Java uncatchable ClassNotFoundException when using java.awt.Clipboard

You should always check what the available clipboard formats are before accessing, as it may not always contain a String. You can do this with:

if (clipboard.isDataFlavorAvailable(DataFlavor.stringFlavor))
...

This will print what formats are available:

Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();

System.out.println("hasString "+clipboard.isDataFlavorAvailable(DataFlavor.stringFlavor));
for(var f : clipboard.getAvailableDataFlavors())
{
    System.out.println(f);
}

It is never a good plan to slip System.exit() into any handler - remove. In theory the clipboard could be changed by another app between checking so you need your exception handler to return "" or throw it as your own exception type.

static String getClipboard() {
    Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();
    try {
        // Note that it is still possible for another app to modify clipboard
        // between "if" and "return", so this could still fail:
        if(clipboard.isDataFlavorAvailable(DataFlavor.stringFlavor)) {
            return (String) clipboard.getData(DataFlavor.stringFlavor);
        }
    } catch (Exception e) {
        // or rethrow as your own exception type
    }
    return "";
}

The solution with isDataFlavorAvailable did not work for me, because the exceptions are fired during loading list of available flavors. Fixed by ignoring all those "intellij idea" formats by:

SystemFlavorMap flavorMap = (SystemFlavorMap) SystemFlavorMap.getDefaultFlavorMap();
flavorMap.setFlavorsForNative(
    "JAVA_DATAFLAVOR:application/x-java-jvm-local-objectref; class=com.intellij.codeInsight.editorActions.FoldingData",
    new DataFlavor[]{DataFlavor.getTextPlainUnicodeFlavor()});