将背景图片加载到JFrame中的JPanel中时出错

Question

我有一个JFrame，我想将其完全用JPanel占据，并在JPanel中放置背景图像。

代码：

public class InicioSesion extends javax.swing.JFrame{
private Image imagenFondo;
private URL fondo;

public InicioSesion(){
    initComponents();
    try{
        fondo = this.getClass().getResource("fondo.jpg");
        imagenFondo = ImageIO.read(fondo);
    }catch(IOException ex){
        ex.printStackTrace();
        System.out.print("Image dont load"); //Dont load the message.
    }

    Container c = getContentPane();
    c.add(PanelFondo);
}

public JPanel panelFondo = new JPanel(){
    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        g.drawImage(imagenFondo, 0, 0, getWidth(), getHeight(), this);
    }
};

为什么不加载图像？我的代码有解决方案吗？

Answer 1

您的问题在这里：

initComponents();

您可能会使用此方法将所有组件添加到GUI中，很可能使用GroupLayout或其他用户不友好的布局管理器，然后在添加所有组件之后添加panelFondo JPanel。

[如果要让GUI显示背景图像，则需要将组件添加到绘制图像的JPanel，并且如果在图像抽屉的顶部添加了任何JPanel，则它们必须是透明的（setOpaque（false）` ），这样背景图片就会显示出来。

---

我猜您正在使用GUI构建器来创建GUI布局并帮助您向GUI添加组件。我自己避免使用它们，更喜欢使用布局管理器（绝不为空的布局）手动创建GUI。如果绝对必须使用GUI构建器，则让该构建器为您创建一个JPanel（而不是JFrame），然后重写此JPanel的paintComponent，在其中绘制图像。否则，您最好像我一样学习Swing布局管理器并手动创建GUI。

您的窗口似乎是一个登录窗口，如果是这样，如果这是我的程序，我什至不使用JFrame而是使用一个[[modal JDialog来显示此窗口，因为这样会更容易这样控制程序流程。

---

使用GridBagLayout的概念验证程序以及太多的“幻数”：

import java.awt.*;
import java.awt.Dialog.ModalityType;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class LoginPanel extends JPanel {
    public static final String TITLE = "INICIO DE SESIÓN";
    public static final String IMG_PATH = "https://upload.wikimedia.org/wikipedia/"
            + "commons/thumb/6/69/MarsSunset.jpg/779px-MarsSunset.jpg";
    private JTextField usuarioField = new JTextField(20);
    private JPasswordField passwordField = new JPasswordField(20);
    private BufferedImage backgroundImg = null;

    public LoginPanel(BufferedImage img) {
        this.backgroundImg = img;
        JCheckBox showPasswordChkBx = new JCheckBox("Show Password");
        showPasswordChkBx.setOpaque(false);
        showPasswordChkBx.addItemListener(new ItemListener() {
            public void itemStateChanged(ItemEvent e) {
                if (e.getStateChange() == ItemEvent.SELECTED) {
                    passwordField.setEchoChar((char) 0);
                } else {
                    passwordField.setEchoChar('*');
                }
            }
        });

        JButton accederBtn = new JButton("Acceder");
        accederBtn.addActionListener(e -> {
            Window win = SwingUtilities.getWindowAncestor(LoginPanel.this);
            win.dispose();
        });

        setForeground(Color.BLACK);

        setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        int row = 0;

        gbc.gridx = 0;
        gbc.gridy = row;
        gbc.gridwidth = 2;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        int ins = 12;
        gbc.insets = new Insets(ins, ins, ins, ins);
        gbc.anchor = GridBagConstraints.CENTER;

        JLabel titleLabel = new JLabel(TITLE);
        titleLabel.setFont(titleLabel.getFont().deriveFont(Font.BOLD, 24f));
        add(titleLabel, gbc);

        row++;
        gbc.gridx = 0;
        gbc.gridy = row;
        gbc.gridwidth = 1;
        gbc.anchor = GridBagConstraints.LINE_START;
        add(new JLabel("Usuario:"), gbc);

        gbc.gridx = 1;
        add(usuarioField, gbc);

        row++;
        gbc.gridx = 0;
        gbc.gridy = row;
        gbc.insets = new Insets(ins, ins, 0, ins);
        add(new JLabel("Password:"), gbc);

        gbc.gridx = 1;
        add(passwordField, gbc);

        row++;
        gbc.gridx = 0;
        gbc.gridy = row;
        gbc.insets = new Insets(0, ins, ins, ins);
        add(new JLabel(""), gbc);

        gbc.gridx = 1;
        add(showPasswordChkBx, gbc);

        row++;
        gbc.gridx = 0;
        gbc.gridy = row;
        gbc.insets = new Insets(ins, ins, ins, ins);
        add(new JLabel(""), gbc);

        gbc.gridx = 1;
        add(accederBtn, gbc);

    }

    public String getUsuario() {
        return usuarioField.getText();
    }

    public char[] getPassword() {
        return passwordField.getPassword();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (backgroundImg != null) {
            g.drawImage(backgroundImg, 0, 0, getWidth(), getHeight(), this);
        }
    }

    @Override
    public Dimension getPreferredSize() {
        Dimension superSize = super.getPreferredSize();
        int width = superSize.width;
        int height = superSize.height;
        if (backgroundImg != null) {
            width = Math.max(width, backgroundImg.getWidth());
            height = Math.max(height, backgroundImg.getHeight());
        }
        return new Dimension(width, height);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }

    private static void createAndShowGui() {
        BufferedImage img = null;
        try {
            URL imgUrl = new URL(IMG_PATH);
            img = ImageIO.read(imgUrl);
        } catch (IOException e) {
            e.printStackTrace();
        }

        LoginPanel mainPanel = new LoginPanel(img);
        JDialog dialog = new JDialog((JFrame) null, LoginPanel.TITLE, ModalityType.APPLICATION_MODAL);
        dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        dialog.add(mainPanel);
        dialog.pack();
        dialog.setLocationByPlatform(true);
        dialog.setVisible(true);

        System.out.println("User Name: " + mainPanel.getUsuario());
        System.out.println("Password: " + new String(mainPanel.getPassword()));
    }
}