使用java apache commons下載文件？

Question

如何使用庫下載文件並打印保存的字節？我嘗試使用

import static org.apache.commons.io.FileUtils.copyURLToFile; 
public static void Download() { 

     URL dl = null; 
     File fl = null; 
     try { 
      fl = new File(System.getProperty("user.home").replace("\\", "/") + "/Desktop/Screenshots.zip"); 
      dl = new URL("http://ds-forums.com/kyle-tests/uploads/Screenshots.zip"); 
      copyURLToFile(dl, fl); 
     } catch (Exception e) { 
      System.out.println(e); 
     } 
    } 

但我不能顯示字節或一個進度條。我應該使用哪種方法？

public class download { 
    public static void Download() { 
     URL dl = null; 
     File fl = null; 
     String x = null; 
     try { 
      fl = new File(System.getProperty("user.home").replace("\\", "/") + "/Desktop/Screenshots.zip"); 
      dl = new URL("http://ds-forums.com/kyle-tests/uploads/Screenshots.zip"); 
      OutputStream os = new FileOutputStream(fl); 
      InputStream is = dl.openStream(); 
      CountingOutputStream count = new CountingOutputStream(os); 
      dl.openConnection().getHeaderField("Content-Length"); 
      IOUtils.copy(is, os);//begin transfer 

      os.close();//close streams 
      is.close();//^ 
     } catch (Exception e) { 
      System.out.println(e); 
     } 
    } 

Answer 1

如果您正在尋找一種在下載前獲取總字節數的方法，您可以從http響應中的Content-Length標頭中獲取該值。

如果你只是想下載後的最終字節數，最簡單的方法是檢查剛剛寫入的文件大小。

但是，如果你想顯示的多少字節已下載的最新進展，您可能要擴展Apache CountingOutputStream包裹FileOutputStream使每次的write方法被調用它計數通過字節和更新的數進度條。

更新

下面是一個簡單的實施DownloadCountingOutputStream。我不確定你是否熟悉使用ActionListener，但它是實現GUI的有用類。

public class DownloadCountingOutputStream extends CountingOutputStream { 

    private ActionListener listener = null; 

    public DownloadCountingOutputStream(OutputStream out) { 
     super(out); 
    } 

    public void setListener(ActionListener listener) { 
     this.listener = listener; 
    } 

    @Override 
    protected void afterWrite(int n) throws IOException { 
     super.afterWrite(n); 
     if (listener != null) { 
      listener.actionPerformed(new ActionEvent(this, 0, null)); 
     } 
    } 

} 

這是使用樣品：

public class Downloader { 

    private static class ProgressListener implements ActionListener { 

     @Override 
     public void actionPerformed(ActionEvent e) { 
      // e.getSource() gives you the object of DownloadCountingOutputStream 
      // because you set it in the overriden method, afterWrite(). 
      System.out.println("Downloaded bytes : " + ((DownloadCountingOutputStream) e.getSource()).getByteCount()); 
     } 
    } 

    public static void main(String[] args) { 
     URL dl = null; 
     File fl = null; 
     String x = null; 
     OutputStream os = null; 
     InputStream is = null; 
     ProgressListener progressListener = new ProgressListener(); 
     try { 
      fl = new File(System.getProperty("user.home").replace("\\", "/") + "/Desktop/Screenshots.zip"); 
      dl = new URL("http://ds-forums.com/kyle-tests/uploads/Screenshots.zip"); 
      os = new FileOutputStream(fl); 
      is = dl.openStream(); 

      DownloadCountingOutputStream dcount = new DownloadCountingOutputStream(os); 
      dcount.setListener(progressListener); 

      // this line give you the total length of source stream as a String. 
      // you may want to convert to integer and store this value to 
      // calculate percentage of the progression. 
      dl.openConnection().getHeaderField("Content-Length"); 

      // begin transfer by writing to dcount, not os. 
      IOUtils.copy(is, dcount); 

     } catch (Exception e) { 
      System.out.println(e); 
     } finally { 
      IOUtils.closeQuietly(os); 
      IOUtils.closeQuietly(is); 
     } 
    } 
} 

Answer 2

commons-io有IOUtils.copy(inputStream, outputStream)。所以：

OutputStream os = new FileOutputStream(fl); 
InputStream is = dl.openStream(); 

IOUtils.copy(is, os); 

而且IOUtils.toByteArray(is)可以用來獲取字節。

獲取總字節數是一個不同的故事。數據流不會給你任何總數 - 它們只能給你當前流中可用的內容。但由於它是一個流，它可能會有更多的來。

這就是爲什麼http有特殊的方式來指定總字節數。它在響應標題Content-Length中。因此，您必須致電url.openConnection()，然後致電URLConnection對象上的getHeaderField("Content-Length")。它將以字符串形式返回字節數。然後使用Integer.parseInt(bytesString)，你會得到你的總數。