多线程从网络下载文件

主类：

public class DownThreadDemo {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int threadNum = 3;
        long fileLen = 0;
        long numPerThread = 0;
        long left = 0;
        URL url = null;
        URLConnection connection = null;
        String fileName = "e:\temp\downlo.jpg";
        RandomAccessFile[] raf = new RandomAccessFile[threadNum];
        InputStream[] is = new InputStream[threadNum];
        try {
            url = new URL(
                    "http://img.bizhi.sogou.com/images/2014/08/22/830201.jpg");
            // 打开网路连接
            connection = url.openConnection();
            // 计算文件大小
            fileLen = connection.getContentLengthLong();
            // 每个线程处理的文件大小
            numPerThread = fileLen / threadNum;
            // 最后一块
            left = fileLen % threadNum;
            // openConnection().getInputStream()
            is[0] = url.openStream();
            raf[0] = new RandomAccessFile(fileName, "rw");
            raf[0].setLength(fileLen);
            // 分别为每个线程创建一个从网络读取的流与写入文件的流。
            for (int i = 0; i < threadNum; i++) {
                if (i != 0) {
                    is[i] = url.openStream();
                    raf[i] = new RandomAccessFile(fileName, "rw");
                }
                if (i == threadNum - 1) {
                    new MultyDown(i * numPerThread, (i + 1) * numPerThread
                            + left, is[i], raf[i]).start();
                } else {
                    new MultyDown(i * numPerThread, (i + 1) * numPerThread,
                            is[i], raf[i]).start();
                }
            }
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("end------");
    }

}

线程类：

public class MultyDown extends Thread {
    private long start, end;
    private InputStream is;
    private RandomAccessFile raf;

    public MultyDown() {
        // TODO Auto-generated constructor stub
    }

    public MultyDown(long start, long end, InputStream is, RandomAccessFile raf) {
        // TODO Auto-generated constructor stub
        this.start = start;
        this.end = end;
        this.is = is;
        this.raf = raf;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub

        int len = 0;
        int size = 0;
        byte[] b = new byte[24];
        try {
            // 跳到文件指定位置
            is.skip(start);
            raf.seek(start);
            while ((len = is.read(b)) != -1 && size < (end - start)) {
                raf.write(b, 0, len);
                size += len;
            }
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            if (raf != null) {
                try {
                    raf.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }

}