【Java】：多线程下载

  1 import java.io.InputStream;

  2 import java.io.RandomAccessFile;

  3 import java.net.URL;

  4 import java.net.URLConnection;

  5 

  6 public class MultithreadsDownload {

  7     public static void main(String[] args) {

  8         final int DOWNLOAD_THREAD_NUM = 4;

  9         final String FILE_NAME = "download.jpg";

 10         InputStream[] is = new InputStream[DOWNLOAD_THREAD_NUM];

 11         RandomAccessFile[] raf = new RandomAccessFile[DOWNLOAD_THREAD_NUM];

 12         

 13         try {

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

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

 16                 is[i] = url.openStream();

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

 18             }

 19             long filelen = getFileLength(url);

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

 21             //create a empty final file

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

 23                 raf[0].write(0);

 24             

 25             //compute the download size for per thread

 26             long numPerThread = filelen / DOWNLOAD_THREAD_NUM;

 27             long left = filelen % DOWNLOAD_THREAD_NUM;

 28             

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

 30                 if (i == DOWNLOAD_THREAD_NUM-1)

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

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

 33                 else

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

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

 36             }

 37         } catch (Exception e)

 38         {

 39             e.printStackTrace();

 40         }

 41     }

 42     

 43     public static long getFileLength(URL url) throws Exception

 44     {

 45         URLConnection conn = url.openConnection();

 46         long size = conn.getContentLengthLong();

 47         return size;

 48     }

 49 }

 50 

 51 class DownloadThread extends Thread {

 52     

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

 54     {

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

 56         this.start = start;

 57         this.end = end;

 58         this.is = is;

 59         this.raf = raf;

 60     }

 61     

 62     @Override

 63     public void run()

 64     {

 65         try {

 66             //seek the appropriate input location of the cur thread

 67             is.skip(start);

 68             //seek the appropriate location for writing

 69             raf.seek(start);

 70             byte[] buff = new byte[BUFF_SIZE];

 71             long contentLen = end - start;

 72             //Ensure the totally download

 73             long times = contentLen / BUFF_SIZE + 4;

 74             int hasRead = 0;

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

 76                 hasRead = is.read(buff);

 77                 if (hasRead < 0)

 78                     break;

 79                 raf.write(buff, 0, hasRead);

 80             }

 81         } catch (Exception e)

 82         {

 83             e.printStackTrace();

 84         }

 85         finally {

 86             try {

 87                 if (is != null)

 88                     is.close();

 89                 if (raf != null)

 90                     raf.close();

 91             }catch (Exception e)

 92             {

 93                 e.printStackTrace();

 94             }

 95         }

 96     }

 97     

 98     private final int BUFF_SIZE = 32;

 99     private long start;

100     private long end;

101     private InputStream is;

102     private RandomAccessFile raf;

103 }

作用JAVA实现对一个文件的多线程下载，基本思想是先读取文件大小，进行分块，创建多个线程分别负责某一部分的下载任务