Android多線程斷點下載簡單實現

HTTP多線程斷點下載

原理：獲取目標文件的大小,在本地建立一個相同大小的文件,並計算每一個線程須要下載的起始位置及大小,而後分配至每一個線程獨立下載,所有下載完畢則自動合併.

實現步驟

1. 查看並計算目標文件的大小

   URL url = new URL(mPath);
   HttpURLConnection connection = (HttpURLConnection) url.openConnection();
   connection.setRequestMethod("GET");
   int code = connection.getResponseCode();
   if (code == 200) {
       int length = connection.getContentLength();
       System.out.println("文件總長度爲：" + length);

2. 設置目標文件在本地的映射

   RandomAccessFile raf = new RandomAccessFile(
           Environment.getExternalStorageDirectory().getAbsolutePath()
                   +"/"+getDownlaodName(mPath), "rw");
   raf.setLength(length);

3. 開啓子線程並分配下載任務

   int blokeSize = length / mTotalCount;
   System.out.println("每一塊大小爲：" + blokeSize);
   runningThreadCount = mTotalCount;
   for (int threadid = 0; threadid < mTotalCount; threadid++) {
       int startPosition = threadid * blokeSize;
       int endPosition = (threadid + 1) * blokeSize - 1;
       //最後一個線程應該下載至末尾
       if (threadid == mTotalCount - 1) {
           endPosition = length - 1;
       }
       System.out.println("線程編號：" + threadid + ""
               + ",下載範圍：" + startPosition + "~~" + endPosition);
       //開啓下載子線程
       new DownloadThread(threadid, startPosition, endPosition).start();

4. 子線程中的具體邏輯

   public void run() {
       System.out.println("線程"+threadid+"開始運行了");
       try{
           //讀存有歷史下載進度的文件,判斷是否已經下載過
           File finfo=new File(Environment.getExternalStorageDirectory().getAbsolutePath()
                   +"/"+mTotalCount + getDownlaodName(mPath)+threadid+".txt");
           if (finfo.exists()&&finfo.length()>0) {
               //獲取文件輸入流
               FileInputStream fis=new FileInputStream(finfo);
               //讀取緩衝
               BufferedReader br=new BufferedReader(new InputStreamReader(fis));
               //獲得文件內容
               String lasrposition=br.readLine();
               //轉化爲int值
               int intlastposition=Integer.parseInt(lasrposition);
               lastLoadSize=intlastposition-startPosition;
               startPosition=intlastposition;
               fis.close();
           }
           URL url=new URL(mPath);
           HttpURLConnection conn=(HttpURLConnection) url.openConnection();
           conn.setRequestMethod("GET");
           System.out.println("線程實際下載："+threadid+"，範圍："+startPosition
                   +"~~"+endPosition);
           //設置http請求頭部參數
           conn.setRequestProperty("Range", "bytes="+startPosition+"-"+endPosition);
           int code=conn.getResponseCode();
           //206表明請求部分數據成功
           if (code==206) {
               //拿到連接返回的輸入流
               InputStream is=conn.getInputStream();
               //指向要寫的文件（abc.exe）
               RandomAccessFile raf=new RandomAccessFile(
                       Environment.getExternalStorageDirectory().getAbsolutePath()
                               +"/"+getDownlaodName(mPath), "rw");
               //指定文件開始寫的位置
               raf.seek(startPosition);
               //下載緩衝區，越大下載越快，對硬盤損耗越小，可是越容易丟失數據
               byte[] buffer=new byte[1024*1024];
               int len=-1;
               int total=0;//當前線程本次下載數據總量
               while ((len=is.read(buffer))!=-1) {
                   raf.write(buffer,0,len);
                   total+=len;
                   //將每次更新的數據同步到底層硬盤
                   RandomAccessFile inforaf=new RandomAccessFile(
                           Environment.getExternalStorageDirectory().getAbsolutePath()
                           +"/"+mTotalCount +getDownlaodName(mPath)+threadid+".txt","rwd");
                   //保存當前線程下載到什麼位置
                   inforaf.write(String.valueOf(startPosition+total).getBytes());
                   inforaf.close();
               }
               is.close();
               raf.close();
               System.out.println("線程"+threadid+"下載完畢");
   
           }
       }catch(Exception e){
           e.printStackTrace();
       }finally{
           //同步代碼塊，保證同時間僅有一個線程執行此區塊代碼
           synchronized (MainActivity.class) {
               runningThreadCount--;
               if (runningThreadCount<=0) {
                   System.out.println("多線程下載完畢");
                   for (int i = 0; i <mTotalCount ; i++) {
                       File f=new File(Environment.getExternalStorageDirectory().getAbsolutePath()
                                       +"/"+mTotalCount +getDownlaodName(mPath)+i+".txt");
                       System.out.println(f.delete());
                       endTime=System.currentTimeMillis();
                   }
                   System.out.println("結束時間："+endTime);
                   System.out.println("總共花費："+(endTime-startTime)/1000+"秒");
                   runOnUiThread(new Runnable() {
                       @Override
                       public void run() {
                           Toast.makeText(getApplicationContext(), "總共花費："
                                   +(endTime-startTime)/1000+"秒",Toast.LENGTH_SHORT).show();
                       }
                   });
               }
           }
       }
   }

備註：若是不使用斷點下載，只須要將判斷和存儲歷史下載信息的邏輯刪除便可。