为什么 JFrame 的背景图像仅在我在 Swing 中调整窗口大小后才显示？

Question

I have noticed some weird behaviour with setting an image as a background for my JFrames. I want to have a background image for both Frames display when they are created. When i create Window1 inside my main method it only shows the background after i manually resize the window. When i click the button in Window1 and create window2 inside Window1, Window2 displays the background image correctly. When i create Window2 inside my main method the background image also does not show correctly.

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class TestStart {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Window1();
            }
        });
    }
}

public class Window1 extends JFrame {

    Image img = Toolkit.getDefaultToolkit().getImage("C:\\Users\\Tim\\Desktop\\water.jpg");

    public Window1() {
        super("gridtest");

        setSize(600, 500);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        this.setContentPane(new JPanel(){
            public void paintComponent(Graphics g){
                super.paintComponent(g);
                g.drawImage(img,0,0,getWidth(),getHeight(),null);
            }
        });

        JButton btn = new JButton("klick");
        add(btn, BorderLayout.CENTER);

        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent actionEvent) {
                new Window2();
            }
        });
        pack();
        setVisible(true);
    }
}

public class Window2 extends JFrame {

    Image img = Toolkit.getDefaultToolkit().getImage("C:\\Users\\Tim\\Desktop\\water.jpg");

    public Window2() {
        super("gridsecond");

        setSize(600, 500);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        this.setContentPane(new JPanel(){
            public void paintComponent(Graphics g){
                super.paintComponent(g);
                g.drawImage(img,0,0,getWidth(),getHeight(),null);
            }
        });
        pack();
        setVisible(true);
    }
}

I tried adding in revalidate()/repaint() calls and calling the paint method at different points in my code, but except for the weird behaviour i described above, i cant get the background image to show without resizing my window.

Disclaimer: I know it's not good practice to create a new JFrame for every window, but i am not able to change the whole project structure, so im stuck with this.

Answer 1

So, a series of issues that might be causing your issue...

First, using Toolkit.getDefaultToolkit().getImage

Image img = Toolkit.getDefaultToolkit().getImage("C:\\Users\\Tim\\Desktop\\water.jpg");

The problem with this is, the image loading is down on a seperate thread. This means that when getImage returns the image may or may not have actually been loaded

Second, when calling drawImage , you pass null as the ImageConsumer , so the component won't be notified of changes to the images "loaded" state and won't be able to schedule updates to the component accordingly...

this.setContentPane(new JPanel(){
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        g.drawImage(img,0,0,getWidth(),getHeight(),null);
    }
});

Three, this is more of a side effect, but, you've set the contentPane manually using a JPanel , which by default uses a FlowLayout , so using BorderLayout constraints is actually kind of pointless.

setLayout(new BorderLayout());

this.setContentPane(new JPanel(){
//...
JButton btn = new JButton("klick");
add(btn, BorderLayout.CENTER);

Besides, BorderLayout is the default layout manager used by window based components ;)

So, how would you fix this issue? Well, immediately, you could pass this as the ImageConsumer ...

g.drawImage(img,0,0,getWidth(),getHeight(),this);

This will allow the component to monitor the loading state of the image and trigger repaints as required.

A longer term solution would be to stop using Toolkit.getDefaultToolkit().getImage (and by extension, ImageIcon ), as they can be annoying, and start using ImageIO instead.

Apart from supporting a wider range of image formats, the API won't return till the image is fully loaded or an error occurred (try diagnose loading issues with the other APIs 🙄)

See Reading/Loading images for more details