如何利用Java实现一个断点续传功能

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/*
 * Encode:UTF-8
 * 
 * Author:zhiming.xu
 * 
 * 多线程的断点下载程序，根据输入的url和指定线程数，来完成断点续传功能。
 * 
 * 每个线程支负责某一小段的数据下载；再通过RandomAccessFile完成数据的整合。
 */
public class MultiTheradDownLoad {

  private String filepath = null;
  private String filename = null;
  private String tmpfilename = null;

  private int threadNum = 0;

  private CountDownLatch latch = null;//设置一个计数器，代码内主要用来完成对缓存文件的删除

  private long fileLength = 0l;
  private long threadLength = 0l;
  private long[] startPos;//保留每个线程下载数据的起始位置。
  private long[] endPos;//保留每个线程下载数据的截止位置。

  private boolean bool = false;

  private URL url = null;

  //有参构造函数，先构造需要的数据
  public MultiTheradDownLoad(String filepath, int threadNum) {
    this.filepath = filepath;
    this.threadNum = threadNum;
    startPos = new long[this.threadNum];
    endPos = new long[this.threadNum];
    latch = new CountDownLatch(this.threadNum);
  }

  /*
   * 组织断点续传功能的方法
   */
  public void downloadPart() {

    File file = null;
    File tmpfile = null;
    HttpURLConnection httpcon = null;

    //在请求url内获取文件资源的名称；此处没考虑文件名为空的情况，此种情况可能需使用UUID来生成一个唯一数来代表文件名。
    filename = filepath.substring(filepath.lastIndexOf('/') + 1, filepath
        .contains("?") ? filepath.lastIndexOf('?') : filepath.length());
    tmpfilename = filename + "_tmp";

    try {
      url = new URL(filepath);
      httpcon = (HttpURLConnection) url.openConnection();

      setHeader(httpcon);
      fileLength = httpcon.getContentLengthLong();//获取请求资源的总长度。

      file = new File(filename);
      tmpfile = new File(tmpfilename);

      threadLength = fileLength / threadNum;//每个线程需下载的资源大小。
      System.out.println("fileName: " + filename + " ," + "fileLength= "
          + fileLength + " the threadLength= " + threadLength);

      if (file.exists() && file.length() == fileLength) {
        System.out
            .println("the file you want to download has exited!!");
        return;
      } else {
        setBreakPoint(startPos, endPos, tmpfile);
        ExecutorService exec = Executors.newCachedThreadPool();
        for (int i = 0; i < threadNum; i++) {
          exec.execute(new DownLoadThread(startPos[i], endPos[i],
              this, i, tmpfile, latch));
        }
        latch.await();//当你的计数器减为0之前，会在此处一直阻塞。
        exec.shutdown();
      }
    } catch (MalformedURLException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }

    if (file.length() == fileLength) {
      if (tmpfile.exists()) {
        System.out.println("delect the temp file!!");
        tmpfile.delete();
      }
    }
  }

  /*
   * 断点设置方法，当有临时文件时，直接在临时文件中读取上次下载中断时的断点位置。没有临时文件，即第一次下载时，重新设置断点。
   * 
   * rantmpfile.seek()跳转到一个位置的目的是为了让各个断点存储的位置尽量分开。
   * 
   * 这是实现断点续传的重要基础。
   */
  private void setBreakPoint(long[] startPos, long[] endPos, File tmpfile) {
    RandomAccessFile rantmpfile = null;
    try {
      if (tmpfile.exists()) {
        System.out.println("the download has continued!!");
        rantmpfile = new RandomAccessFile(tmpfile, "rw");
        for (int i = 0; i < threadNum; i++) {
          rantmpfile.seek(8 * i + 8);
          startPos[i] = rantmpfile.readLong();

          rantmpfile.seek(8 * (i + 1000) + 16);
          endPos[i] = rantmpfile.readLong();

          System.out.println("the Array content in the exit file: ");
          System.out.println("thre thread" + (i + 1) + " startPos:"
              + startPos[i] + ", endPos: " + endPos[i]);
        }
      } else {
        System.out.println("the tmpfile is not available!!");
        rantmpfile = new RandomAccessFile(tmpfile, "rw");
        
        //最后一个线程的截止位置大小为请求资源的大小
        for (int i = 0; i < threadNum; i++) {
          startPos[i] = threadLength * i;
          if (i == threadNum - 1) {
            endPos[i] = fileLength;
          } else {
            endPos[i] = threadLength * (i + 1) - 1;
          }

          rantmpfile.seek(8 * i + 8);
          rantmpfile.writeLong(startPos[i]);

          rantmpfile.seek(8 * (i + 1000) + 16);
          rantmpfile.writeLong(endPos[i]);

          System.out.println("the Array content: ");
          System.out.println("thre thread" + (i + 1) + " startPos:"
              + startPos[i] + ", endPos: " + endPos[i]);
        }
      }
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      try {
        if (rantmpfile != null) {
          rantmpfile.close();
        }
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
  
  /*
   * 实现下载功能的内部类，通过读取断点来设置向服务器请求的数据区间。
   */
  class DownLoadThread implements Runnable {

    private long startPos;
    private long endPos;
    private MultiTheradDownLoad task = null;
    private RandomAccessFile downloadfile = null;
    private int id;
    private File tmpfile = null;
    private RandomAccessFile rantmpfile = null;
    private CountDownLatch latch = null;

    public DownLoadThread(long startPos, long endPos,
        MultiTheradDownLoad task, int id, File tmpfile,
        CountDownLatch latch) {
      this.startPos = startPos;
      this.endPos = endPos;
      this.task = task;
      this.tmpfile = tmpfile;
      try {
        this.downloadfile = new RandomAccessFile(this.task.filename,
            "rw");
        this.rantmpfile = new RandomAccessFile(this.tmpfile, "rw");
      } catch (FileNotFoundException e) {
        e.printStackTrace();
      }
      this.id = id;
      this.latch = latch;
    }

    @Override
    public void run() {

      HttpURLConnection httpcon = null;
      InputStream is = null;
      int length = 0;

      System.out.println("the thread " + id + " has started!!");

      while (true) {
        try {
          httpcon = (HttpURLConnection) task.url.openConnection();
          setHeader(httpcon);
          
          //防止网络阻塞，设置指定的超时时间；单位都是ms。超过指定时间，就会抛出异常
          httpcon.setReadTimeout(20000);//读取数据的超时设置
          httpcon.setConnectTimeout(20000);//连接的超时设置

          if (startPos < endPos) {
            
            //向服务器请求指定区间段的数据，这是实现断点续传的根本。
            httpcon.setRequestProperty("Range", "bytes=" + startPos
                + "-" + endPos);

            System.out
                .println("Thread " + id
                    + " the total size:---- "
                    + (endPos - startPos));

            downloadfile.seek(startPos);

            if (httpcon.getResponseCode() != HttpURLConnection.HTTP_OK
                && httpcon.getResponseCode() != HttpURLConnection.HTTP_PARTIAL) {
              this.task.bool = true;
              httpcon.disconnect();
              downloadfile.close();
              System.out.println("the thread ---" + id
                  + " has done!!");
              latch.countDown();//计数器自减
              break;
            }

            is = httpcon.getInputStream();//获取服务器返回的资源流
            long count = 0l;
            byte[] buf = new byte[1024];

            while (!this.task.bool && (length = is.read(buf)) != -1) {
              count += length;
              downloadfile.write(buf, 0, length);
              
              //不断更新每个线程下载资源的起始位置，并写入临时文件；为断点续传做准备
              startPos += length;
              rantmpfile.seek(8 * id + 8);
              rantmpfile.writeLong(startPos);
            }
            System.out.println("the thread " + id
                + " total load count: " + count);
            
            //关闭流
            is.close();
            httpcon.disconnect();
            downloadfile.close();
            rantmpfile.close();
          }
          latch.countDown();//计数器自减
          System.out.println("the thread " + id + " has done!!");
          break;
        } catch (IOException e) {
          e.printStackTrace();
        } finally {
          try {
            if (is != null) {
              is.close();
            }
          } catch (IOException e) {
            e.printStackTrace();
          }
        }
      }
    }
  }

  /*
   * 为一个HttpURLConnection模拟请求头，伪装成一个浏览器发出的请求
   */
  private void setHeader(HttpURLConnection con) {
    con.setRequestProperty(
        "User-Agent",
        "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3) Gecko/2008092510 Ubuntu/8.04 (hardy) Firefox/3.0.3");
    con.setRequestProperty("Accept-Language", "en-us,en;q=0.7,zh-cn;q=0.3");
    con.setRequestProperty("Accept-Encoding", "aa");
    con.setRequestProperty("Accept-Charset",
        "ISO-8859-1,utf-8;q=0.7,*;q=0.7");
    con.setRequestProperty("Keep-Alive", "300");
    con.setRequestProperty("Connection", "keep-alive");
    con.setRequestProperty("If-Modified-Since",
        "Fri, 02 Jan 2009 17:00:05 GMT");
    con.setRequestProperty("If-None-Match", "\"1261d8-4290-df64d224\"");
    con.setRequestProperty("Cache-Control", "max-age=0");
    con.setRequestProperty("Referer",
        "http://www.skycn.com/soft/14857.html");
  }
}

public class DownLoadTest {

  /**
   * @param args
   */
  public static void main(String[] args) {
    
    String filepath = "http://127.0.0.1:8080/file/loadfile.mkv";
    MultiTheradDownLoad load = new MultiTheradDownLoad(filepath ,4);  
    load.downloadPart();  
  }
}