Question

所以我有一个JTextPane对象需要在运行时将其背景更改为特定图像的背景。我所拥有的似乎非常多（JComboBox用于更改背景并调用repaintBackground()似乎不会在选择时自动关闭等），它也会抛出一个nullpointer而我不知道为什么背景变化。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.text.BoxView.paint(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI$RootView.paint(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI.paintSafely(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI.paint(Unknown Source)
at javax.swing.plaf.synth.SynthEditorPaneUI.paint(Unknown Source)
at javax.swing.plaf.synth.SynthEditorPaneUI.update(Unknown Source)
at javax.swing.JComponent.paintComponent(Unknown Source)
etc etc etc....

这是我的目标：

public class PreviewPane extends JTextPane  {
    private String _name = "bg3";   

    public PreviewPane() {
        super();
        setOpaque(false);
        StyledDocument document = this.getStyledDocument();
        SimpleAttributeSet center = new SimpleAttributeSet();
        StyleConstants.setAlignment(center, StyleConstants.ALIGN_CENTER);
        document.setParagraphAttributes(0, document.getLength(), center, false);
    }

    @Override
    protected void paintComponent(Graphics g)  throws RuntimeException{
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, getWidth(), getHeight());

        BufferedImage img = null;

            try {
                img = ImageIO.read(new File(getClass().getResource("/icons/"+_name+".png").toURI()));
            } catch (IOException | URISyntaxException e) {
                // TODO Auto-generated catch block
                //e.printStackTrace();
            }

         g.drawImage(img, 0, 0, this);


        super.paintComponent(g);
    }

    public void repaintBackground(String bgName){

        _name = bgName;

        paintComponent(this.getGraphics());

    }

}

任何帮助都将不胜感激。

Answer 1

paintComponent(this.getGraphics()); - 否。您永远不必明确调用paintComponent。而是致电repaint()。

super.paintComponent(g);

开头

paintComponent，或者至少在使用Graphics上下文进行任何绘制之前调用。
请勿使用paintComponent方法加载图片。一种选择是将缓存保留在Map<String, Image>中。这样您就可以轻松地引用它们，而无需在每次想要更改时加载它们。总的来说，无论你是否决定缓存它们，它都不是的大问题。您只需在repaintBackground方法中加载它。
保留班级成员Image image;。这将是您用来绘制的Image。您的repaintBackground，我会接受Image而不是字符串。您传递的Image将是用于绘画的类成员Image image。如果您决定从该方法加载图像，您仍然可以让方法接受String。
```
classs MyPanel extends JPanel {
    Image image;

    public void repaintBackground(Image image) {
        this.image = image;
        repaint();
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(image);
    }
}
```
paintComponent不应抛出RuntimeException

这是一个完整的例子。我决定使用Map缓存。由你决定如何做到这一点。有很多方法可以解决这个问题。

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

public class ImageChangeDemo {

    private ImagePanel imagePanel = new ImagePanel();

    public ImageChangeDemo() {
        JFrame frame = new JFrame();
        frame.add(imagePanel);
        frame.add(createCombo(), BorderLayout.PAGE_START);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private JComboBox createCombo() {
        String[] items = {
            ImagePanel.DIRECTORY,
            ImagePanel.COMPUTER,
            ImagePanel.FILE,
            ImagePanel.FLOPPY,
            ImagePanel.HARD_DRIVE
        };
        JComboBox comboBox = new JComboBox(items);
        comboBox.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                imagePanel.repaintBackground(comboBox.getSelectedItem().toString());
            }
        });
        return comboBox;
    }

    private class ImagePanel extends JPanel {
        public static final String DIRECTORY = "directory";
        public static final String FILE = "file";
        public static final String COMPUTER = "computer";
        public static final String HARD_DRIVE = "harddrive";
        public static final String FLOPPY = "floppy";

        Map<String, Image> images = new HashMap<>();

        private Image currentImage;

        public ImagePanel() {
            initImageMap();
            repaintBackground(DIRECTORY);
        }

        private void initImageMap() {
            ImageIcon dirIcon = (ImageIcon)UIManager.getIcon("FileView.directoryIcon");
            ImageIcon fileIcon =(ImageIcon)UIManager.getIcon("FileView.fileIcon");
            ImageIcon compIcon = (ImageIcon)UIManager.getIcon("FileView.computerIcon");
            ImageIcon hdIcon = (ImageIcon)UIManager.getIcon("FileView.hardDriveIcon");
            ImageIcon flopIcon = (ImageIcon)UIManager.getIcon("FileView.floppyDriveIcon");
            images.put(DIRECTORY, dirIcon.getImage());
            images.put(FILE, fileIcon.getImage());
            images.put(COMPUTER, compIcon.getImage());
            images.put(HARD_DRIVE, hdIcon.getImage());
            images.put(FLOPPY, flopIcon.getImage());
        }

        protected void repaintBackground(String key) {
            currentImage = images.get(key);
            repaint();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(currentImage, 0, 0, getWidth(), getHeight(), this);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(150, 150);
        }
    }

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

Answer 2

首先单独进行图像初始化和绘图。将img装载移出油漆。也不要尝试创建文件。如果无法创建JAR文件中的图像。

public PreviewPane() {
    super();
    setOpaque(false);
    StyledDocument document = this.getStyledDocument();
    SimpleAttributeSet center = new SimpleAttributeSet();
    StyleConstants.setAlignment(center, StyleConstants.ALIGN_CENTER);
    document.setParagraphAttributes(0, document.getLength(), center, false);
}

BufferedImage img = null;

private void initImg() {
    if( img==null) {
        img = ImageIO.read(getClass().getResourceAsStream("/icons/"+_name+".png")));
//process missing img here
    }
}

@Override
protected void paintComponent(Graphics g)  throws RuntimeException{
    g.setColor(Color.WHITE);
    g.fillRect(0, 0, getWidth(), getHeight());
    initImg();

    BufferedImage img = null;

     g.drawImage(img, 0, 0, this);


    super.paintComponent(g);
}

尝试在运行时更改JTextPane的背景，但是成功但有错误

2 个答案: