Error loading background image into JPanel in a JFrame

问题

I have a JFrame in which I want to occupy it entirely with a JPanel and put a background image in the JPanel.

Code:

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);
    }
};

Why doesn't the image load? Any solution to my code?

回答1:

Your problem is here:

initComponents();

You likely add all components to the GUI in this method, quite possibly using GroupLayout or other user-unfriendly layout manager, and then add the panelFondo JPanel after all components have been added.

If you want a GUI to show a background image, the components need to be added to the image-drawing JPanel, and if any JPanels are added on top of the image drawer, they need to be transparent (setOpaque(false)`) so that the background image shows through.

---

I'm guessing that you're using a GUI builder to create your GUI layouts and to assist you in adding components to the GUI. Myself, I avoid using them and much prefer creating my GUI's by hand using layout managers (never null layouts). If you absolutely must use a GUI builder, then have the builder create a JPanel for you, not a JFrame, and then override this JPanel's paintComponent, drawing the image within it. Otherwise you may be better off learning the Swing layout managers and creating your GUI's by hand like I do.

Your window appears to be a sign-in window, and if so, if this were my program, I wouldn't even use a JFrame but rather a modal JDialog to display this since it would be much easier to control program flow in this way.

---

Proof of concept program using GridBagLayout and way too many "magic numbers":

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()));
    }
}

来源：https://stackoverflow.com/questions/61851372/error-loading-background-image-into-jpanel-in-a-jframe