我尝试了不同的方法来让它工作，但它们要么不能使用进度条，要么不能按照我想要的方式工作。
我已经创建了一个带有进度条的新窗口，并且需要创建一个方法来允许我在下载文件的同时更新JProgressBar. 有一个 Apache Commons 方法非常容易用于下载文件，但它似乎与JProgressBar's.
在另一个线程中运行它时我也遇到了麻烦，SwingUtilities.invokeLater似乎没有更新到进度条，但它确实运行，因为我可以让它打印到控制台。我什至尝试过这种progressBar.repaint()方法。
所以我想要的是一种可以下载文件同时更新 aJProgressBar以反映下载状态的方法。
提前致谢！凯尔
根据这篇文章，我可以建议你写一个Download class，可以很容易地更新进度条。
这是Download课程：
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();
}
}
正如你所看到的，这个Download类有一些特定的字段，比如size和downloaded。
在其他一些方法中，您可以编写：
JProgressBar j = new JProgressBar(0,download.getSize());
在此之后，您可以开始一个新的Thread，它会以一定的间隔更新您的进度条，比如每 10 毫秒，
j.setValue(download.getDownloaded());
希望这可以帮到你。
在新线程中下载文件工作正常：
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(imxylz@gmail.com)
* @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);
}
}