HttpURLConnection多线程下载

package cn.itcast.net.download;

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();
}
}
}

}