JLabel lbBg = new JLabel(imageIcon);
lbBg.setBounds(0, 0, frameSize.width, frameSize.height);
this.getContentPane().add(lbBg);

@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawImage(image, 0, 0, d.width, d.height, this);
MainFrame.instance().repaint();
}

package frame;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class ImageFrame extends JFrame {
  class ImagePanel extends JPanel {
    Dimension d;
    Image image;
    public ImagePanel(Dimension d, Image image) {
      super();
      this.d = d;
      this.image = image;
    }
    @Override
    public void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.drawImage(image, 0, 0, d.width, d.height, this);
      MainFrame.instance().repaint();
    }
  }
  Dimension frameSize = new Dimension(500, 300);
  ImageIcon imageIcon = new ImageIcon(this.getClass().getResource(
      "/images/bg.jpg"));
  public ImageFrame() {
    // 设置窗体属性
    setSize(frameSize);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setIconImage(imageIcon.getImage());
    setUndecorated(true);
  }
  public void addImageByJLable() {
    setLayout(null);
    // 设置背景
    JLabel lbBg = new JLabel(imageIcon);
    lbBg.setBounds(0, 0, frameSize.width, frameSize.height);
    this.getContentPane().add(lbBg);
    addComponents();
    setVisible(true);
  }
  public void addImageByRepaint() {
    ImagePanel imagePanel = new ImagePanel(frameSize, imageIcon.getImage());
    setContentPane(imagePanel);
    addComponents();
    setVisible(true);
  }
  private void addComponents() {
    JButton btn1 = new JButton("haha");
    btn1.setBounds(10, 20, 60, 30);
    this.getContentPane().add(btn1);
    JTextField jtf = new JTextField("22222222222");
    jtf.setBounds(200, 100, 80, 30);
    this.getContentPane().add(jtf);
  }
  public static void main(String[] args) {
    // TODO Auto-generated method stub
    ImageFrame imageFrame = new ImageFrame();
    // imageFrame.addImageByJLable();
    imageFrame.addImageByRepaint();
  }
}

package awtDemo;

import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
@SuppressWarnings("serial")
public class Ss extends JFrame {
    Container con = this.getContentPane();
    JButton jb1 = new JButton("jb1");
    JButton jb2 = new JButton("jb2");
    JLabel jl1 = new JLabel("jl1");
    FlowLayout gly = new FlowLayout();
    JPanel jp = new JPanel(gly);
    public Ss() {
        con.add(jp);
        jp.add(jb1);
        jp.add(jb2);
        MyListener ml = new MyListener();
        jb1.addMouseListener(ml);
        this.setSize(300, 200);
        this.setVisible(true);
    }
    private class MyListener extends MouseAdapter {
      @Override
      public void mouseClicked(MouseEvent e) {
          // TODO Auto-generated method stub
          try {
              JButton jb3 = new JButton("jb3");
              jp.add(jb3);
//              jp.updateUI();//可以正常显示
              jp.repaint();//API中对repaint()方法是这样描述的，调度完当前所有未完成的事件后重新绘制该组件，repaint方法不总是马上执行，所以只有调整大小才可以显示。
          //主要就是下面的invalidate和validate
          //当然，用jp来invalidate和validatae也是可以的
//                jp.invalidate();
          } catch (Exception ex) {
              ex.printStackTrace();
          }
      }
  }
    public static void main(String s[]) {
      Ss sss = new Ss();
      sss.setVisible(true);
  }
}