java - 为什么JPanel的paintComponent(Graphics g)不运行？

Question

根据我之前的经验，面板的paintComponent(Graphics g)会在我初始化它时运行，并且我可以通过调用repaint()方法使其重新绘制。但我下面的演示并不像我的经验那样工作。这是怎么回事？

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;



public class BackgroundTest {
    public static void main(String[] args) {
        BackgroundTest backgroundTest = new BackgroundTest();
        try {
            backgroundTest.createUI();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public void createUI() throws IOException{
        JFrame frame = new JFrame("Background Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(true);
        JPanel mainPanel = new JPanel();

        JLabel backgroundLabel = new JLabel(new ImageIcon("background.png"));       
        JPanel imagePanel = new ImagePanel();
        imagePanel.setPreferredSize(new Dimension(626, 434)); // it's the size of house.png
        JScrollPane scrollPane = new JScrollPane(imagePanel);
        backgroundLabel.add(scrollPane,BorderLayout.CENTER);


        mainPanel.add(backgroundLabel); 
        frame.add(mainPanel,BorderLayout.CENTER);   

        frame.getContentPane().add(backgroundLabel,BorderLayout.CENTER);    
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }   

    @SuppressWarnings("serial")
    class ImagePanel extends JPanel {
        protected void paintComponent(Graphics g) {
            System.out.println("I'm not be run");
            super.paintComponent(g);
            BufferedImage image = null;
            try {
                image = ImageIO.read(new File("house.png"));
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            g.drawImage(image, 0, 0, null);
        }
    }
}

Answer 1

But my demo below doesn't work like my experience. What's wrong with it?

永远不要加载，在paint/paintComponent中提供任何FileIO，将此Object准备为局部变量，意味着代码行

BufferedImage image = null;
try {
    image = ImageIO.read(new File("house.png"));
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}