Go: Converting JSON string to map[string]interface{}

Question

I'm trying to create a JSON representation within Go using a map[string]interface{} type. I'm dealing with JSON strings and I'm having a hard time figuring out how to avoid the JSON unmarshaler to automatically deal with numbers as float64s. As a result the following error occurs.

Ex. "{ 'a' : 9223372036854775807}" should be map[string]interface{} = [a 9223372036854775807 but in reality it is map[string]interface{} = [a 9.2233720368547758088E18]

I searched how structs can be used to avoid this by using json.Number but I'd really prefer using the map type designated above.

JSON defaults to using float64 for all numbers: golang.org/pkg/encoding/json/#Unmarshal. If you want it to be an integer, you need to unmarshal it to a destination with an integer value (example, a map[string]int or a struct{ a int }). That, or accept that it'll be a float and simply type-convert it to an int. — Kaedys
– Kaedys, Commented Nov 20, 2017 at 20:24
I'm aware that Go's encoding/json defaults to use float64s for json numbers. Type conversion would be the easiest solution if it worked with large int64 values. I guess I would have to write a custom marshaler or specify the map type. Thanks — Steven
– Steven, Commented Nov 21, 2017 at 21:17

maerics · Accepted Answer · 2017-11-20 20:54:20Z

The go json.Unmarshal(...) function automatically uses float64 for JSON numbers. If you want to unmarshal numbers into a different type then you'll have to use a custom type with a custom unmarshaler. There is no way to force the unmarshaler to deserialize custom values into a generic map.

For example, here's how you could parse values of the "a" property as a big.Int.

package main

import (
  "encoding/json"
  "fmt"
  "math/big"
)

type MyDoc struct {
  A BigA `json:"a"`
}

type BigA struct{ *big.Int }

func (a BigA) UnmarshalJSON(bs []byte) error {
  _, ok := a.SetString(string(bs), 10)
  if !ok {
    return fmt.Errorf("invalid integer %s", bs)
  }
  return nil
}

func main() {
  jsonstr := `{"a":9223372036854775807}`
  mydoc := MyDoc{A: BigA{new(big.Int)}}

  err := json.Unmarshal([]byte(jsonstr), &mydoc)
  if err != nil {
    panic(err)
  }

  fmt.Printf("OK: mydoc=%#v\n", mydoc)
  // OK: mydoc=main.MyDoc{A:9223372036854775807}
}

Omkesh Sajjanwar · Accepted Answer · 2022-07-05 18:23:25Z

2

func jsonToMap(jsonStr string) map[string]interface{} {
    result := make(map[string]interface{})
    json.Unmarshal([]byte(jsonStr), &result)
    return result
}

Example - https://goplay.space/#ra7Gv8A5Heh
Related questions - create a JSON data as map[string]interface with the given data

answered Jul 5, 2022 at 18:23

Omkesh Sajjanwar

92312 silver badges16 bronze badges

Comments

kadir.seckin · Accepted Answer · 2024-04-21 09:45:55Z

0

You can still use map[string]interface{} but you need to change your json decode way. Here is an example with json.NewDecoder().

jsonData := `{ "a" : 9223372036854775807}`
jsonDecoder := json.NewDecoder(bytes.NewReader([]byte(jsonData)))
jsonDecoder.UseNumber() // enable useNumber for preventing float

var decodedData map[string]interface{}
err := jsonDecoder.Decode(&decodedData)
if err != nil {
    panic(err)
}

fmt.Println(decodedData) // prints map[a:9223372036854775807]

answered Apr 21, 2024 at 9:45

kadir.seckin

215 bronze badges

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