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);//設置內容面板為透明       this.setContentPane(rp);       this.setUndecorated(true);       this.setSize(rp.img.getIconWidth(),rp.img.getIconHeight());       AWTUtilities.setWindowOpaque(this, false);//設置為JFrame為透明       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) {     /**      * 設置背景圖和背景圖的透明度，0為全透明，1.0f為不透明。      */     myJFrame f = new myJFrame("ab.png",0.7f);      f.setLayout(null);     Font font = new Font("宋體",Font.PLAIN,30);     JLabel user = new JLabel("用戶名");     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("密碼");     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);   } }