Android 断点续传原理以及实现

public class FileInfo implements Serializable{ 
  private String url; //URL 
  private int length; //长度或结束位置 
  private int start; //开始位置 
  private int now;//当前进度 
//构造方法，set/get略 
} 
//开始按钮逻辑，停止逻辑大致相同 
strat.setOnClickListener(new View.OnClickListener() { 
   @Override 
   public void onClick(View view) { 
    Intent intent = new Intent(MainActivity.this,DownLoadService.class); 
    intent.setAction(DownLoadService.ACTION_START); 
    intent.putExtra("fileUrl",info); 
    startService(intent); 
  } 
}); 

public void run() { 
      HttpURLConnection urlConnection = null; 
      RandomAccessFile randomFile = null; 
      try { 
        URL url = new URL(fileInfo.getUrl()); 
        urlConnection = (HttpURLConnection) url.openConnection(); 
        urlConnection.setConnectTimeout(3000); 
        urlConnection.setRequestMethod("GET"); 
        int length = -1; 
        if (urlConnection.getResponseCode() == HttpStatus.SC_OK) { 
          //获得文件长度 
          length = urlConnection.getContentLength(); 
        } 
        if (length <= 0) { 
          return; 
        } 
        //创建相同大小的本地文件 
        File dir = new File(DOWNLOAD_PATH); 
        if (!dir.exists()) { 
          dir.mkdir(); 
        } 
        File file = new File(dir, FILE_NAME); 
        randomFile = new RandomAccessFile(file, "rwd"); 
        randomFile.setLength(length); 
        //长度给fileInfo对象 
        fileInfo.setLength(length); 
        //通过Handler将对象传递给Service 
        mHandle.obtainMessage(0, fileInfo).sendToTarget(); 
      } catch (Exception e) { 
        e.printStackTrace(); 
      } finally { //流的回收逻辑略 
      } 
    } 
  } 

downLoadUtil = new DownLoadUtil(DownLoadService.this,info);

downLoadUtil.download();

public interface ThreadDAO { 
  //插入一条数据 
  public void insert(FileInfo info); 
  //根据URL删除一条数据 
  public void delete(String url); 
  //根据URL更新一条进度 
  public void update(String url,int finished); 
  //根据URL找到一条数据 
  public List<FileInfo> get(String url); 
  //是否存在 
  public boolean isExits(String url); 
} 

public class DownLoadUtil { 
  //构造方法略 
  public void download(){ 
    List<FileInfo> lists = threadDAO.get(fileInfo.getUrl()); 
    FileInfo info = null; 
    if(lists.size() == 0){ 
      //第一次下载，创建子线程下载 
      new MyThread(fileInfo).start(); 
    }else{ 
      //中间开始的 
      info = lists.get(0); 
      new MyThread(info).start(); 
    } 
  } 
 
  class MyThread extends Thread{ 
    private FileInfo info = null; 
    public MyThread(FileInfo threadInfo) { 
      this.info = threadInfo; 
    } 
    @Override 
    public void run() { 
      //向数据库添加线程信息 
      if(!threadDAO.isExits(info.getUrl())){ 
        threadDAO.insert(info); 
      } 
      HttpURLConnection urlConnection = null; 
      RandomAccessFile randomFile =null; 
      InputStream inputStream = null; 
      try { 
        URL url = new URL(info.getUrl()); 
        urlConnection = (HttpURLConnection) url.openConnection(); 
        urlConnection.setConnectTimeout(3000); 
        urlConnection.setRequestMethod("GET"); 
        //设置下载位置 
        int start = info.getStart() + info.getNow(); 
        urlConnection.setRequestProperty("Range","bytes=" + start + "-" + info.getLength()); 
 
        //设置文件写入位置 
        File file = new File(DOWNLOAD_PATH,FILE_NAME); 
        randomFile = new RandomAccessFile(file, "rwd"); 
        randomFile.seek(start); 
 
        //向Activity发广播 
        Intent intent = new Intent(ACTION_UPDATE); 
        finished += info.getNow(); 
 
        if (urlConnection.getResponseCode() == HttpStatus.SC_PARTIAL_CONTENT) { 
          //获得文件流 
          inputStream = urlConnection.getInputStream(); 
          byte[] buffer = new byte[512]; 
          int len = -1; 
          long time = System.currentTimeMillis(); 
          while ((len = inputStream.read(buffer))!= -1){ 
            //写入文件 
            randomFile.write(buffer,0,len); 
            //把进度发送给Activity 
            finished += len; 
            //看时间间隔，时间间隔大于500ms再发 
            if(System.currentTimeMillis() - time >500){ 
              time = System.currentTimeMillis(); 
              intent.putExtra("now",finished *100 /fileInfo.getLength()); 
              context.sendBroadcast(intent); 
            } 
            //判断是否是暂停状态 
            if(isPause){ 
              threadDAO.update(info.getUrl(),finished); 
              return; //结束循环 
            } 
          } 
          //删除线程信息 
          threadDAO.delete(info.getUrl()); 
        } 
      }catch (Exception e){ 
        e.printStackTrace(); 
      }finally {//回收工作略 
      } 
    } 
  } 
}