Copying Values from JSON to Struct using reflect Package for Golang

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Copying Values from JSON to Struct using reflect Package

This is a sample script for copying values from JSON to a struct using reflect package.

Script :

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
)

type obj struct {
	Key1 string `json:"k1"`
	Key2 string `json:"k2"`
	Key3 int64  `json:"k3"`
	Key4 int    `json:"k4"`
	Key5 bool   `json:"k5"`
}

func main() {
	data := `{"k1": "v1", "k2": "v2", "k3": 1234567890, "k4": 456, "k5": true}`
	d := map[string]interface{}{}
	json.Unmarshal([]byte(data), &d)
	obj := &obj{}
	s := reflect.ValueOf(obj).Elem()
	typeOfT := s.Type()
	for i := 0; i < s.NumField(); i++ {
		for j, f := range d {
			if typeOfT.Field(i).Tag.Get("json") == j {
				fl := s.FieldByName(typeOfT.Field(i).Name)
				switch fl.Kind() {
				case reflect.Bool:
					fl.SetBool(f.(bool))
				case reflect.Int, reflect.Int64:
					c, _ := f.(float64)
					fl.SetInt(int64(c))
				case reflect.String:
					fl.SetString(f.(string))
				}
			}
		}
	}
	fmt.Printf("%+v\n", obj) // &{Key1:v1 Key2:v2 Key3:1234567890 Key4:456 Key5:true}
}

Result :

&{Key1:v1 Key2:v2 Key3:1234567890 Key4:456 Key5:true}

The Go Playground

https://play.golang.org/p/Rz-GNbFyDfh

Why need the nested loop? I tried directly fetch value of map and got the same output.

	for i := 0; i < s.NumField(); i++ {
		v, err := d[typeOfT.Field(i).Tag.Get("json")]
		if err {
			fl := s.FieldByName(typeOfT.Field(i).Name)
			switch fl.Kind() {
			case reflect.Bool:
				fl.SetBool(v.(bool))
			case reflect.Int, reflect.Int64:
				c, _ := v.(float64)
				fl.SetInt(int64(c))
			case reflect.String:
				fl.SetString(v.(string))
			}
		}
	}