import java.io.InputStream;

 import java.io.RandomAccessFile;

 import java.net.URL;

 import java.net.URLConnection;

 public class MultithreadsDownload {

     public static void main(String[] args) {

         final int DOWNLOAD_THREAD_NUM = 4;

         final String FILE_NAME = "download.jpg";

         InputStream[] is = new InputStream[DOWNLOAD_THREAD_NUM];

         RandomAccessFile[] raf = new RandomAccessFile[DOWNLOAD_THREAD_NUM];

         try {

             URL url = new URL("http://p.qpic.cn/ninja/0/ninja1393807134/0");

             for (int i=0; i<DOWNLOAD_THREAD_NUM; i++) {

                 is[i] = url.openStream();

                 raf[i] = new RandomAccessFile(FILE_NAME, "rw");

             }

             long filelen = getFileLength(url);

             System.out.println("The size of the file is : " + filelen);

             //create a empty final file

             for (int i=0; i<filelen; i++)

                 raf[0].write(0);

             //compute the download size for per thread

             long numPerThread = filelen / DOWNLOAD_THREAD_NUM;

             long left = filelen % DOWNLOAD_THREAD_NUM;

             for (int i=0; i<DOWNLOAD_THREAD_NUM; i++) {

                 if (i == DOWNLOAD_THREAD_NUM-1)

                     new DownloadThread(i*numPerThread, (i+1)*numPerThread + left,

                             is[i], raf[i]).start();

                 else

                     new DownloadThread(i*numPerThread, (i+1)*numPerThread,

                             is[i], raf[i]).start();

             }

         } catch (Exception e)

         {

             e.printStackTrace();

         }

     }

     public static long getFileLength(URL url) throws Exception

     {

         URLConnection conn = url.openConnection();

         long size = conn.getContentLengthLong();

         return size;

     }

 }

 class DownloadThread extends Thread {

     DownloadThread(long start, long end, InputStream is, RandomAccessFile raf)

     {

         System.out.println("Begin downloading : " + start + "--->" + end);

         this.start = start;

         this.end = end;

         this.is = is;

         this.raf = raf;

     }

     @Override

     public void run()

     {

         try {

             //seek the appropriate input location of the cur thread

             is.skip(start);

             //seek the appropriate location for writing

             raf.seek(start);

             byte[] buff = new byte[BUFF_SIZE];

             long contentLen = end - start;

             //Ensure the totally download

             long times = contentLen / BUFF_SIZE + 4;

             int hasRead = 0;

             for (int i=0; i<times; i++) {

                 hasRead = is.read(buff);

                 if (hasRead < 0)

                     break;

                 raf.write(buff, 0, hasRead);

             }

         } catch (Exception e)

         {

             e.printStackTrace();

         }

         finally {

             try {

                 if (is != null)

                     is.close();

                 if (raf != null)

                     raf.close();

             }catch (Exception e)

             {

                 e.printStackTrace();

             }

         }

     }

     private final int BUFF_SIZE = 32;

     private long start;

     private long end;

     private InputStream is;

     private RandomAccessFile raf;

 }