多线程从网络下载文件

主类：

 1 public class DownThreadDemo {
 2 
 3     public static void main(String[] args) {
 4         // TODO Auto-generated method stub
 5         int threadNum = 3;
 6         long fileLen = 0;
 7         long numPerThread = 0;
 8         long left = 0;
 9         URL url = null;
10         URLConnection connection = null;
11         String fileName = "e:\temp\downlo.jpg";
12         RandomAccessFile[] raf = new RandomAccessFile[threadNum];
13         InputStream[] is = new InputStream[threadNum];
14         try {
15             url = new URL(
16                     "http://img.bizhi.sogou.com/images/2014/08/22/830201.jpg");
17             // 打开网路连接
18             connection = url.openConnection();
19             // 计算文件大小
20             fileLen = connection.getContentLengthLong();
21             // 每个线程处理的文件大小
22             numPerThread = fileLen / threadNum;
23             // 最后一块
24             left = fileLen % threadNum;
25             // openConnection().getInputStream()
26             is[0] = url.openStream();
27             raf[0] = new RandomAccessFile(fileName, "rw");
28             raf[0].setLength(fileLen);
29             // 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。
30             for (int i = 0; i < threadNum; i++) {
31                 if (i != 0) {
32                     is[i] = url.openStream();
33                     raf[i] = new RandomAccessFile(fileName, "rw");
34                 }
35                 if (i == threadNum - 1) {
36                     new MultyDown(i * numPerThread, (i + 1) * numPerThread
37                             + left, is[i], raf[i]).start();
38                 } else {
39                     new MultyDown(i * numPerThread, (i + 1) * numPerThread,
40                             is[i], raf[i]).start();
41                 }
42             }
43         } catch (MalformedURLException e) {
44             // TODO Auto-generated catch block
45             e.printStackTrace();
46         } catch (IOException e) {
47             // TODO Auto-generated catch block
48             e.printStackTrace();
49         }
50         System.out.println("end------");
51     }
52 
53 }

线程类：

 1 public class MultyDown extends Thread {
 2     private long start, end;
 3     private InputStream is;
 4     private RandomAccessFile raf;
 5 
 6     public MultyDown() {
 7         // TODO Auto-generated constructor stub
 8     }
 9 
10     public MultyDown(long start, long end, InputStream is, RandomAccessFile raf) {
11         // TODO Auto-generated constructor stub
12         this.start = start;
13         this.end = end;
14         this.is = is;
15         this.raf = raf;
16     }
17 
18     @Override
19     public void run() {
20         // TODO Auto-generated method stub
21 
22         int len = 0;
23         int size = 0;
24         byte[] b = new byte[24];
25         try {
26             // 跳到文件指定位置
27             is.skip(start);
28             raf.seek(start);
29             while ((len = is.read(b)) != -1 && size < (end - start)) {
30                 raf.write(b, 0, len);
31                 size += len;
32             }
33         } catch (IOException e) {
34             // TODO Auto-generated catch block
35             e.printStackTrace();
36         } finally {
37             if (is != null) {
38                 try {
39                     is.close();
40                 } catch (IOException e) {
41                     // TODO Auto-generated catch block
42                     e.printStackTrace();
43                 }
44             }
45             if (raf != null) {
46                 try {
47                     raf.close();
48                 } catch (IOException e) {
49                     // TODO Auto-generated catch block
50                     e.printStackTrace();
51                 }
52             }
53         }
54     }
55 
56 }