Prevent alphabetically ordering json at Marshal

Question

I want to prevent alphabetically reordering at Marshal. My script is below. {"key3": "value3", "key2": "value2", "key1": "value1"} is reordered to {"key1":"value1","key2":"value2","key3":"value3"} by Marshal. I thought this may be close issue. But I could not solve my issue. Is there way to solve this?

Script

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    obj := `{"key3": "value3", "key2": "value2", "key1": "value1"}`
    var o map[string]interface{}
    json.Unmarshal([]byte(obj), &o)
    fmt.Println(o)
    r, _ := json.Marshal(o)
    fmt.Println(string(r))
}

play.golang.org

• fmt.Println(o) is map[key3:value3 key2:value2 key1:value1].
• fmt.Println(string(r)) is {"key1":"value1","key2":"value2","key3":"value3"}.

Thank you so much for your time. And I'm sorry for my immature question.

Answer 1

You can preserve the order like this:

type OrderedMap struct {
    Order []string
    Map map[string]string
}

func (om *OrderedMap) UnmarshalJSON(b []byte) error {
    json.Unmarshal(b,&om.Map)

    index:=make(map[string]int)
    for key:=range om.Map {
        om.Order=append(om.Order,key)
        esc,_:=json.Marshal(key) //Escape the key
        index[key]=bytes.Index(b,esc)
    }

    sort.Slice(om.Order, func(i,j int) bool { return index[om.Order[i]]<index[om.Order[j]] })
    return nil
}

func (om OrderedMap) MarshalJSON() ([]byte, error) {
    var b []byte
    buf:=bytes.NewBuffer(b)
    buf.WriteRune('{')
    l:=len(om.Order)
    for i,key:=range om.Order {
        km,err:=json.Marshal(key)
        if err!=nil { return nil,err }
        buf.Write(km)
        buf.WriteRune(':')
        vm,err:=json.Marshal(om.Map[key])
        if err!=nil { return nil,err }
        buf.Write(vm)
        if i!=l-1 { buf.WriteRune(',') }
        fmt.Println(buf.String())
    }
    buf.WriteRune('}')
    fmt.Println(buf.String())
    return buf.Bytes(),nil
}

Playground: https://play.golang.org/p/TxenZEuy_u0

Please note that json spec says objects are unordered, which means other client may not respect the order you preserve.