Thread: Getting Image reader to change source

I'm building an image gallery type program. I want to be able to abort the loading of an image and start loading another. Here's some of the code I've got so far.

ImageReader reader = null;
File image1 = "myFile.jpg";
File image2 = "myFile2.jpg;
ImagePanel imgPnl = new ImagePanel();
 
public void loadImage(File f) {
    if(reader != null) reader.abort();
 
    //get file extention
    String filename = f.getName();
    int pos = filename.lastIndexOf('.');
    String ext = filename.substring(pos+1);
 
    Iterator readers = ImageIO.getImageReadersByFormatName(ext);
    ImageInputStream iis = null;
 
    try { 
        iis = ImageIO.createImageInputStream(f);
    } catch (IOException ex) { 
        ex.printStackTrace();
    }
 
    reader = (ImageReader) readers.next();
    reader.setInput(iis);
    reader.addIIOReadProgressListener(new MyProgressListener());
    reader.addIIOReadUpdateListener(new MyUpdateListener());
 
    Thread thread = new Thread(new Runnable() {
        public void run() {
            try {
                reader.read(0);
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    });
 
    thread.start();
}
 
// Component that draw a BufferedImage
class ImagePanel extends JPanel {
    private BufferedImage img;
 
    public void setImage(BufferedImage img) {
        this.img = img;
        this.repaint();
    }
 
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if(img != null)
            g.drawImage(img, 0, 0, null);
    }
}
 
class MyProgressListener implements IIOReadProgressListener {
    ...
}
 
class MyUpdateListener implements IIOReadUpdateListener {
    ...
    public void imageUpdate(ImageReader source, BufferedImage theImage, int minX, int minY, int width, int height, int periodX, int periodY, int[] bands) {
        imgPnl.setImage(theImage);
    }
    ...
}

So, if the image is still loading (from the new thread) I can call loadImage() with a different file and it aborts and then begins to work on the new file. This does work, but pretty quickly I run into errors like this:

Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException: Attempt to use instance of com.sun.imageio.plugins.jpeg.JPEGImageReader@6c1989b5 locked on thread Thread[Thread-10,6,main] from thread Thread[AWT-EventQueue-0,6,main]

What's causing this and how do I fix it?

The message says you are trying to use the same instance of a class on two threads.
Perhaps the class is not thread safe in some way.
Can you make a new instance for each thread?
Or synchronize the method that uses it.

I thought about using only one instance, but if my next file is in a different format, I need a new ImageReader.

I was just looking into Thread.join() and I thought the problem could be that I'm not waiting for the thread using the ImageReader to terminate properly. I now store a reference to the reader thread and interrupt() + join() on it when trying to load a different image. I also put an interrupted check in one of the frequently called methods in the IIORead listeners.

So far so good, no errors yet. Hope this is a decent way of handling this situation; I'm new to thread programming.

ImageReader reader = null;
File image1 = "myFile.jpg";
File image2 = "myFile2.jpg;
ImagePanel imgPnl = new ImagePanel();
Thread activeThread = null;
 
public void loadImage(File f) {
 
    if(activeThread != null) {
        if(activeThread.getState() != Thread.State.TERMINATED) {
            try {
                activeThread.interrupt();
                activeThread.join();
            } catch (InterruptedException ex) {
                ex.printStackTrace();
            }
        }
    }
 
    //get file extention
    String filename = f.getName();
    int pos = filename.lastIndexOf('.');
    String ext = filename.substring(pos+1);
 
    Iterator readers = ImageIO.getImageReadersByFormatName(ext);
    ImageInputStream iis = null;
 
    try { 
        iis = ImageIO.createImageInputStream(f);
    } catch (IOException ex) { 
        ex.printStackTrace();
    }
 
    reader = (ImageReader) readers.next();
    reader.setInput(iis);
    reader.addIIOReadProgressListener(new MyProgressListener());
    reader.addIIOReadUpdateListener(new MyUpdateListener());
 
    activeThread = new Thread(new Runnable() {
        public void run() {
            try {
                reader.read(0);
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    });
 
    activeThread.start();
}
 
// Component that draw a BufferedImage
class ImagePanel extends JPanel {
    private BufferedImage img;
 
    public void setImage(BufferedImage img) {
        this.img = img;
        this.repaint();
    }
 
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if(img != null)
            g.drawImage(img, 0, 0, null);
    }
}
 
class MyProgressListener implements IIOReadProgressListener {
    ...
}
 
class MyUpdateListener implements IIOReadUpdateListener {
    ...
    public void imageUpdate(ImageReader source, BufferedImage theImage, int minX, int minY, int width, int height, int periodX, int periodY, int[] bands) {
        if(Thread.interrupted()) {
            source.abort();
            return;
        }
        imgPnl.setImage(theImage);
 
    }
    ...
}