Jython Swing: Randomly Occurring Java ImageIO IOException

Question

I've built a Jython swing app using eclipse and pydev. I've used simple images for the backgrounds of all of the buttons. 95% of the time, everything works wonderfully. However, about 5% of the time when adding or removing content from the screen, this occurs:

Exception in thread "AWT-EventQueue-0"  at com.sun.imageio.plugins.png.PNGImageReader.readImage(PNGImageReader.java:1310)
    at com.sun.imageio.plugins.png.PNGImageReader.read(PNGImageReader.java:1579)
    at javax.imageio.ImageIO.read(ImageIO.java:1438)
    at javax.imageio.ImageIO.read(ImageIO.java:1342)
    at sun.reflect.GeneratedMethodAccessor33.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:616)

javax.imageio.IIOException: javax.imageio.IIOException: Error reading PNG image data

The frustrating thing is that it's not readily reproducable, and can happen anywhere in the application that I draw a PNG. I can certainly catch the exception and have it reimport the image, but I'm hoping to figure out the root cause.

I call the resources like so:

notPressed = ImageIO.read(pkg_resources.resource_stream('images', "button-blue.png")).getScaledInstance(width,height, Image.SCALE_SMOOTH )

My only guess is that sometimes pkg_resources might fail to open a filestream, causing the exception. Any suggestions on how to track this down, or should I just be happy that I can make it fail quietly?

Are you calling `ImageIO.read()` on the [event dispatch thread](http://download.oracle.com/javase/tutorial/uiswing/concurrency/initial.html)? — trashgod, Dec 08 '11 at 02:16
Exactly -- this intermittent NPE smells very suspicious for a concurrency problem. I think that it's fine to call ImageIO.read() on a background thread as long as you don't use the images until they are complete and as long as all Swing calls (such as adding an ImageIcon to a JLabel) are done on the EDT. — Hovercraft Full Of Eels, Dec 08 '11 at 03:10
I am. Assuming that's the issue, what's more practical solution; having a thread cache all of the images somewhere accessible on application start-up or sending a thread for each individual image. I use roughly 2-3 MBs of images throughout the application. — sarwar, Dec 08 '11 at 04:17

score 3 · Accepted Answer · edited May 23 '17 at 12:20

What's a more practical solution: having a thread cache all of the images somewhere accessible on application start-up or sending a thread for each individual image. I use roughly 2-3 MBs of images throughout the application.

Much depends on the anticipated latency and image size. This simple example caches images as acuired in a List<ImageIcon>; the slight delay for a new image is imperceptible, as the images are part of the distribution JAR. In contrast, this example lags palpably as the application acquires images from the network at startup.

For long latency or a large number of large images, you may want to consider SwingWorker, as discussed in this Oracle article on how to Improve Application Performance With SwingWorker.

Thanks. I'm going to cache everything, probably as a Jython dictionary of imageicons. The images are already distributed along with the application. I just hadn't seen the need to load them off of the UI thread until now. — sarwar, Dec 08 '11 at 16:18

Jython Swing: Randomly Occurring Java ImageIO IOException

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