public class UnitDownloader implements Runnable {
  private int from;
  private int to;
  private File target;
  private String uri;
  private int id;

  public UnitDownloader(int from, int to, File target, String uri, int id) {
    this.from = from;
    this.to = to;
    this.target = target;
    this.uri = uri;
    this.id = id;
  }

  public int getFrom() {
    return from;
  }

  public int getTo() {
    return to;
  }

  @Override
  public void run() {
    //download and save data
    try {
      HttpURLConnection connection = (HttpURLConnection) new URL(uri).openConnection();
      connection.setRequestProperty("Range", "bytes=" + from + "-" + to);
      connection.connect();
      int totalSize = connection.getContentLength();
      InputStream inputStream = connection.getInputStream();
      RandomAccessFile randomAccessFile = new RandomAccessFile(target, "rw");
      randomAccessFile.seek(from);
      byte[] buffer = new byte[1024 * 1024];
      int readCount = inputStream.read(buffer, 0, buffer.length);
      while (readCount > 0) {
        totalSize -= readCount;
        System.out.println("分片：" + this.id + "的剩余：" + totalSize);
        randomAccessFile.write(buffer, 0, readCount);
        readCount = inputStream.read(buffer, 0, buffer.length);
      }
      inputStream.close();
      randomAccessFile.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

public class MultipleThreadDownloadManager implements Runnable {
  private String uri;
  private File target;

  public MultipleThreadDownloadManager(String uri, File target) {
    this.target = target;
    this.uri = uri;
    if (target.exists() == false) {
      try {
        target.createNewFile();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }

  /**
   * 開始下載
   */
  public void start() {
    new Thread(this).start();
  }


  /**
   * 根據(jù)文件總大小計算線程數(shù)量
   *
   * @param totalSize
   * @return
   */
  public int threadCount(int totalSize) {
    if (totalSize < 30 * 2014 * 1024) {
      return 1;
    }
    return 30;
  }


  @Override
  public void run() {
    //獲取文件總大小
    int totalSize = 0;
    try {
      HttpURLConnection connection = (HttpURLConnection) new URL(uri).openConnection();
      connection.connect();
      int contentLength = connection.getContentLength();
      totalSize = contentLength;
    } catch (IOException e) {
      e.printStackTrace();
    }
    //將文件分片并分開下載
    int threadCount = threadCount(totalSize);
    int perThreadSize = totalSize / threadCount;//每一個線程分到的任務(wù)下載量
    int id = 0;
    int from = 0, to = 0;
    while (totalSize > 0) {
      id++;
      //計算分片
      if (totalSize < perThreadSize) {
        from = 0;
        to = totalSize;
      } else {
        from = totalSize;
        to = from + perThreadSize;
      }
      //開始下載
      UnitDownloader downloader = new UnitDownloader(from, to, target, uri, id);
      new Thread(downloader).start();
    }
  }
}