func JSONFieldsFromStruct(v reflect.Value) []string {
	if v.Kind() != reflect.Struct {
		return nil
	}

	m := getType2jfm(v.Type())
	m.lock.Lock()
	defer m.lock.Unlock()

	l := make([]string, 0, len(m.fields))
	for k := range m.fields {
		l = append(l, k)
	}
	return l
}
func JSONFieldsFromStruct(v reflect.Value) []string {
	if v.Kind() != reflect.Struct {
		return nil
	}

	m := getType2jfm(v.Type())
	m.lock.Lock()
	defer m.lock.Unlock()

	l := make([]string, 0, len(m.fields))
	for k := range m.fields {
		l = append(l, k)
	}
	return l
}
// JSONFieldsFromStruct returns the names of JSON fields associated
// with the given struct. Returns nil if v is not a struct