問題（下載ftp檔案）：

寫一個ftp下載程式，出現很怪異的錯誤，寫得過程中會出現空指標， 

有時候又出現java.net.SocketTimeoutException: Read timed out，

還會出現org.apache.commons.net.io.CopyStreamException: IOException caught while copying. 

1. public void run() {  
2.         ftpClient.enterLocalPassiveMode();  
3.         OutputStream outputStream = null
   ;  
4.         try {  
5.             ftpClient.setFileType(FTPClient.BINARY_FILE_TYPE);  
6.             ftpClient.changeWorkingDirectory(path);// 轉移到FTP伺服器目錄  
7.             FTPFile[] fs = ftpClient.listFiles(); // 得到目錄的相應檔案列表  
8.             for (int i = 0; i < fs.length; i++) {  
9.                 FTPFile ff = fs[i];  
10.                 if (ff.getName().equals(fileName)) {  
11.                     System.out.println(ff.getName());  
12.                     String filename = fileName;  
13.                     // 這個就就是彈出下載對話方塊的關鍵程式碼  
14.                     // response.setContentType("application/octet-stream");  
15.                     response.setHeader("Content-disposition"
    ,  
16.                     "attachment;filename="  
17.                     + URLEncoder.encode(filename, "utf-8"));  
18.                     // 將檔案儲存到輸出流outputStream中  
19.                     InputStream in = ftpClient.retrieveFileStream(new String(ff  
20.                             .getName().getBytes("GBK"), "ISO-8859-1"));  
21.                     outputStream = response.getOutputStream();  
22.                     int len = 0;  
23.                     long size = 0;  
24.                     long t = 16364;  
25.                     byte[] bt = new byte[1024];  
26.                     while ((len = in.read(bt)) > 0) {  
27.                         outputStream.write(bt, 0, len); // outputStream.flush();  
28.                         size = size + len;  
29.                         System.out.println(this.fileName + "已xiazai ：" + size);  
30.                     }  
31.                     /* 
32.                      * // response.s File file = new File("d:/123456.com"); // 
33.                      * response.reset();response //response. outputStream = 
34.                      * response.getOutputStream(); // new 
35.                      * FileOutputStream(file); boolean boo = false; if 
36.                      * (outputStream != null) { boo = ftpClient 
37.                      * .retrieveFile(ff.getName(), outputStream); } 
38.                      */  
39.                     // System.out.println(boo);  
40.                     // in.close();  
41.                     // os.close();  
42.                     outputStream.flush();  
43.                 }  
44.             }  
45.         } catch (IOException e) {  
46.             // TODO Auto-generated catch block  
47.             e.printStackTrace();  
48.         } finally {  
49.             try {  
50.                 outputStream.close();  
51.             } catch (IOException e) {  
52.                 // TODO Auto-generated catch block  
53.                 e.printStackTrace();  
54.             }  
55.             FTPUtil.disconnect(ftpClient);  
56.         }  

at com.yniso.filesystem.DownloadFTPFile.run(DownloadFTPFile.java:120) 

分析原因：經反覆測試這異常好像都是因為網路的問題，因為網路不穩定下載直接被迫中斷了