使JLabel背景在有背景的框架中透明

Question

我在JFrame上有一个JLabel和一个带有图片背景的JFrame。问题是，虽然我的JLabel是不透明的，但它仍然有一个灰色的背景，这是恼人的。
下面是我的代码:

import java.awt.Graphics;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Image;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPasswordField;
import javax.swing.JTextField;
public class MainFrame extends JFrame {
    private JTextField username;
    private JPasswordField password;
    private JLabel passwordLbl;
    private JLabel usernameLbl;
    private GridBagConstraints gc;
    public MainFrame() {
        username = new JTextField(10);
        password = new JPasswordField(10);
        passwordLbl = new JLabel("Password: ");
        usernameLbl = new JLabel("Username: ");
        usernameLbl.setOpaque(true);
        passwordLbl.setOpaque(true);
        setLayout(new GridBagLayout());
        gc = new GridBagConstraints();
        gc.weightx = 1;
        gc.weighty = 1;
        gc.gridx = 1;
        gc.gridy = 0;
        add(username, gc);
        gc.gridx = 1;
        gc.gridy = 1;
        add(password, gc);
        gc.gridx = 0;
        gc.gridy = 0;
        add(usernameLbl, gc);
        gc.gridx = 0;
        gc.gridy = 1;
        add(passwordLbl, gc);
        setSize(400, 600);
        setLocation(400, 50);
        setUndecorated(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }
    @Override
    public void paint(Graphics arg0) {
        Image img = getToolkit().getImage("pics/blue_and_red.jpg");
        arg0.drawImage(img, 0, 0, getWidth(), getHeight(), this);
        username.repaint();
        password.repaint();
        usernameLbl.setOpaque(true);
        usernameLbl.repaint();
        passwordLbl.repaint();
    }
}

Answer 1

您的绘制方法完全分叉是您的问题的原因。不要用油漆画标签，但是要在那里调用super.paint(g)。

更好的是，不要在JFrame的paint方法中绘制，而是在JPanel的paintComponent方法中绘制，并在那里调用super的方法。

。

public class MainPanel extends JPanel {
    private JTextField username;
    private JPasswordField password;
    private JLabel passwordLbl;
    private JLabel usernameLbl;
    private GridBagConstraints gc;
    private Image img;
    public MainPanel() {
        img = getToolkit().getImage("pics/blue_and_red.jpg");
        username = new JTextField(10);
        password = new JPasswordField(10);
        passwordLbl = new JLabel("Password: ");
        usernameLbl = new JLabel("Username: ");
        // usernameLbl.setOpaque(true);
        // passwordLbl.setOpaque(true);
        setLayout(new GridBagLayout());
        gc = new GridBagConstraints();
        gc.weightx = 1;
        gc.weighty = 1;
        gc.gridx = 1;
        gc.gridy = 0;
        add(username, gc);
        gc.gridx = 1;
        gc.gridy = 1;
        add(password, gc);
        gc.gridx = 0;
        gc.gridy = 0;
        add(usernameLbl, gc);
        gc.gridx = 0;
        gc.gridy = 1;
        add(passwordLbl, gc);
    }
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (img != null) {
            g.drawImage(img, 0, 0, getWidth(), getHeight(), this);
        }
        // username.repaint();
        // password.repaint();
        // usernameLbl.setOpaque(true);
        // usernameLbl.repaint();
        // passwordLbl.repaint();
    }
    private static void createAndShowGui() {
      // create JFrame
      JFrame frame = new JFrame("BUI");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      // add our MainPanel to the JFrame
      frame.getContentPane().add(new MainPanel());
      frame.pack(); // pack it
      frame.setLocationByPlatform(true);
      frame.setVisible(true); // show it
   }
   public static void main(String[] args) {
      // this is for starting our Swing app on the event thread
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

最重要的是，阅读Swing标准Swing教程，因为那里有很好的解释。