来自构造函数的repaint（）调用

Question

I'm trying to display loaded resource-image. And it works, but only after user resized the window.

import javax.swing.*; import java.awt.*;

class MyMainWindow extends JFrame{
    Image img;

    public MyMainWindow(){
        setSize(300,300);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        img = MyResourceLoader.getImage("res.jpg");
        repaint();

        setVisible(true);
    }
    @Override
    public void paint (Graphics g){
        super.paint(g);
        if (img != null)
            g.drawImage(img, 0,0, null);
    } 
}

public class ResourcesProgram {
    public static void main(String[] args) {
        new MyMainWindow();
    } 
}

Why the constructor's repaint() call doesn't work? And what should I do to show this image from the start?

Resource Loader class:

import java.awt.*;

public class MyResourceLoader{
    static MyResourceLoader rl = new MyResourceLoader();
    static public Image getImage(String fileName){
        return Toolkit.getDefaultToolkit().getImage(rl.getClass().getResource("images/" + fileName));
    }
}

Answer 1

Okay, so a bunch of things...

First...

Because the window has not yet been realised on the screen (attached to a native peer), there is nothing for repaint to paint...

Second...

Toolkit.getDefaultToolkit().getImage(rl.getClass().getResource("images/" + fileName));

Passes the actually loading of the image to a background thread, meaning that while you get a non-null value returned, the actual image data might not have been fully loaded.

You should consider using ImageIO.read instead, it will only return AFTER the image data has been fully loaded and will also throw an exception if something goes wrong (unlike Toolkit.getImage ). See Reading/Loading an Image for more details...

Third...

All Swing components implement the ImageObserver interface, this means when you call Graphics#drawImage , you should pass the object as the ImageObserver parameter

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

This allows the component to monitor updates from the loading of the image and reschedule repaint as required.

Fourth...

You should avoid overriding paint of top level containers, apart from not been double buffered, there is a bunch of components between the frame and the user, which can interfere with the painting process.

Instead, you should extend from something like JPanel and override it's paintComponent method, performing your custom painting there. You should then add this panel to an instance of JFrame (or some other component as needed)

Take a look at Painting in AWT and Swing and Performing Custom Painting for more details

Fifth...

You should be making sure that you create and modify your UI only from within the context of the Event Dispatching Thread, see Initial Threads for more details

Answer 2

Paint is immediately called when the code is initialized. To see this, get rid of repaint(); in your constructor, and add in your paint method, g.drawLine(0, 0, 300, 300) ;

Answer 3

Try this, you may have to add some imports:

import javax.swing.*; import java.awt.*;

class MyMainWindow extends JFrame{
    Image img;

    public MyMainWindow(){
        setSize(300,300);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        img = MyResourceLoader.getImage("res.jpg");
        setVisible(true);
    }

    @Override
    public void paint (Graphics g){
        super.paint(g);
        if (img != null)
            g.drawImage(img, 0,0, this);
    } 
}

public class ResourcesProgram {
    public static void main(String[] args) {
        new MyMainWindow();
    } 
}

Resource Loader:

import javax.swing.*; import java.awt.*;

public class MyResourceLoader {
    public static Image getImage(String fileName){
        ImageIcon icon = new ImageIcon(getClass().getResource(fileName));
        return icon.getImage();
    }
}