下载文件时同时更新JProgressBar

Question

我已经尝试了很多不同的方法来让这个工作，但它们要么不能与进度条一起使用，要么不能按我想要的方式工作。

我已经创建了一个带有进度条的新窗口，并需要创建一种方法，允许我在下载文件的同时更新 JProgressBar。有一个非常容易使用的Apache Commons方法可用于下载文件，但似乎不兼容 JProgressBar 。

当在另一个线程中运行时，SwingUtilities.invokeLater 似乎无法更新进度条，但可以正常运行，因为我可以将其输出到控制台。我甚至尝试过 progressBar.repaint() 方法。

所以我想要的是一种方法，可以在下载文件的同时更新 JProgressBar ，以反映下载的状态。

提前感谢！ Keir

- Keir Nellyer

Answer 1

根据这篇文章，我建议您编写一个Download类，可以轻松更新进度条。

这是Download类：

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

// This class downloads a file from a URL.
class Download extends Observable implements Runnable {

// Max size of download buffer.
private static final int MAX_BUFFER_SIZE = 1024;

// These are the status names.
public static final String STATUSES[] = {"Downloading",
"Paused", "Complete", "Cancelled", "Error"};

// These are the status codes.
public static final int DOWNLOADING = 0;
public static final int PAUSED = 1;
public static final int COMPLETE = 2;
public static final int CANCELLED = 3;
public static final int ERROR = 4;

private URL url; // download URL
private int size; // size of download in bytes
private int downloaded; // number of bytes downloaded
private int status; // current status of download

// Constructor for Download.
public Download(URL url) {
    this.url = url;
    size = -1;
    downloaded = 0;
    status = DOWNLOADING;

    // Begin the download.
    download();
}

// Get this download's URL.
public String getUrl() {
    return url.toString();
}

// Get this download's size.
public int getSize() {
    return size;
}

// Get this download's progress.
public float getProgress() {
    return ((float) downloaded / size) * 100;
}

// Get this download's status.
public int getStatus() {
    return status;
}

// Pause this download.
public void pause() {
    status = PAUSED;
    stateChanged();
}

// Resume this download.
public void resume() {
    status = DOWNLOADING;
    stateChanged();
    download();
}

// Cancel this download.
public void cancel() {
    status = CANCELLED;
    stateChanged();
}

// Mark this download as having an error.
private void error() {
    status = ERROR;
    stateChanged();
}

// Start or resume downloading.
private void download() {
    Thread thread = new Thread(this);
    thread.start();
}

// Get file name portion of URL.
private String getFileName(URL url) {
    String fileName = url.getFile();
    return fileName.substring(fileName.lastIndexOf('/') + 1);
}

// Download file.
public void run() {
    RandomAccessFile file = null;
    InputStream stream = null;

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

        // Specify what portion of file to download.
        connection.setRequestProperty("Range",
                "bytes=" + downloaded + "-");

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

        // Make sure response code is in the 200 range.
        if (connection.getResponseCode() / 100 != 2) {
            error();
        }

        // Check for valid content length.
        int contentLength = connection.getContentLength();
        if (contentLength < 1) {
            error();
        }

  /* Set the size for this download if it
     hasn't been already set. */
        if (size == -1) {
            size = contentLength;
            stateChanged();
        }

        // Open file and seek to the end of it.
        file = new RandomAccessFile(getFileName(url), "rw");
        file.seek(downloaded);

        stream = connection.getInputStream();
        while (status == DOWNLOADING) {
    /* Size buffer according to how much of the
       file is left to download. */
            byte buffer[];
            if (size - downloaded > MAX_BUFFER_SIZE) {
                buffer = new byte[MAX_BUFFER_SIZE];
            } else {
                buffer = new byte[size - downloaded];
            }

            // Read from server into buffer.
            int read = stream.read(buffer);
            if (read == -1)
                break;

            // Write buffer to file.
            file.write(buffer, 0, read);
            downloaded += read;
            stateChanged();
        }

  /* Change status to complete if this point was
     reached because downloading has finished. */
        if (status == DOWNLOADING) {
            status = COMPLETE;
            stateChanged();
        }
    } catch (Exception e) {
        error();
    } finally {
        // Close file.
        if (file != null) {
            try {
                file.close();
            } catch (Exception e) {}
        }

        // Close connection to server.
        if (stream != null) {
            try {
                stream.close();
            } catch (Exception e) {}
        }
    }
}

// Notify observers that this download's status has changed.
private void stateChanged() {
    setChanged();
    notifyObservers();
}
}

如您所见，这个Download类有一些特定的字段，比如size和downloaded。

在其他方法中，您可以这样编写：

JProgressBar j = new JProgressBar(0,download.getSize());

在此之后，您可以启动一个新的线程，以每10毫秒为间隔更新进度条。

j.setValue(download.getDownloaded());

希望这能对你有所帮助。

- SomeJavaGuy

---

我知道回答帖子的时间有点晚了，但是请问getFileName(url)在哪一行中？因为我替换掉getFileName(url)为我想要保存下载文件的路径后，它并没有做任何事情。我不明白如何保存下载的文件，请帮忙解决一下。 - Vali Maties

我成功解决了我的问题，所以没事了。是我的错，保存文件的路径设置错误了。 - Vali Maties

Answer 2

在新线程中下载文件正常工作：

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.LockSupport;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JProgressBar;

/**
 * 
 * @author adyliu([e-mail])
 * @since 2012-12-28
 */
public class JProgressBarDemo {

    public static void main(String[] args) {
        final JProgressBar pbFile = new JProgressBar();
        pbFile.setValue(0);
        pbFile.setMaximum(100);
        pbFile.setStringPainted(true);
        pbFile.setBorder(BorderFactory.createTitledBorder("Download file"));

        JFrame theFrame = new JFrame("ProgressBar Demo");
        theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Container contentPane = theFrame.getContentPane();
        contentPane.add(pbFile, BorderLayout.SOUTH);
        final JButton btnDownload = new JButton("Download");
        contentPane.add(btnDownload);
        final AtomicBoolean running = new AtomicBoolean(false);
        btnDownload.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                running.set(!running.get());
                btnDownload.setText(running.get() ? "Pause" : "Continue");
                if (running.get()) {
                    new Thread() {
                        public void run() {
                            //download file in a thread
                            int v = 0;
                            while (running.get() && v < pbFile.getMaximum()) {
                                pbFile.setValue(++v);
                                LockSupport.parkNanos(TimeUnit.MILLISECONDS.toNanos(200));
                            }
                        }
                    }.start();
                }
            }
        });
        theFrame.setSize(300, 150);
        theFrame.setLocationRelativeTo(null);
        theFrame.setVisible(true);
    }

}

- imxylz