HttpUrlConnection实现多线程下载网络资源

/**
 * 
 * @create_time:2016年11月1日  下午4:45:05
 * @类描述：多线程下载工具类
 * @version:
 *
 */
public class DownUtil {
    private String path;//下载资源的路径
    private String targetFile;//下载文件的保存位置
    private int threadNum;//线程数
    private DownloadThread[] threads;
    private int fileSize;//文件大小

    public DownUtil(String path,String targetFile,int threadNum){
        this.path = path;
        this.targetFile = targetFile;
        this.threadNum = threadNum;
        threads = new DownloadThread[threadNum];//初始化threads数据
    }
    /**
      *下载
      */
    public void download() throws Exception{
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setConnectTimeout(5*1000);
        conn.setRequestMethod("GET");
        conn.setRequestProperty("Accept", "image/gif,image/jpeg,image/pjpeg,application/x-shockwaveflash,application/xaml+xml,application/vnd.ms-xpsdocument,application/x-ms-xbap,application/x-ms-application,application/vnd.ms-excel,application/vnd.ms-powerpoint,application/msword,*/*");
        conn.setRequestProperty("Accept-Language", "zh-CN");
        conn.setRequestProperty("Charset", "UTF-8");
        conn.setRequestProperty("User-Agent", "Mozilla/4.0(compatible;MSIE 7.0;Windows NT 5.2;Trident/4.0;.NET CLR 1.1.4322;.NET CLR 2.0.50727;.NET CLR 3.0.04506;.NET CLR 3.0.4506.2152;.NET CLR 3.5.30729)");
        conn.setRequestProperty("Connection", "Keep-Alive");
        fileSize = conn.getContentLength();//获取文件大小
        conn.disconnect();
        int currentPartSize = fileSize/threadNum +1;
        RandomAccessFile file = new RandomAccessFile(targetFile, "rw");
        file.setLength(fileSize);
        file.close();
        for (int i = 0; i < threadNum; i++) {
            //每个线程下载的起始位置
            int startPos = i*currentPartSize;
            //每一个线程使用一个RandomAccessFile进行下载
            RandomAccessFile currentPart = new RandomAccessFile(targetFile, "rw");
            //定位该线程的下载位置
            currentPart.seek(startPos);
            threads[i] = new DownloadThread(startPos, currentPartSize, currentPart);
            threads[i].start();
        }

    }
    //获取下载完成的百分比
    public double getCompleteRate(){
        int sumSize = 0;
        for (int i = 0; i < threadNum; i++) {
            sumSize +=threads[i].length;
        }
        return sumSize*1.0/fileSize;
    }

    private class DownloadThread extends Thread{
        private int startPos;//当前线程的下载位置
        private int currentPartSize;
        //当前线程需要下在的文件块
        private RandomAccessFile currentPart;
        //定义已下载的字节数
        public int length;

        public DownloadThread(int startPos,int currentPartSize,RandomAccessFile currentPart){
            this.startPos = startPos;
            this.currentPart = currentPart;
            this.currentPartSize = currentPartSize;
        }
        @Override
        public void run() {
            super.run();
            try {
                URL url = new URL(path);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setConnectTimeout(5*1000);
                conn.setRequestMethod("GET");
                conn.setRequestProperty("Accept", "image/gif,image/jpeg,image/pjpeg,application/x-shockwaveflash,application/xaml+xml,application/vnd.ms-xpsdocument,application/x-ms-xbap,application/x-ms-application,application/vnd.ms-excel,application/vnd.ms-powerpoint,application/msword,*/*");
                conn.setRequestProperty("Accept-Language", "zh-CN");
                conn.setRequestProperty("Charset", "UTF-8");
                InputStream is = conn.getInputStream();
                is.skip(this.startPos);
                byte[] buffer = new byte[1024];
                int hasRead = 0;
                while (length1) {
                    currentPart.write(buffer);
                    length +=hasRead;
                }
                currentPart.close();
                is.close();
            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (ProtocolException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}