Extension resources

It's a fairly common use case for extensions to package some resource that needs to be loaded. For example, an icon to display on a button, or an audio file to play when certain actions happen. Of course, these resources can be packaged into the jar file and then loaded via getClass().getResourceAsStream().

However! Due to the way extension jars are dynamically loaded, you can only load resources in your constructor, or in the loadJarResources() method. The class loader that loads the jar is closed after the extension is loaded, so any attempt to do getClass().getResourceAsStream() will return null after your extension is instantiated.

Here's the load order:

1. ExtensionManager opens the extension jar file
2. Extension main class is instantiated (your constructor fires)
3. ExtensionManager invokes createConfigProperties() exactly once
4. ExtensionManager invokes loadJarResources() exactly once
5. ExtensionManager closes the jar file (class loader is gone)

So, any resources that you need to load should be loaded either in your constructor or in the loadJarResources() method. This also applies to loading the extInfo.json file as a resource in your extension. For example:

public class MyExtension extends MyAppExtension {
    private final AppExtensionInfo extInfo;
    private BufferedImage someIcon;
    
    public MyExtension() {
        // We can safely load resources here in the constructor:
        extInfo = AppExtensionInfo.fromExtensionJar(getClass(), "/path/to/extInfo.json");
        if (extInfo == null) {
            throw new RuntimeException("MyExtension: can't load extInfo.json!");
        }
    }
     
    @Override
    public void loadJarResources() {
        try {
            // It's also safe to load jar resources here in the loadJarResources method:
            someIcon = ImageUtil.loadFromResource(getClass(), "/path/to/icon.png", 32, 32);
        }
        catch (IOException ioe) {
            throw new RuntimeException("MyExtension: can't load icon resource!", ioe);
        }
    }
}

Attempting to lazy-load any resource outside of your constructor or the loadJarResources() method will fail!

private void someExtensionMethod() {
    // Hmm, I think I'll load another icon now...
    InputStream inStream = getClass().getResourceAsStream("/some/icon.png");
    
    // Oh no - inStream is null! We should have done this in loadJarResources()...
}