = start + bufferLength;
 //如果分割點(diǎn)超出文件大小，回退分割點(diǎn)
 if (end > file.size) {end = fileSize;}
 //切割文件
 var chunk = file.slice(start, end);
 //創(chuàng)建 formData 對(duì)象并添加數(shù)據(jù)
 let formData = new FormData();
 formData.set("file", chunk);
 //如果是第一次上傳，連同文件塊數(shù)量也上傳
 if (start == 0) { 
  //計(jì)算文件切片總數(shù),向上取整
  let chunkNum = Math.ceil(file.size / bufferLength);
  formData.set("total", chunkNum);
 }
 //上傳文件的api，此處使用axios發(fā)送請(qǐng)求
 doUpload(formData)
  //發(fā)送成功，則上傳下一片，遞歸調(diào)用方法
  .then(function() {
   retry = xx;//刷新重試次數(shù)
   uploadPart(retry, file, ++idx);
  })
  //發(fā)送失敗
  .catch(function() {
   retry--;//重試次數(shù)減一
   //重試上傳這一片
   uploadPart(retry, file, idx);
  });
},

/**合并文件的實(shí)際操作*/
public static void doMergeFiles(String outFile, String[] files) {
  //設(shè)置緩存大小
  int BUFSIZE = 1024 * 1024;
  //排序。文件后綴名是文件的順序。
  Arrays.sort(files);
  //輸出流
  FileChannel outChannel = null;
  //標(biāo)記最后的一個(gè)文件
  String lastFlag = files[files.length-1];
  try {
    outChannel = new FileOutputStream(outFile).getChannel();
    //遍歷文件列表
    for(String f : files){
      //最后一塊文件用真實(shí)大小設(shè)置緩存，避免自動(dòng)填充數(shù)據(jù)造成的md5不一致
      if(lastFlag.equals(f)){
        File last = new File(f);
        BUFSIZE = (int) last.length();//獲取文件的大小并設(shè)置成緩存的大小
      }
      FileChannel fc = new FileInputStream(f).getChannel();
      //用ByteBuffer創(chuàng)建緩存
      ByteBuffer bb = ByteBuffer.allocate(BUFSIZE);
      while(fc.read(bb) != -1){//把數(shù)據(jù)讀到緩存
        bb.flip();//重置游標(biāo)
        outChannel.write(bb);//寫(xiě)入數(shù)據(jù)
        bb.clear();//清空數(shù)據(jù)
      }
      fc.close();//關(guān)閉流
    }
  } catch (IOException ioe) {
    ioe.printStackTrace();
  } finally {
    try {if (outChannel != null) {outChannel.close();}} catch (IOException ignore) {}
  }
}