缩放后，如果没有不相关的imageIcon，将无法显示图像

Question

I'm trying to get some images to scale for a chess program I'm writing. I tried writing up a small program just to get a feel for scaling images and I ran into a problem with the image displaying. Basically the image will only display properly if that ImageIcon (icon) is there and it is above pawn in the code. If it doesn't exist, it is below pawn in the code, or the image sources are different pawn displays as a black square. If I don't try and scale pawn then the whole thing works just fine regardless of whether icon is there or not. This whole thing just really perplexes me because the only association pawn and icon have is that they share a source image. Can anyone shed some light on this?

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import javax.swing.*;



public class resizeImg extends JFrame{

     public void generateGui(){
        JPanel mainPanel=new JPanel();
        getContentPane().add(mainPanel);

        JPanel main=new JPanel();
        main.setBorder(BorderFactory.createLineBorder(new Color(0,0,0)));
        main.setSize(400,400);
        mainPanel.add(main);
        mainPanel.setLayout(null);

        ImageIcon icon=new ImageIcon(Toolkit.getDefaultToolkit().getImage("resources/pawn.jpg"));
        //if you remove the above ImageIcon the image just displays as a black square

        Image pawn=scale(Toolkit.getDefaultToolkit().getImage("resources/pawn.jpg"));
        JLabel pawnContainer=new JLabel(new ImageIcon(pawn));
        main.add(pawnContainer);

        setSize(400,400);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public BufferedImage scale(Image i){
        BufferedImage resized=new BufferedImage(50,50,BufferedImage.TYPE_INT_RGB);
        Graphics2D g=resized.createGraphics();
        g.drawImage(i,0,0,50,50,null);
        g.dispose();
        return resized;
    }
    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable(){
            public void run(){
                resizeImg imgObject=new resizeImg();
                imgObject.generateGui();
                imgObject.setVisible(true);
            }
        });
    }
}

Answer 1

The original image loading code was designed to load to images over slow networks, so it uses a background thread to load the image. This means that when Toolkit.getImage returns, the image may not have actually begin loaded.

When you included the ImageIcon call, ImageIcon is using a MediaTracker to waitFor the image to become loaded. Because of the caching mechanism of Toolkit.getImage , repeated calls for the same image can use the cached image instead.

This is, also, why it's important to use an ImageObserver when painting images, but it may not help you in this case.

Instead, use ImageIO

public class Scale extends JFrame {

    public void generateGui() {
        JPanel main = new JPanel();
        main.setBorder(BorderFactory.createLineBorder(new Color(0, 0, 0)));
        main.setLayout(new BorderLayout());

        try {
            BufferedImage image = ImageIO.read(new File("path/to/image"));

            Image pawn = scale(image);
            JLabel pawnContainer = new JLabel(new ImageIcon(pawn));
            main.add(pawnContainer);
        } catch (IOException exp) {
            exp.printStackTrace();
        }

        add(main);

        setSize(400, 400);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public BufferedImage scale(Image i) {
        BufferedImage resized = new BufferedImage(50, 50, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = resized.createGraphics();
        g.drawImage(i, 0, 0, 50, 50, this);
        g.dispose();
        return resized;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                Scale imgObject = new Scale();
                imgObject.generateGui();
                imgObject.setVisible(true);
            }
        });
    }
}