Java实现多线程断点下载

JAVA多线程断点下载原理如图：

代码如下：

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 MutileThreadDownload {
  /**
   * 线程的数量
   */
  private static int threadCount = 3; 

  /**
   * 每个下载区块的大小
   */
  private static long blocksize; 

  /**
   * 正在运行的线程的数量
   */
  private static int runningThreadCount; 

  /**
   * @param args
   * @throws Exception
   */
  public static void main(String[] args) throws Exception {
    // 服务器文件的路径
    String path = "http://192.168.1.100:8080/ff.exe";
    URL url = new URL(path);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setRequestMethod("GET");
    conn.setConnectTimeout(5000);
    int code = conn.getResponseCode();
    if (code == 200) {
      long size = conn.getContentLength();// 得到服务端返回的文件的大小
      System.out.println("服务器文件的大小：" + size);
      blocksize = size / threadCount;
      // 1.首先在本地创建一个大小跟服务器一模一样的空白文件。
      File file = new File("temp.exe");
      RandomAccessFile raf = new RandomAccessFile(file, "rw");
      raf.setLength(size);
      // 2.开启若干个子线程分别去下载对应的资源。
      runningThreadCount = threadCount;
      for (int i = 1; i <= threadCount; i++) {
        long startIndex = (i - 1) * blocksize;
        long endIndex = i * blocksize - 1;
        if (i == threadCount) {
          // 最后一个线程
          endIndex = size - 1;
        }
        System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~"
            + endIndex);
        new DownloadThread(path, i, startIndex, endIndex).start();
      }
    }
    conn.disconnect();
  } 

  private static class DownloadThread extends Thread {
    private int threadId;
    private long startIndex;
    private long endIndex;
    private String path; 

    public DownloadThread(String path, int threadId, long startIndex,
        long endIndex) {
      this.path = path;
      this.threadId = threadId;
      this.startIndex = startIndex;
      this.endIndex = endIndex;
    } 

    @Override
    public void run() {
      try {
        // 当前线程下载的总大小
        int total = 0;
        File positionFile = new File(threadId + ".txt");
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection) url
            .openConnection();
        conn.setRequestMethod("GET");
        // 接着从上一次的位置继续下载数据
        if (positionFile.exists() && positionFile.length() > 0) {// 判断是否有记录
          FileInputStream fis = new FileInputStream(positionFile);
          BufferedReader br = new BufferedReader(
              new InputStreamReader(fis));
          // 获取当前线程上次下载的总大小是多少
          String lasttotalstr = br.readLine();
          int lastTotal = Integer.valueOf(lasttotalstr);
          System.out.println("上次线程" + threadId + "下载的总大小："
              + lastTotal);
          startIndex += lastTotal;
          total += lastTotal;// 加上上次下载的总大小。
          fis.close();
        } 

        conn.setRequestProperty("Range", "bytes=" + startIndex + "-"
            + endIndex);
        conn.setConnectTimeout(5000);
        int code = conn.getResponseCode();
        System.out.println("code=" + code);
        InputStream is = conn.getInputStream();
        File file = new File("temp.exe");
        RandomAccessFile raf = new RandomAccessFile(file, "rw");
        // 指定文件开始写的位置。
        raf.seek(startIndex);
        System.out.println("第" + threadId + "个线程：写文件的开始位置："
            + String.valueOf(startIndex));
        int len = 0;
        byte[] buffer = new byte[512];
        while ((len = is.read(buffer)) != -1) {
          RandomAccessFile rf = new RandomAccessFile(positionFile,
              "rwd");
          raf.write(buffer, 0, len);
          total += len;
          rf.write(String.valueOf(total).getBytes());
          rf.close();
        }
        is.close();
        raf.close(); 

      } catch (Exception e) {
        e.printStackTrace();
      } finally {
        // 只有所有的线程都下载完毕后 才可以删除记录文件。
        synchronized (MutileThreadDownload.class) {
          System.out.println("线程" + threadId + "下载完毕了");
          runningThreadCount--;
          if (runningThreadCount < 1) {
            System.out.println("所有的线程都工作完毕了。删除临时记录的文件");
            for (int i = 1; i <= threadCount; i++) {
              File f = new File(i + ".txt");
              System.out.println(f.delete());
            }
          }
        } 

      }
    }
  }
}

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