Question

In the following code, I call JOptionPane.showMessageDialog, inside a try/catch block. But when the error is caught, my JOptionPane is visible but without any message !!! Does someone knows why and how I can correct the problem ?

Regards

MyBoardJPannel.java

package experimentations.gui;

import java.awt.Graphics;
import java.awt.Image;
import java.io.InputStream;

import javax.imageio.ImageIO;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class MyBoardPannel extends JPanel {

@Override
public void paint(Graphics grahics) {
            if (imageToShow == null)
        imageToShow = loadImage("sampleImage");
}

/**
 * In fact, there are not any image in project => will go to catch clause.
 * @param imageName
 */
private void loadImage(String imageName) {
    InputStream imageStream = getClass().getResourceAsStream("/"+imageName+".png");
    try {
        imageToShow = ImageIO.read(imageStream);
    }
    catch (Exception e) {
        String errorMessage = "Failed to load image "+imageName;
        System.err.println(errorMessage);
        JOptionPane.showMessageDialog(this, errorMessage,
                "Image loading error", JOptionPane.ERROR_MESSAGE);
        imageToShow = null;
        System.exit(1);
    }
}

private Image imageToShow;



}

JOptionPaneErrorShowing.java

package experimentations.gui;

import javax.swing.JFrame;

public class JOptionPaneErrorShowing extends JFrame {

public JOptionPaneErrorShowing(){
    setTitle("JOptionPane experimentation");
    setSize(300, 300);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLocationRelativeTo(null);

    add(new MyBoardPannel());
}

/**
 * @param args
 */
public static void main(String[] args) {
    new JOptionPaneErrorShowing().setVisible(true);
}

}

Answer 1

It’s likely a Swing concurrency issue. But more importantly, you should never load an image from within a paint or paintComponent method, ever. Read it in the constructor or elsewhere but paint/paintComponent need to be lean and blazingly fast.

To solve your issue, consider reading in the image in SwingWorker object. If you call a JOptionPane from within the SwingWorker’s doInBackground method though, be sure to call it on the Swing event thread, the EDT, using SwingUtilities.invokeLater(Runnable).

Also, you will hardly ever want to draw in a JPanel’s paint method unless you are taking care of painting borders and children. Instead paint in a paintComponent method, and don’t forget to call the super.paintComponent(g) method in that paintComponent override. You’ll want to read the Swing graphics tutorials as this is all spelled out there.

For example:

import java.awt.Graphics;
import java.awt.Image;
import java.io.InputStream;
import java.util.concurrent.ExecutionException;

import javax.imageio.ImageIO;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.SwingWorker;

public class MyBoardPannel extends JPanel {
   protected static final String SAMPLE_IMAGE = "sampleImage";
   Image imageToShow = null;

   public MyBoardPannel() {
      SwingWorker<Image, Void> mySW = new SwingWorker<Image, Void>() {

         @Override
         protected Image doInBackground() throws Exception {
            return loadImage(SAMPLE_IMAGE);
         }

         @Override
         protected void done() {
            try {
               imageToShow = get();
            } catch (InterruptedException e) {
               e.printStackTrace();
            } catch (ExecutionException e) {
               e.printStackTrace();
            }
         }
      };

      mySW.execute();
   }

   @Override
   public void paintComponent(Graphics grahics) {
      super.paintComponent(grahics);
      if (imageToShow != null) {
         grahics.drawImage(imageToShow, 0, 0, null);
      }
   }

   private Image loadImage(String imageName) {
      InputStream imageStream = getClass().getResourceAsStream(
            "/" + imageName + ".png");
      try {
         return ImageIO.read(imageStream);
      } catch (Exception e) {
         final String errorMessage = "Failed to load image " + imageName;
         System.err.println(errorMessage);
         SwingUtilities.invokeLater(new Runnable() {
            public void run() {
               JOptionPane.showMessageDialog(MyBoardPannel.this, errorMessage,
                     "Image loading error", JOptionPane.ERROR_MESSAGE);
               System.exit(1);
            }
         });
      }

      return null;
   }

}