java 如何恢复中断的下载

how to resume an interrupted download

提问by gregS

I'm trying to download a large file from my Yahoo! web site server which apparently is setup (not by me) to disconnect downloads if they are not completed within 100 seconds. The file is small enough to usually successfully transfer. On the occasions when the data rate is slow and the download gets disconnected, is there a way to resume the URLConnection at the file offset where the disconnection occurred? Here's the code:

我正在尝试从我的 Yahoo! 下载一个大文件。如果下载未在 100 秒内完成，该网站服务器显然已设置（不是我设置的）以断开下载。该文件足够小，通常可以成功传输。在数据速率慢且下载断开的情况下，有没有办法在断开连接的文件偏移处恢复 URLConnection？这是代码：

// Setup connection.
URL url = new URL(strUrl[0]);
URLConnection cx = url.openConnection();
cx.connect();

// Setup streams and buffers.
int lengthFile = cx.getContentLength();
InputStream input = new BufferedInputStream(url.openStream());
OutputStream output = new FileOutputStream(strUrl[1]);
byte data[] = new byte[1024];

// Download file.
for (total=0; (count=input.read(data, 0, 1024)) != -1; total+=count) {
    publishProgress((int)(total*100/lengthFile));
    output.write(data, 0, count);
    Log.d("AsyncDownloadFile", "bytes: " + total);
}

// Close streams.
output.flush();
output.close();
input.close();

回答by naikus

Try using a "Range" request header:

尝试使用“范围”请求标头：

// Open connection to URL.
HttpURLConnection connection = (HttpURLConnection) url.openConnection();

// Specify what portion of file to download.
connection.setRequestProperty("Range", "bytes=" + downloaded + "-");
// here "downloaded" is the data length already previously downloaded.

// Connect to server.
connection.connect();

Having done that, you can seekat a given point (just before the length of your download data, say X) and start writing the newly downloaded data there. Be sure to use the same value Xfor the range header.

完成后，您可以seek在给定点（就在下载数据的长度之前，例如X）并开始在那里写入新下载的数据。确保对X范围标题使用相同的值。

Details about 14.35.2 Range Retrieval Requests

有关14.35.2 范围检索请求的详细信息

More details and source code can be found here

更多细节和源代码可以在这里找到

回答by Mark Amabile

Here's an example code that you can use:

这是您可以使用的示例代码：

import java.io.*;
import java.net.*;


public class HttpUrlDownload {

    public static void main(String[] args) {
       String strUrl = "http://VRSDLSCEN001:80//DLS//lib//clics.jar";
       String DESTINATION_PATH = "clics.jar";

       int count = 0;
       while (true) {
          count++;
          if (download(strUrl, DESTINATION_PATH) == true || count > 20) {
             break;
          }         
       }
    }

    public static boolean download(String strUrl, String DESTINATION_PATH) {
        BufferedInputStream in = null;
        FileOutputStream fos = null;
        BufferedOutputStream bout = null;
        URLConnection connection = null;

        int downloaded = 0;

        try {
           System.out.println("mark ... download start");
           URL url = new URL(strUrl);

           connection = url.openConnection();

           File file=new File(DESTINATION_PATH);
           if(file.exists()){
               downloaded = (int) file.length();
           }
           if (downloaded == 0) {
               connection.connect();
           }
           else {
               connection.setRequestProperty("Range", "bytes=" + downloaded + "-");
               connection.connect();
           }

           try {
               in = new BufferedInputStream(connection.getInputStream());
           } catch (IOException e) {
               int responseCode = 0;
               try {
                   responseCode = ((HttpURLConnection)connection).getResponseCode();
               } catch (IOException e1) {
                 e1.printStackTrace();
               }

               if (responseCode == 416) {         
                   return true;
               } else {
                   e.printStackTrace();
                   return false;
               }
           }

           fos=(downloaded==0)? new FileOutputStream(DESTINATION_PATH): new FileOutputStream(DESTINATION_PATH,true);
           bout = new BufferedOutputStream(fos, 1024);

           byte[] data = new byte[1024];
           int x = 0;
           while ((x = in.read(data, 0, 1024)) >= 0) {
              bout.write(data, 0, x);
           }

           in.close();
           bout.flush();
           bout.close();
           return false;

       } catch (IOException e) {
           e.printStackTrace();
           return false;
       } finally {
           if (in != null) {
               try {
                   in.close();
               } catch (IOException e) {
               }
           }
           if (fos != null) {
               try {
                   fos.close();
               } catch (IOException e) {
               }
           }
           if (bout != null) {
               try {
                   bout.close();
               } catch (IOException e) {
               }
           }

           if (connection != null) {
              ((HttpURLConnection)connection).disconnect();
           }
       }
    } 
}