Stack printing System.exit(int) values

StackPrinting.java

// Authors: xdark, Col-E

AccessPatcher.patch();
MethodHandles.Lookup lookup = LookupUtil.lookup();
Runnable hook = () -> {
	try {
		Class<?> stackFrameClass = Class.forName("java.lang.LiveStackFrame");

		MethodHandle getLocals = lookup.findVirtual(stackFrameClass, "getLocals", MethodType.methodType(Object[].class))
				.asType(MethodType.methodType(Object[].class, Object.class));
		Method getStackWalker = stackFrameClass.getDeclaredMethod("getStackWalker", Set.class);
		getStackWalker.setAccessible(true);
		StackWalker stackWalker = (StackWalker) getStackWalker.invoke(null, Set.of(StackWalker.Option.RETAIN_CLASS_REFERENCE, StackWalker.Option.SHOW_HIDDEN_FRAMES));
		stackWalker.forEach(frame -> {
			try {
				if (frame.getDeclaringClass() == System.class && frame.getMethodName().equals("exit")) {
					Object[] locals = (Object[]) getLocals.invoke(frame);

					// Value is stored as a long with for some reason (which messes with negative values), so map back to int.
					String local0 = locals[0].toString().replaceAll("\\D+", "");
					int exit = (int) Long.parseLong(local0);

					System.err.println("The exit code was: " + exit);
				}
			} catch (Throwable t) {
				throw new RuntimeException(t);
			}
		});
	} catch (Throwable t) {
		t.printStackTrace();
		logger.error("Failed to create stack-walker");
	}
};

try {
	// We use this instead of the Runtime shutdown hook thread because this will run on the same thread
	// as the call to System.exit(N)
	Class<?> shutdown = lookup.findClass("java.lang.Shutdown");
	MethodHandle add = lookup.findStatic(shutdown, "add", MethodType.methodType(void.class, int.class, boolean.class, Runnable.class));
	add.invoke(9, false, hook); // the int param is 'slot' which can be 0-9 (but 0-3 are already occupied)
} catch (Throwable t) {
	t.printStackTrace();
	logger.error("Failed to add shutdown hook");
}