为什么我的下载进度条会多次触发同一个事件？

Question

我正在练习 Swing，我编写了一个下载进度条，以便在用户按下 "Start download" 按钮时下载图像。下载有效。问题是在我的终端中，我可以看到同一事件 (propertyChange) 被多次触发，每次后续下载的次数都在增加。我已经用检查点调试了我的代码，但我仍然不确定为什么会这样。

更具体地说，在我的终端中，我看到类似

的内容

...100% completed 
...100% completed 
...100% completed 
...100% completed 
...100% completed 
...100% completed 
...100% completed

当我希望只看到一次“...100% 完成”时。显示的“...100% 完成”的数量随着每次下载而累积。我不确定这是否会影响我的下载性能，但我想知道为什么会这样。

ProgressBar.java:

package download_progress_bar;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ProgressBar {
    private JFrame frame;
    private JPanel gui;
    private JButton button;
    private JProgressBar progressBar;

    public ProgressBar() {
        customizeFrame();
        createMainPanel();
        createProgressBar();
        createButton();
        addComponentsToFrame();
        frame.setVisible(true);
    }

    private void customizeFrame() {
        // Set the look and feel to the cross-platform look and feel
        try {
            UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
        } catch (Exception e) {
            System.err.println("Unsupported look and feel.");
            e.printStackTrace();
        }

        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
    }

    private void createMainPanel() {
        gui = new JPanel();
        gui.setLayout(new BorderLayout());
    }

    private void createProgressBar() {
        progressBar = new JProgressBar(0, 100);
        progressBar.setStringPainted(true);  // renders a progress string
    }

    private void createButton()  {
        button = new JButton("Start download");
    }

    private void addComponentsToFrame() {
        gui.add(progressBar, BorderLayout.CENTER);
        gui.add(button, BorderLayout.SOUTH);
        frame.add(gui);
        frame.pack();
    }

    // Add passed ActionListener to the button
    void addButtonListener(ActionListener listener) {
        button.addActionListener(listener);
    }

    // Get progress bar
    public JProgressBar getProgressBar() {
        return progressBar;
    }

    // Enable or disable button
    public void turnOnButton(boolean flip) {
        button.setEnabled(flip);
    }
}

Downloader.java:

package download_progress_bar;

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

public class Downloader {
    private URL url;
    private int percentCompleted;
    private PropertyChangeSupport pcs;

    public Downloader() {
        pcs = new PropertyChangeSupport(this);
    }

    // Set URL object
    public void setURL(String src) throws MalformedURLException {
        url = new URL(src);
    }

    // Add passed PropertyChangeListener to pcs
    public void addListener(PropertyChangeListener listener) {
        pcs.addPropertyChangeListener(listener);
    }

    public void download() throws IOException {
        // Open connection on URL object
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();

        // Check response code (always do this first)
        int responseCode = connection.getResponseCode();
        System.out.println("response code: " + responseCode);
        if (responseCode == HttpURLConnection.HTTP_OK) {
            // Open input stream from connection
            BufferedInputStream in = new BufferedInputStream(connection.getInputStream());
            // Open output stream for file writing
            BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream("cat.jpg"));

            int totalBytesRead = 0;
            //int percentCompleted = 0;
            int i = -1;
            while ((i = in.read()) != -1) {
                out.write(i);
                totalBytesRead++;

                int old = percentCompleted;
                percentCompleted = (int)(((double)totalBytesRead / (double)connection.getContentLength()) * 100.0);
                pcs.firePropertyChange("downloading", old, percentCompleted);

                System.out.println(percentCompleted);  // makes download a bit slower, comment out for speed
            }

            // Close streams
            out.close();
            in.close();
        }
    }
}

Controller.java:

package download_progress_bar;

import java.util.concurrent.ExecutionException;
import javax.swing.*;
import java.awt.event.*;
import java.util.List;
import java.net.*;
import java.io.*;
import java.beans.*;

public class Controller {
    private ProgressBar view;
    private Downloader model;
    private JProgressBar progressBar;
    private SwingWorker<Void, Integer> worker;

    public Controller(ProgressBar theView, Downloader theModel) {
        view = theView;
        model = theModel;
        progressBar = view.getProgressBar();

        // Add button listener to the "Start Download" button
        view.addButtonListener(new ButtonListener());
    }

    class ButtonListener implements ActionListener {
        /**
         * Invoked when user clicks the button.
         */
        public void actionPerformed(ActionEvent evt) {
            view.turnOnButton(false);
            progressBar.setIndeterminate(true);
            // NOTE: Instances of javax.swing.SwingWorker are not reusable, 
            // so we create new instances as needed
            worker = new Worker();
            worker.addPropertyChangeListener(new PropertyChangeListener() {
                @Override
                public void propertyChange(PropertyChangeEvent evt) {
                    if (evt.getPropertyName().equals("progress")) {
                        progressBar.setIndeterminate(false);
                        progressBar.setValue(worker.getProgress());
                    }
                }
            });
            worker.execute();
        }
    }

    class Worker extends SwingWorker<Void, Integer> implements PropertyChangeListener {
        /* 
         * Download task. Executed in worker thread.
         */
        @Override
        protected Void doInBackground() throws MalformedURLException {
            model.addListener(this);
            try {
                String src = "https://lh3.googleusercontent.com/l6JAkhvfxbP61_FWN92j4ulDMXJNH3HT1DR6xrE7MtwW-2AxpZl_WLnBzTpWhCuYkbHihgBQ=s640-h400-e365";
                model.setURL(src);
                model.download();
            } catch (IOException ex) {
                System.out.println(ex);
                this.cancel(true);
            }   
            return null;
        }

        /*
         * Executed in event dispatching thread
         */
        @Override
        protected void done() {
            try {
                if (!isCancelled()) {
                    get();  // throws an exception if doInBackground throws one
                    System.out.println("File has been downloaded successfully!");
                }
            } catch (InterruptedException x) {
                x.printStackTrace();
                System.out.println("There was an error in downloading the file.");
            } catch (ExecutionException x) {
                x.printStackTrace();
                System.out.println("There was an error in downloading the file.");
            }

            view.turnOnButton(true);
        }

        /**
         * Invoked in the background thread of Downloader.
         */
        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            this.setProgress((int) evt.getNewValue());
            System.out.println("..." + this.getProgress() + "% completed");
        }
    }
}

Main.java:

package download_progress_bar;

import javax.swing.SwingUtilities;

/**
 * Runs the download progress bar application.
 */
public class Main {
    public static void main(String[] args) {
        // Schedule a job for the event-dispatching thread:
        // creating and showing this application's GUI.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                // Create view
                ProgressBar view = new ProgressBar();
                // NOTE: Should model/controller be created outside invokeLater?
                // Create model
                Downloader model = new Downloader();
                // Create controller
                Controller controller = new Controller(view, model);
            }
        });
    }
}

编辑：我更新了我的代码以反映建议的更改。但是即使进行了更改，问题仍然存在。我仍然看到多次调用“...100% 完成”，调用次数随着每次后续下载而增加。例如，我运行应用程序并第一次按下下载按钮，我会看到

...100% completed

我再次按下下载按钮。我明白了

...100% completed
...100% completed

我再次按下下载按钮...

...100% completed
...100% completed
...100% completed

等等。为什么会这样？

Answer 1

如图所示 here and here, SwingWorker maintains two bound properties: state and progress. Invoking setProgress() 确保“PropertyChangeListeners 在 事件调度线程 上被异步通知。”只需将 PropertyChangeListener 添加到进度条并在 doInBackground() 的实现中调用 setProgress() 或它调用的方法，例如 download()。方便地，"For performance purposes, all these invocations are coalesced into one invocation with the last invocation argument only."

Answer 2

有可能，由于计算百分比的方式，当还有一些工作要完成时，它会报告 100%

在我的测试过程中，我观察到...

//...
98
...
99
99
...
100

所以在代码完成之前重复了很多值。

我在您的下载代码中注意到一些 issues/oddities，主要是您完全忽略了 percentCompleted 属性，所以我将其更改为更像...

public void download() throws IOException {
    // Open connection on URL object
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();

    // Check response code (always do this first)
    int responseCode = connection.getResponseCode();
    System.out.println("response code: " + responseCode);
    if (responseCode == HttpURLConnection.HTTP_OK) {
        // Open input stream from connection
        BufferedInputStream in = new BufferedInputStream(connection.getInputStream());
        // Open output stream for file writing
        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream("cat.jpg"));

        int totalBytesRead = 0;
        //int percentCompleted = 0;
        int i = -1;
        while ((i = in.read()) != -1) {
            out.write(i);
            totalBytesRead++;

            int old = percentCompleted;
            percentCompleted = (int) (((double) totalBytesRead / (double) connection.getContentLength()) * 100.0);
            pcs.firePropertyChange("downloading", old, percentCompleted);

            System.out.println(percentCompleted);  // makes download a bit slower, comment out for speed
        }

        // Close streams
        out.close();
        in.close();
    }
}

对我来说，我会稍微更改代码，而不是...

@Override
protected void process(List<Integer> chunks) {
    int percentCompleted = chunks.get(chunks.size() - 1); // only interested in the last value reported each time
    progressBar.setValue(percentCompleted);

    if (percentCompleted > 0) {
        progressBar.setIndeterminate(false);
        progressBar.setString(null);
    }
    System.out.println("..." + percentCompleted + "% completed");
}

/**
 * Invoked when a progress property of "downloading" is received.
 */
@Override
public void propertyChange(PropertyChangeEvent evt) {
    if (evt.getPropertyName().equals("downloading")) {
        publish((Integer) evt.getNewValue());
    }
}

您应该利用 SwingWorker 的内置进度支持，例如...

/**
 * Invoked when a progress property of "downloading" is received.
 */
@Override
public void propertyChange(PropertyChangeEvent evt) {
    setProgress((int)evt.getNewValue());
}

这意味着您需要将 PropertyChangeListener 附加到 SwingWorker

/**
 * Invoked when user clicks the button.
 */
public void actionPerformed(ActionEvent evt) {
    view.turnOnButton(false);
    progressBar.setIndeterminate(true);
    // NOTE: Instances of javax.swing.SwingWorker are not reusable, 
    // so we create new instances as needed
    worker = new Worker();
    worker.addPropertyChangeListener(new PropertyChangeListener() {
        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            if ("progress".equals(evt.getPropertyName())) {
                progressBar.setIndeterminate(false);
                progressBar.setValue(worker.getProgress());
            }
        }
    });
    worker.execute();
}

这样做的副作用是，您知道有一种方法可以在 SwingWorker 的 state 更改时也收到通知，以检查 DONE

已更新

好的，在检查代码之后，我再次看到您每次执行 SwingWorker[=34= 时都会向 model 添加一个新的 PropertyChangeListener ]

/* 
 * Download task. Executed in worker thread.
 */
@Override
protected Void doInBackground() throws MalformedURLException, InterruptedException {
    model.addListener(this); // Add another listener...
    try {
        String src = "https://lh3.googleusercontent.com/l6JAkhvfxbP61_FWN92j4ulDMXJNH3HT1DR6xrE7MtwW-2AxpZl_WLnBzTpWhCuYkbHihgBQ=s640-h400-e365";
        model.setURL(src);
        model.download();
    } catch (IOException ex) {
        System.out.println(ex);
        this.cancel(true);
    }
    return null;
}

因为model是Controller的一个实例字段，这是有累加效应的。

一个解决方案可能是将 Downloader 添加为 model 的侦听器，但这需要您确保对 UI 执行的任何更新都是同步的正确。

一个更好的通用解决方案是添加支持以在工作程序完成后删除侦听器

public class Downloader {
    //...        
    public void removeListener(PropertyChangeListener listener) {
        pcs.removePropertyChangeListener(listener);
    }

然后在SwingWorkers done方法中，移除监听器...

/*
 * Executed in event dispatching thread
 */
@Override
protected void done() {
    model.removeListener(this);

为什么我的下载进度条会多次触发同一个事件？

Why is my download progress bar firing the same event multiple times?

java

swing

swingworker

jprogressbar

已更新