static Field findField(Object o, String field) {
  if (o instanceof Class) return findField_static((Class) o, field);
  
  Class c = o.getClass();
  for (Field f : c.getDeclaredFields())
    if (f.getName().equals(field))
      return f;
  throw new RuntimeException("Field '" + field + "' not found in " + c.getName());
}

static Field findField_static(Class<?> c, String field) {
  for (Field f : c.getDeclaredFields())
    if (f.getName().equals(field) && (f.getModifiers() & Modifier.STATIC) != 0)
      return f;
  throw new RuntimeException("Static field '" + field + "' not found in " + c.getName());
}

Began life as a copy of #1000619