Question

如何使用库下载文件并打印保存的字节？我尝试使用

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);
        }
    }

但我无法显示字节或进度条。我应该使用哪种方法？

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

如果您正在寻找一种在下载前获取总字节数的方法，可以从http响应中的Content-Length标题中获取此值。

如果您只想在下载后获得最终字节数，最简单的方法是检查您刚写入的文件大小。

但是，如果要显示已下载的字节数的当前进度，您可能希望扩展apache CountingOutputStream以包装FileOutputStream，以便每次write方法都是调用它计算通过的字节数并更新进度条。

<强>更新

以下是DownloadCountingOutputStream的简单实现。我不确定您是否熟悉使用ActionListener，但它是实现GUI的有用类。

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));
        }
    }

}

这是用法示例：

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);

并且IOUtils.toByteArray(is)可用于获取字节。

获取总字节数是另一回事。 Streams不会给你任何总数 - 它们只能提供流中当前可用的内容。但由于它是一个流，它可以有更多的来。

这就是为什么http有其特殊的方式来指定总字节数。它位于响应标头Content-Length中。因此，您必须致电url.openConnection()，然后致电getHeaderField("Content-Length")对象上的URLConnection。它将以字符串形式返回字节数。然后使用Integer.parseInt(bytesString)，您将获得总数。

使用java apache commons下载文件？

2 个答案: