java - JPanel 背景图像不适用于其中的 JPanel

Question

我正在开发一个简单的注册窗口，它会在 Java 应用程序打开时出现。

这是一个 JFrame，里面有一个 JPanel，它有文本字段、标签，另一个面板也包含文本字段和标签。我的问题是外部面板有背景图像，但它不适用于内部面板，如下所示:

这是整个窗口代码:

public void start() {

    try {
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (Exception ex) {
        ex.printStackTrace();
    }

    Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
    frame = new JFrame("Chat");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    //frame.setMaximumSize(new Dimension((int)screenSize.getWidth()-1000, (int)screenSize.getHeight()-1000));
    frame.setMinimumSize(new Dimension((int)screenSize.getWidth()/2-200,(int) screenSize.getHeight()/2));
    frame.setResizable(false);

        welcome = new LoginPanel();
        welcome.setLayout(new BoxLayout(welcome, BoxLayout.Y_AXIS));
        welcome.setBorder(BorderFactory.createEmptyBorder(50, welcome.getWidth()/2-500, 50, welcome.getWidth()/2 -500));

        //repaintThread = new Thread(new RepaintRunnable(frame, welcome));
        //repaintThread.start();


            request = new JLabel("Please fill the required fields below:");
            request.setFont(new Font("Serif", Font.BOLD, 20));
            request.setBorder(BorderFactory.createEmptyBorder(0, 0, 10, 0));
            request.setAlignmentX(Component.CENTER_ALIGNMENT);

            userInfo = new JPanel();
            userInfo.setLayout(new BoxLayout(userInfo, BoxLayout.Y_AXIS));
            userInfo.setAlignmentX(Component.CENTER_ALIGNMENT);

                Font fieldType = new Font("Serif", Font.PLAIN, 15);

                PlainDocument doc = new PlainDocument();
                doc.setDocumentFilter(new NameDocument());

                enterFirstName = new JLabel("First name:");
                enterFirstName.setAlignmentX(Component.LEFT_ALIGNMENT);
                enterFirstName.setFont(fieldType);
                firstName = new JTextField(20);
                firstName.setMaximumSize(firstName.getPreferredSize());
                firstName.setDocument(NameDocument.getNewNameDocument(NameDocument.NO_SPACE));
                firstName.getDocument().addDocumentListener(new ChangeDocumentListener());
                firstName.addActionListener(new ConfirmListener());
                firstName.setAlignmentX(Component.LEFT_ALIGNMENT);

                enterSecName= new JLabel("Surname:");
                enterSecName.setAlignmentX(Component.LEFT_ALIGNMENT);
                enterSecName.setFont(fieldType);
                secName = new JTextField(30);
                secName.setMaximumSize(secName.getPreferredSize());
                secName.setDocument(NameDocument.getNewNameDocument(NameDocument.HAS_SPACE));
                secName.getDocument().addDocumentListener(new ChangeDocumentListener());
                secName.addActionListener(new ConfirmListener());
                secName.setAlignmentX(Component.LEFT_ALIGNMENT);

                enterNickname = new JLabel("Nickname (how other people will see you in chat):");
                enterNickname.setAlignmentX(Component.LEFT_ALIGNMENT);
                enterNickname.setFont(fieldType);
                nickname = new JTextField(30);
                nickname.setMaximumSize(nickname.getPreferredSize());
                nickname.setDocument(NameDocument.getNewNameDocument(NameDocument.NO_SPACE));
                nickname.addActionListener(new ConfirmListener());
                nickname.setAlignmentX(Component.LEFT_ALIGNMENT);

            userInfo.add(enterFirstName);
            userInfo.add(firstName);
            userInfo.add(enterSecName);
            userInfo.add(secName);
            userInfo.add(enterNickname);
            userInfo.add(nickname);

            confirm = new JButton("Submit");
            confirm.setAlignmentX(Component.CENTER_ALIGNMENT);
            confirm.setEnabled(false);
            confirm.addActionListener(new ConfirmListener());

        welcome.add(request);
        welcome.add(userInfo);
        welcome.add(new Box.Filler(new Dimension(10, 10), new Dimension(10, 10), new Dimension(10, 10)));
        welcome.add(confirm);

    frame.getContentPane().add(BorderLayout.CENTER, welcome);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

这是 LoginPanel 代码(外部 JPanel):

public class LoginPanel extends JPanel {
public void paintComponent(Graphics g) {
    try {
        super.paintComponent(g);
        BufferedImage background = ImageIO.read(new File("Background.jpg"));
        g.drawImage(background, 0, 0, getWidth(), getHeight(), null);
    } catch(Exception ex) {
        ex.printStackTrace();
    }
}
}

因为我是 Java 的初学者，如果有人能给我建议如何使这段代码更好，那就太好了。

Answer 1

请记住在任何覆盖显示图像的 JPanel 的内部 JPanel(以及一些其他组件——尽管不是全部)上调用 setOpaque(false);。这将允许背景图像显示出来。您不必对 JLabel 执行此操作，因为默认情况下它们是透明的(非不透明的)，但您可以对 JPanel 执行此操作。

所以对你来说它将是:

userInfo = new JPanel();
userInfo.setOpaque(false);

另一个问题是您不应该永远从 paintComponent 方法中读取图像。这个方法可能会经常被调用，为什么在图像可以而且应该只读一次的时候重新读取图像。更重要的是，这种方法应该尽可能快，因为不必要地减慢它会减慢程序的感知响应速度。一次读取图像，并将其存储在一个变量中，然后在 paintComponent 中显示该变量。

例如，

public class LoginPanel extends JPanel {
    private BufferedImage background;

    public LoginPanel(BufferedImage background) {
        this.background = background;
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (background != null) {
            g.drawImage(background, 0, 0, getWidth(), getHeight(), this);
        }
    }
}

读入图像，然后将其传递给 LoginPanel 类的构造函数。