Android通过HTTP协议实现多线程下载

import java.io.File;  import java.io.InputStream;  import java.io.RandomAccessFile;  import java.net.HttpURLConnection;  import java.net.URL;    public class MulThreadDownload {     /**    * @param args    */   public static void main(String[] args) {    String path = "http://net.itcast.cn/QQWubiSetup.exe";    try {     new MulThreadDownload().download(path, 3);    } catch (Exception e) {     e.printStackTrace();    }   }   /**    * 从路径中获取文件名称    * @param path 下载路径    * @return    */   public static String getFilename(String path){    return path.substring(path.lastIndexOf('/')+1);   }   /**    * 下载文件    * @param path 下载路径    * @param threadsize 线程数    */   public void download(String path, int threadsize) throws Exception{    URL url = new URL(path);    HttpURLConnection conn = (HttpURLConnection)url.openConnection();    conn.setRequestMethod("GET");    conn.setConnectTimeout(5 * 1000);    int filelength = conn.getContentLength();//获取要下载的文件的长度    String filename = getFilename(path);//从路径中获取文件名称    File saveFile = new File(filename);    RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");    accessFile.setLength(filelength);//设置本地文件的长度和下载文件相同    accessFile.close();    //计算每条线程下载的数据长度    int block = filelength%threadsize==0? filelength/threadsize : filelength/threadsize+1;    for(int threadid=0 ; threadid < threadsize ; threadid++){     new DownloadThread(url, saveFile, block, threadid).start();    }   }      private final class DownloadThread extends Thread{    private URL url;    private File saveFile;    private int block;//每条线程下载的数据长度    private int threadid;//线程id      public DownloadThread(URL url, File saveFile, int block, int threadid) {     this.url = url;     this.saveFile = saveFile;     this.block = block;     this.threadid = threadid;    }      @Override    public void run() {     //计算开始位置公式：线程id*每条线程下载的数据长度= ？        //计算结束位置公式：（线程id +1）*每条线程下载的数据长度-1 =?     int startposition = threadid * block;     int endposition = (threadid + 1 ) * block - 1;     try {      RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");      accessFile.seek(startposition);//设置从什么位置开始写入数据      HttpURLConnection conn = (HttpURLConnection)url.openConnection();      conn.setRequestMethod("GET");      conn.setConnectTimeout(5 * 1000);      conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);      InputStream inStream = conn.getInputStream();      byte[] buffer = new byte[1024];      int len = 0;      while( (len=inStream.read(buffer)) != -1 ){       accessFile.write(buffer, 0, len);      }      inStream.close();      accessFile.close();      System.out.println("线程id:"+ threadid+ "下载完成");     } catch (Exception e) {      e.printStackTrace();     }    }     }    }