code: public static Class<?> getMapKeyFieldType(Field mapField, int nestingLevel) {
  		return ResolvableType.forField(mapField).getNested(nestingLevel).asMap().resolveGeneric(0);
  	}
docs: Determine the generic key type of the given Map field.
  @param mapField the map field to introspect
  @param nestingLevel the nesting level of the target type
  (typically 1; e.g. in case of a List of Lists, 1 would indicate the
  nested List, whereas 2 would indicate the element of the nested List)
  @return the generic type, or {@code null} if none