Java绘画到BufferedImage或ImageIO.write（）不会阻塞，并且生成的png文件要么空白，要么空白

Question

I'm creating small png files with simple black and white line drawings within the servlet running on Tomcat 7 on Linux CentOS and using frame buffer Xvfb to provide graphic resources. My goal is then to show this png file on the html page returned by the server. What I'm finding is that once in a while (say one picture out of 10) is shown as blank or sometimes even half-drawn. I checked the corresponding .png files on the server and they are indeed generated in these cases either as blank or as half-drawn.

This is how my code looks (additional complication is that ChimePro is part of the proprietary class library, I don't have its code; it just extends Panel or Canvas):

Frame f = new Frame();
ChimePro cp = new ChimePro();
f.add(cp);
f.addNotify();
cp.setBounds(0,0,200,200);

BufferedImage bi = new BufferedImage (200,200,BufferedImage.TYPE_BYTE_INDEXED); 
Graphics g = bi.createGraphics()
cp.paint(g);
File ffjj = new File("file.png");


ImageIO.write(bi, "png", ffjj);

So it looks as if paint(g) or, perhaps less probable, ImageIO.write() methods are not blocking as they should be doing, and the program goes forward even though the proper file has not yet been created. It is also interesting that depending on how I point DISPLAY variable before the Tomcat startup - either to Xvfb screen emulating the X environment on the same server, or to some real X display running on the remote machine - the probability of blank or corrupted pictures changes.

Any ideas of how to deal with this issue to insure that my users do not see blank or corrupted drawings? I'm actually writing small number of short files, so this is not about speed, just about avoiding showing incorrectly drawn pictures. Would be thankful for any suggestions.

This is the exception (happens only in some drawings arbitrarily - not depending on the contents of drawing but on some tinming probably):

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerExceptio
at com.mdli.chime.ChimePro.paint(ChimePro.java)
at java.awt.Canvas.update(Canvas.java:142)
at sun.awt.RepaintArea.updateComponent(RepaintArea.java:255)
at sun.awt.X11.XRepaintArea.updateComponent(XRepaintArea.java:60)
at sun.awt.RepaintArea.paint(RepaintArea.java:232)
at sun.awt.X11.XComponentPeer.handleEvent(XComponentPeer.java:591)
at java.awt.Component.dispatchEventImpl(Component.java:4937)

Answer 1

Have g.dispose() before ImageIO.write and see if it helps:

Frame f = new Frame();
ChimePro cp = new ChimePro();
f.add(cp);
f.addNotify();
cp.setBounds(0,0,200,200);

BufferedImage bi = new BufferedImage (200,200,BufferedImage.TYPE_BYTE_INDEXED); 
Graphics g = bi.createGraphics()
cp.paint(g);
File ffjj = new File("file.png");
g.dispose();

ImageIO.write(bi, "png", ffjj);

Answer 2

You need to make sure that you...

1. Set the size of the component
2. Layout the child components of the container if required
3. Use printAll rather the print as it will attempt to ensure the content is painted fully and doesn't double buffer the process (making it faster)
4. dispose of the Graphics context once you are completed.

The following shows the panel when displayed on a frame and when printed using the sample code...

import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class PrintComponent {

    public static void main(String[] args) {
        new PrintComponent();
    }

    public PrintComponent() {

        Example example = new Example();
        example.setSize(example.getPreferredSize());
        example.doLayout();
        BufferedImage bi = new BufferedImage (example.getWidth(), example.getHeight(), BufferedImage.TYPE_BYTE_INDEXED);
        Graphics2D g2d = bi.createGraphics();
        example.printAll(g2d);
        g2d.dispose();

        try {
            ImageIO.write(bi, "png", new File("D:/PrintExample.png"));
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    public class Example extends JPanel {

        public Example() {
            setLayout(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;
            add(new JLabel("Hello"), gbc);
            add(new JTextField("'ello", 20), gbc);
            add(new JButton("g'day"), gbc);
        }

    }

}