How to obtain pid from Process without illegal access warning with Java 9+?

I need to obtain the underlying OS PID for a Process I start. The solution I'm using now involves access to a private field through reflection using code like this:

private long getLongField(Object target, String fieldName) throws NoSuchFieldException, IllegalAccessException {
    Field field = target.getClass().getDeclaredField(fieldName);
    field.setAccessible(true);
    long value = field.getLong(target);
    field.setAccessible(false);
    return value;
}

It works but there are several problems with this approach, one being that you need to do extra work on Windows because the Windows-specific Process subclass doesn't store a "pid" field but a "handle" field (so you need to do a bit of JNA to get the actual pid), and another being that starting with Java 9 it triggers a bunch of scary warnings like "WARNING: An illegal reflective access operation has occurred".

So the question: is there a better way (clean, OS independent, garanteed not to break in a future release of Java) to obtain the pid? Shouldn't this be exposed by Java in the first place?