Java Swing ImageIcon 未在 JLabel 中显示

Question

我发现以下内容MRE每次执行时都会重现相同的错误:

import java.awt.Component;
import java.awt.Graphics;
import java.awt.GraphicsConfiguration;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.Image;
import java.awt.MediaTracker;
import java.awt.image.BufferedImage;
import java.util.Objects;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class IconDelegate implements Icon {
    private final Icon delegated;

    public IconDelegate(final Icon delegated) {
        this.delegated = Objects.requireNonNull(delegated);
    }

    @Override
    public int getIconWidth() {
        return delegated.getIconWidth();
    }

    @Override
    public int getIconHeight() {
        return delegated.getIconHeight();
    }

    @Override
    public void paintIcon(final Component c, final Graphics g, final int x, final int y) {
        delegated.paintIcon(c, g, x, y);
        //Some other drawing operations would be here...
    }

    public static void main(final String[] args) {
        final ImageIcon ii = new ImageIcon();
        final GraphicsEnvironment genv = GraphicsEnvironment.getLocalGraphicsEnvironment();
        final GraphicsDevice gdev = genv.getDefaultScreenDevice();
        final GraphicsConfiguration gcnf = gdev.getDefaultConfiguration();
        final BufferedImage bimg = gcnf.createCompatibleImage(300, 400);
        final Image img = bimg.getScaledInstance(150, 200, Image.SCALE_SMOOTH);
        //img.getWidth(null); //Uncomment this line and the image will be drawn normally!
        ////img.flush(); //Uncomment this line and the image will not be drawn.
        ii.setImage(img);
        System.out.println(ii.getImageLoadStatus() == MediaTracker.ABORTED);
        final JLabel lbl = new JLabel(new IconDelegate(ii));
        final JFrame frame = new JFrame("ImageIcon delegate.");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(lbl);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

我希望任何人都可以重现的错误是 ImageIcon的图像永远不会绘制在显示的标签/框架中。

当我尝试调试它时，我发现图像加载状态( ImageIcon )等于 MediaTracker.ABORTED其委托(delegate)给 ImageObserver.ABORT目标Component (实现 ImageObserver ) MediaTracker ImageIcon的实例，这又(根据 ImageObserver 的文档)与其 imageUpdate 有关。方法。

我还注意到，如果我调用 img.getWidth(null);就在将图像设置为 ImageIcon 之前然后图像正常绘制(即错误停止再现)并且图像加载状态不等于MediaTracker.ABORTED不再了。阅读 ImageObserver.imageUpdate 的文档后我见过的方法指出 Image.getWidth(ImageObserver)方法是异步？...

我还注意到，如果我调用 img.flush(); img.getWidth(null);之后调用，然后错误重现。调用img.getWidth(null);刷新图像后，错误将不再重现。这种模式可以重复。因此，我归结为我在 MediaTracker 的文档中找到的可能最重要的信息。 :

If no ImageObservers are observing the image when the first frame has finished loading, the image might flush itself to conserve resources (see Image.flush()).

我必须指出，我确实关心事件的顺序。例如我需要 ImageIcon创建为空，然后使用setImage更新其图像。我这么说是因为，正如我注意到的，创建 ImageIcon并在构建时为其提供图像似乎可以正确绘制图像而不是重现错误。查看 ImageIcon 的差异后的构造函数接受 Image和 ImageIcon.setImage我发现构造函数还从图像中请求属性“comment”，提供 Image.getProperty方法 ImageObserver ...所以我也猜测它是异步？...

我还打过System.out.println(img.getClass());但它最终出现在sun.*中的一个类中包含编译代码但没有文档的包。所以我没能跟上。

我希望我的调试努力不会产生误导。

所以我的问题是:

1. 什么原因导致 MRE 中未绘制图像？
2. 如何避免发生此错误，而不必进行任意调用(例如 img.getWidth(null); 之前的 setImage )？

我还发现了this相关问题，但我看到 ImageIcon类内部使用 MediaTracker那么重复代码有什么意义呢？原因MediaTracker ImageIcon的我不知道中止图像的加载(根据其加载状态)。

编辑 1

更现实且不太最小的 MRE 仍然会重现错误，如下所示:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Objects;
import javax.imageio.ImageIO;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class ImageThumbnail implements Icon {
    private final ImageIcon delegated;
    private final int w, h;
    private String text;

    public ImageThumbnail(final int width, final int height, final String text) {
        w = width;
        h = height;
        this.text = Objects.toString(text).trim();
        delegated = new ImageIcon();
    }

    @Override
    public int getIconWidth() {
        return w;
    }

    @Override
    public int getIconHeight() {
        return h;
    }

    public void setText(final String text) {
        this.text = Objects.toString(text).trim();
    }

    //Scales the given image to fit this icon's width and height maintaining aspect ratio:
    public void setImage(final BufferedImage bimg) {
        final int bimgw = bimg.getWidth(),
                  bimgh = bimg.getHeight();
        if (bimgw > bimgh)
            delegated.setImage(bimg.getScaledInstance(Math.min(w, bimgw), -1, Image.SCALE_SMOOTH));
        else
            delegated.setImage(bimg.getScaledInstance(-1, Math.min(h, bimgh), Image.SCALE_SMOOTH));
        //Alternatively you can try the following code if you don't trust the -1 arguments above:
        //delegated.setImage(bimg.getScaledInstance(w, h, Image.SCALE_SMOOTH));
    }

    @Override
    public void paintIcon(final Component c, final Graphics g, final int x, final int y) {
        delegated.paintIcon(c, g, x, y);
        //Other drawing operations here, like drawing a String, at the center, over the ImageIcon:
        g.drawString(text, x + w / 2 - g.getFontMetrics().stringWidth(text) / 2, y + h / 2);
    }

    private static JPanel centered(final Component c) {
        /*Creating a JPanel with GridBagLayout and a single component
        in it, will layout the component in the center of the JPanel.*/
        final JPanel panel = new JPanel(new GridBagLayout());
        panel.add(c);
        return panel;
    }

    public static void main(final String[] args) {
        SwingUtilities.invokeLater(() -> {

            final ImageThumbnail thumb = new ImageThumbnail(200, 200, "Select an image...");

            final JLabel lbl = new JLabel(thumb);

            final JFileChooser chooser = new JFileChooser();
            chooser.setFileSelectionMode(JFileChooser.FILES_ONLY);
            chooser.setMultiSelectionEnabled(false);

            final JButton load = new JButton("Load image");
            load.addActionListener(e -> {
                if (chooser.showOpenDialog(load) == JFileChooser.APPROVE_OPTION) {
                    try {
                        thumb.setImage(ImageIO.read(chooser.getSelectedFile()));
                        thumb.setText("Image-001");
                        lbl.setForeground(Color.GREEN.darker());
                        //lbl.repaint(); //Not needed, as I call 'lbl.setForeground(Color.GREEN.darker());' above...
                    }
                    catch (final IOException iox) {
                        iox.printStackTrace();
                    }
                }
            });

            final JPanel contents = new JPanel(new BorderLayout());
            contents.add(centered(lbl), BorderLayout.CENTER);
            contents.add(centered(load), BorderLayout.PAGE_END);

            final JFrame frame = new JFrame("Thumbnail of images.");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.getContentPane().add(contents);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}

我知道我可以使用SwingWorker在按钮上加载图像，这样用户就不会遇到延迟，但为了让事情更简单，我更喜欢阻止 EDT 一段时间。

Answer 1

How to prevent this error from happening without having to make arbitrary calls

使用您的原始代码，我得到了 NPE。

以下似乎有效。我现在看到一个黑色图像:

//final ImageIcon ii = new ImageIcon();
…
//ii.setImage(img);
ImageIcon ii = new ImageIcon( img );

编辑:

我尝试使用不同的方法，包括确保标签具有 ImageObserver。对我来说没有任何作用。

import java.awt.*;
import javax.swing.*;
import javax.swing.text.*;
import java.awt.image.*;

public class IconMRE extends JPanel
{
    private ImageIcon icon = new ImageIcon( new BufferedImage(200, 200, BufferedImage.TYPE_INT_RGB) );
    private JLabel label1 = new JLabel( icon );

    public IconMRE()
    {
        setLayout( new BorderLayout(20, 20) );

        label1 = new JLabel( icon );
        add(label1, BorderLayout.LINE_START);

        JLabel label2 = new JLabel();
        icon.setImageObserver( label2 );
        label2.setIcon( icon );
        add(label2, BorderLayout.LINE_END);


        JComboBox<Color>comboBox = new JComboBox<Color>();
        comboBox.addItem( Color.RED );
        comboBox.addItem( Color.GREEN );
        comboBox.addItem( Color.BLUE );
        add(comboBox, BorderLayout.PAGE_START);

        comboBox.addActionListener( (e) ->
        {
            Color background = (Color)comboBox.getSelectedItem();
            BufferedImage image = new BufferedImage(200, 200, BufferedImage.TYPE_INT_RGB);
            Graphics2D g2d = image.createGraphics();
            g2d.setColor( background );
            g2d.fillRect(0, 0, 200, 200);
            g2d.dispose();
//          image.getWidth(null);
            icon.setImage( image );
//          label1.repaint();
        });

        comboBox.setSelectedIndex(2);
    }

    private static void createAndShowUI()
    {
        JFrame frame = new JFrame("IconMRE");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( new IconMRE() );
        frame.pack();
        frame.setLocationRelativeTo( null );
        frame.setVisible( true );
    }

    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                createAndShowUI();
            }
        });
    }
}

唯一有效的是在标签上强制重绘()。