reflect by Tag

gistfile1.txt

package main

import (
	"fmt"
	"reflect"
	"strings"
)

type Human struct {
	Head string `json:"a1" xml:"x1"`
	Body string `json:"a2" xml:"x2"`
	Leg  string `json:"a3,omitempty" xml:"x3"`
}

func getFieldName(tag, key string, s interface{}) (fieldname string) {
	rt := reflect.TypeOf(s)
	if rt.Kind() != reflect.Struct {
		panic("bad type")
	}
	for i := 0; i < rt.NumField(); i++ {
		f := rt.Field(i)
		v := strings.Split(f.Tag.Get(key), ",")[0] // use split to ignore tag "options"
		if v == tag {
			return f.Name
		}
	}
	return ""
}

func main() {
	fmt.Println(getFieldName("a1", "json", Human{}))
	fmt.Println(getFieldName("a3", "json", Human{}))
	fmt.Println(getFieldName("a2", "json", Human{}))
	fmt.Println(getFieldName("x2", "xml", Human{}))
	fmt.Println(getFieldName("a99", "json", Human{}))
	fmt.Println(getFieldName("a99", "json", "Human{}")) // panic if not struct
}