Handling an exception not happening on my code, what should I do?

Question

I have a JavaFX application that takes a screenshot/snapshot of a node. The code that takes the screenshot looks something like this:

WritableImage image = webView.snapshot(null, null);

where webView is a javafx.scene.web.WebView. Normally this works fine, but when the WebView is too big (and by that I mean, in the order of 10240x5548 pixels), then, I get this exception:

java.lang.NullPointerException
    at com.sun.prism.impl.ps.BaseShaderContext.initLCDBuffer(BaseShaderContext.java:703)
    at com.sun.prism.impl.ps.BaseShaderContext.validateLCDBuffer(BaseShaderContext.java:725)
    at com.sun.prism.impl.ps.BaseShaderGraphics.initLCDSampleRT(BaseShaderGraphics.java:1925)
    at com.sun.prism.impl.ps.BaseShaderGraphics.drawString(BaseShaderGraphics.java:2056)
    at com.sun.javafx.webkit.prism.WCGraphicsPrismContext$10.doPaint(WCGraphicsPrismContext.java:936)
    at com.sun.javafx.webkit.prism.WCGraphicsPrismContext$Composite.paint(WCGraphicsPrismContext.java:1500)
    at com.sun.javafx.webkit.prism.WCGraphicsPrismContext$Composite.paint(WCGraphicsPrismContext.java:1485)
    at com.sun.javafx.webkit.prism.WCGraphicsPrismContext.drawString(WCGraphicsPrismContext.java:948)
    at com.sun.webkit.graphics.GraphicsDecoder.decode(GraphicsDecoder.java:299)
    at com.sun.webkit.graphics.WCRenderQueue.decode(WCRenderQueue.java:92)
    at com.sun.webkit.WebPage.paint2GC(WebPage.java:734)
    at com.sun.webkit.WebPage.paint(WebPage.java:701)
    at com.sun.javafx.sg.prism.web.NGWebView.renderContent(NGWebView.java:96)
    at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:2053)
    at com.sun.javafx.sg.prism.NGNode.render(NGNode.java:1945)
    at com.sun.javafx.tk.quantum.QuantumToolkit$5.draw(QuantumToolkit.java:1393)
    at com.sun.javafx.tk.quantum.QuantumToolkit$5.run(QuantumToolkit.java:1429)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
    at com.sun.javafx.tk.RenderJob.run(RenderJob.java:58)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at com.sun.javafx.tk.quantum.QuantumRenderer$PipelineRunnable.run(QuantumRenderer.java:125)
    at java.lang.Thread.run(Thread.java:748)

This wouldn't be a common occurrence but I still would like to handle the situation even if only to tell the user that such resolution is not supported. The problem is that that stack trace doesn't touch my code at all. Not a single line is in my code. I guess it's a thread that's being triggered by something I call that returns immediately and later on the thread crashes.

My application is already heavily multi-threaded and thus, I have a generic error handler that looks like this:

public class ErrorHandler implements Thread.UncaughtExceptionHandler {
    public ErrorHandler(String appName) {
        super();
        // init stuff.
    }

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        // Oh my! something went wrong.
        e.printStackTrace(); // We still want to see the exception, specially while developing.
    }
}

which I set up right after my app gets started like this:

ErrorHandler errorHandler = new ErrorHandler(APP_NAME);
Thread.setDefaultUncaughtExceptionHandler(errorHandler);

which tends to work, but for some reason, it doesn't work for that thread.

Any ideas how can I catch the situation to do something about it?

Note: I posted a related question in which I was actually getting exception reports from production in what looked like a big screenshot, but I haven't been able to trigger that exception and instead I trigger this one. That question is here: How to work around Unrecognized image loader: null in JavaFX?

Pushing harder to figure out what's going on, eventually I got this to stderr:

Outstanding resource locks detected:
D3D Vram Pool: 402,867,712 used (75.0%), 500,823,400 target (93.3%), 536,870,912 max
21 total resources being managed
average resource age is 37.2 frames
0 resources at maximum supported age (0.0%)
11 resources marked permanent (52.4%)
5 resources have had mismatched locks (23.8%)
5 resources locked (23.8%)
12 resources contain interesting data (57.1%)
0 resources disappeared (0.0%)

I'm not sure what's going on there, but it might be related.

Answer 1

You could introduce a simple verification before performing the snapshot.

if (webView.getHeight() > 1000 || webView.getWidth() > 1000){
    throw new Exception("Web view dimension too big for capturing a snapshot!");
}
WritableImage writImage = webView.snapshot(null, null);

If you want to not hard-code the maximum values that the webView can have, you can just get the current screen bounds and compare against them.

Rectangle2D primaryScreenBounds = Screen.getPrimary().getVisualBounds();
if (webView.getHeight() > primaryScreenBounds.getHeight() || webView.getWidth() > primaryScreenBounds.getWidth()){
    throw new Exception("Webview exceeds screen dimensions!");
}