Java多線程下載文件

import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class MultiThreadDownFile {
    public static void main(String[] args) {
        try {
            String filePath = "http://dl_dir.qq.com/qqfile/qd/QQ2012Beta3_QQProtect2.8.exe";
            int threadNum = 5;
            new MultiThreadDown().down(filePath, threadNum);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
class MultiThreadDown extends Thread {

private int threadId;
private int block;
private URL url;
private File file;

public MultiThreadDown() {
}

public MultiThreadDown(int threadId, int block, URL url, File file) {
    super();
    this.threadId = threadId;
    this.block = block;
    this.url = url;
    this.file = file;
}

public void down(String path, int threadNum) throws Exception {
    URL url = new URL(path);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setConnectTimeout(5000);
    conn.setRequestMethod("GET");
    try {
        if (conn.getResponseCode() == 200) {
            int fileLength = conn.getContentLength();// Get the length of
                                                        // NetFile
            System.out.println("網絡文件大小：" + fileLength);
            File file = new File(getFilename(path));
            RandomAccessFile raf = new RandomAccessFile(file, "rwd");
            raf.setLength(fileLength);
            raf.close();
            // 計算每條線程的下載量
            int block = fileLength % threadNum == 0 ? fileLength
                    / threadNum : fileLength / threadNum + 1;
            for (int threadId = 0; threadId < threadNum; threadId++) {
                new MultiThreadDown(threadId, block, url, file).start();
                System.out.println("線程" + threadId + "開始");
            }
        } else {
            System.out.println("Connection failed.");
        }
    } catch (Exception e) {
        System.out.println("連接失敗.");
        e.printStackTrace();
    }
}

private String getFilename(String path) {
    return path.substring(path.lastIndexOf("/") + 1);
}

@Override
public void run() {
    super.run();
    int start = threadId * block;// 計算線程下載的網絡文件的文件位置
    int end = (threadId + 1) * block - 1;// 線程的結束位置
    try {
        RandomAccessFile raf = new RandomAccessFile(file, "rwd");
        raf.seek(start);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setConnectTimeout(5000);
        conn.setRequestMethod("GET");
        conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
        if (conn.getResponseCode() == 206) {
            InputStream is = conn.getInputStream();
            byte[] buf = new byte[1024];
            int len = 0;
            while ((len = is.read(buf)) != -1) {
                raf.write(buf, 0, len);
            }
            raf.close();
            is.close();
        }
        System.out.println("第" + threadId + "線程下載完畢");
    } catch (Exception e) {
        e.printStackTrace();
    }
}

}</pre>