java BuffereImage没有在JPanel中显示

Question

我试图测试->this answer <-中介绍的技术

    private void jButton1MouseClicked(java.awt.event.MouseEvent evt) {

        BufferedImage img = new BufferedImage(plotPanel.getWidth(),plotPanel.getHeight(),BufferedImage.TYPE_4BYTE_ABGR);
        Graphics2D g2d = img.createGraphics();

        g2d.setColor(Color.red);        
        g2d.drawLine(0, 0, plotPanel.getWidth(), plotPanel.getHeight());

        JLabel picLabel = new JLabel(new ImageIcon( img ));

        plotPanel.add(picLabel);
        plotPanel.revalidate();
        plotPanel.repaint();
    }

为什么绘图面板仍然完好无损

更新

以下是备受期待的SSCCE：

package javaapplication10;

import javax.swing.SwingUtilities;


public class JavaApplication10 {

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

    }
}

---

package javaapplication10;

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import javax.swing.ImageIcon;
import javax.swing.JLabel;


public class NewJFrame extends javax.swing.JFrame {

    /**
     * Creates new form NewJFrame
     */
    public NewJFrame() {
        initComponents();
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        plotPanel = new javax.swing.JPanel();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        plotPanel.setBackground(new java.awt.Color(255, 255, 255));

        javax.swing.GroupLayout plotPanelLayout = new javax.swing.GroupLayout(plotPanel);
        plotPanel.setLayout(plotPanelLayout);
        plotPanelLayout.setHorizontalGroup(
            plotPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 0, Short.MAX_VALUE)
        );
        plotPanelLayout.setVerticalGroup(
            plotPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 379, Short.MAX_VALUE)
        );

        jButton1.setText("jButton1");
        jButton1.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseClicked(java.awt.event.MouseEvent evt) {
                jButton1MouseClicked(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(plotPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jButton1, javax.swing.GroupLayout.PREFERRED_SIZE, 116, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addGap(0, 758, Short.MAX_VALUE)))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(plotPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jButton1)
                .addContainerGap(124, Short.MAX_VALUE))
        );

        plotPanel.getAccessibleContext().setAccessibleName("plotPanel");
        plotPanel.getAccessibleContext().setAccessibleDescription("");

        pack();
    }// </editor-fold>

    private void jButton1MouseClicked(java.awt.event.MouseEvent evt) {
           BufferedImage img = new BufferedImage(plotPanel.getWidth(),plotPanel.getHeight(),BufferedImage.TYPE_4BYTE_ABGR);
        Graphics2D g2d = img.createGraphics();

        g2d.setColor(Color.red);        
        g2d.drawLine(0, 0, plotPanel.getWidth(), plotPanel.getHeight());

        JLabel picLabel = new JLabel(new ImageIcon( img ));

        plotPanel.add(picLabel);
        plotPanel.revalidate();
        plotPanel.repaint();

    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }


        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewJFrame().setVisible(true);
            }
        });
    }

    private javax.swing.JButton jButton1;
    private javax.swing.JPanel plotPanel;

}

Answer 1

plotPanel正在使用组布局：

plotPanel = new javax.swing.JPanel();
javax.swing.GroupLayout plotPanelLayout = new javax.swing.GroupLayout(plotPanel);
plotPanel.setLayout(plotPanelLayout);

在不指定约束的情况下，不能只向GroupLayout添加组件：

plotPanel.add(picLabel);

我不知道GroupLayout是如何工作的，所以我建议您在GUI生成器中创建JLabel，并将其添加到plotPanel中。也就是说，让GUI生成器为您创建所有约束

然后，当您想要在标签上添加图标时，只需执行以下操作：

picLabel.setIcon(new ImageIcon( img ));

现在，布局管理器将自动布局面板，以包含新图像

Answer 2

确保在添加JLabelpicLabel后调用revalidate：

plotPanel.revalidate();

此外，您正在使用GroupLayout，这要求将水平和垂直顺序组添加到布局中，以便任何新组件都可见。这种布局不是最容易手工编码的，而且会产生很多锅炉板代码。使用更传统的布局管理器熟悉代码。见A Visual Guide to Layout Managers