java创建以任意图片为背景的窗口

import com.sun.awt.AWTUtilities; 
import javax.swing.*; 
import java.awt.*; 
public class myJFrame extends JFrame{ 
  private float alpha; 
    public myJFrame(String bgPath,float alpha){ 
      super(); 
      myContentPane rp = new myContentPane(bgPath); 
      rp.setOpaque(false);//设置内容面板为透明 
      this.setContentPane(rp); 
      this.setUndecorated(true); 
      this.setSize(rp.img.getIconWidth(),rp.img.getIconHeight()); 
      AWTUtilities.setWindowOpaque(this, false);//设置为JFrame为透明 
      this.alpha = alpha; 
    } 
  private class myContentPane extends JPanel{ 
    public ImageIcon img; 
    public myContentPane(String bgPath) { 
      super(); 
      img = new ImageIcon(Test.class.getResource(bgPath)); 
    } 
    @Override 
    protected void paintComponent(Graphics g) { 
      AlphaComposite ac = AlphaComposite.getInstance(AlphaComposite.SRC_OVER, alpha); 
      Composite old = ((Graphics2D) g).getComposite(); 
      ((Graphics2D) g).setComposite(ac); 
      if(img!=null){ 
        g.drawImage(img.getImage(), 0, 0, getWidth(), getHeight(), this); 
      } 
      ((Graphics2D) g).setComposite(old); 
      super.paintComponent(g); 
    } 
  } 
} 

import javax.swing.*; 
import java.awt.*; 
public class Test { 
  public static void main(String[] args) { 
    /** 
     * 设置背景图和背景图的透明度，0为全透明，1.0f为不透明。 
     */ 
    myJFrame f = new myJFrame("ab.png",0.7f); 
 
    f.setLayout(null); 
    Font font = new Font("宋体",Font.PLAIN,30); 
    JLabel user = new JLabel("用户名"); 
    user.setFont(font); 
    user.setBounds(100,150,100,30); 
    JTextField userInput = new JTextField(); 
    userInput.setFont(font); 
    userInput.setBounds(200,145,250,40); 
    JLabel ps = new JLabel("密码"); 
    ps.setFont(font); 
    ps.setBounds(110,200,90,30); 
    JTextField psInput = new JTextField(); 
    psInput.setFont(font); 
    psInput.setBounds(200,195,250,40); 
 
    f.add(user); 
    f.add(userInput); 
    f.add(ps); 
    f.add(psInput); 
    f.setLocation(300,100); 
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    f.setVisible(true); 
  } 
}