import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
public class DownLoad {
  // 声明下载路径“红色警戒2”
  public static final String PATH = "http://soft3.xzstatic.com/2015/10/hsjj2ghgzh.rar";
  public static int threadCount = 0;// 声明线程数量
  public static void main(String[] args) {
    try {
      URL url = new URL(PATH);
      // 获取连接
      HttpURLConnection conn = (HttpURLConnection) url.openConnection();
      // 通过获取连接定义文件名
      String[] str = PATH.split("/");
      String fileName = str[5];
      // 获取下载文件大小
      int fileLength = conn.getContentLength();
      System.out.println(fileName);
      // 在本地创建一个与服务器大小一致的可随机写入文件
      RandomAccessFile raf = new RandomAccessFile(fileName, "rwd");
      System.out.println(fileLength);// 测试用
      raf.setLength(fileLength);
      // 自定义线程数量
      threadCount = 3;
      // 计算每条线程下载数据的大小
      int blockSize = fileLength / threadCount;
      // 启动线程下载
      for (int threadId = 1; threadId <= threadCount; threadId++) {
        // 核心代码，定义每个线程开始以及结束的下载位置
        int startPos = (threadId - 1) * blockSize;// 开始下载的位置
        int endPos = (threadId * blockSize) - 1;// 结束下载的位置（不包含最后一块）
        if (threadCount == threadId) {
          endPos = fileLength;
        }
        new Thread(new DownLoadThread(threadId, startPos, endPos, PATH))
            .start();
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  // 实现下载线程
  static class DownLoadThread implements Runnable {
    private int threadId;
    private int startPos;
    private int endPos;
    private String path;
    public DownLoadThread(int threadId, int startPos, int endPos,
        String path) {
      super();
      this.threadId = threadId;
      this.startPos = startPos;
      this.endPos = endPos;
      this.path = path;
    }
    public void run() {
      try {
        URL url = new URL(path);
        String[] str = PATH.split("/");
        String fileName = str[5];
        HttpURLConnection conn = (HttpURLConnection) url
            .openConnection();
        // 设置URL请求的方法（具体参考API）
        conn.setRequestMethod("GET");
        // 设置500毫秒为超时值
        conn.setReadTimeout(5000);
        File file = new File(threadId + ".txt");
        if (file.exists() && file.length() > 0) {
          BufferedReader br = new BufferedReader(
              new InputStreamReader(new FileInputStream(file)));
          String saveStartPos = br.readLine();
          if (saveStartPos != null && saveStartPos.length() > 0) {
            startPos = Integer.parseInt(saveStartPos);
          }
        }
        // 注意双引号内的格式，不能包含空格（等其他字符），否则报416
        conn.setRequestProperty("Range", "bytes=" + startPos + "-"
            + endPos);
        RandomAccessFile raf = new RandomAccessFile(fileName, "rwd");// 存储下载文件的随机写入文件
        raf.seek(startPos);// 设置开始下载的位置
        System.out.println("线程" + threadId + ":" + startPos + "~~"
            + endPos);
        InputStream is = conn.getInputStream();
        byte[] b = new byte[1024 * 1024 * 10];
        int len = -1;
        int newPos = startPos;
        while ((len = is.read(b)) != -1) {
          RandomAccessFile rr = new RandomAccessFile(file, "rwd");// 存储下载标记的文件
          raf.write(b, 0, len);
          // 将下载标记存入指定文档
          String savaPoint = String.valueOf(newPos += len);
          rr.write(savaPoint.getBytes());
          rr.close();
        }
        is.close();
        raf.close();
        System.out.println("下载完成");
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }
}