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Java Swing - 如何在JPanel上显示作为背景的图像

jpanel 【jpanel】
我们想知道如何在JPanel上显示作为背景的图像。
代码展示 运行结果 
import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.Image;
import java.net.URL;

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

public class Main {
  public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
      @Override
      public void run() {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new LoadImage());
        f.pack();
        f.setVisible(true);
      }
    });
  }
}

class LoadImage extends JPanel {

  private Image image;

  public LoadImage() {
    super(new GridLayout());
    try {
      image = ImageIO.read(new URL("http://www.w3cschool.cn/style/download.png"));
    } catch (Exception ex) {
      ex.printStackTrace(System.err);
    }
    int w = image.getWidth(null) / 2;
    int h = image.getHeight(null) / 2;
    this.add(new JLabel(new ImageIcon(image.getScaledInstance(w, h,
        Image.SCALE_SMOOTH))));
  }
}