Overview

encoding/json package provides an Unmarshal method that can be used to convert the file bytes into struct or map in golang.

json.Unmarshal function can be used to convert from JSON to a struct or app. Below is the signature of the method

func Unmarshal(data []byte, v interface{}) error

Let’s see examples of

• JSON parse a file into a struct

• JSON parse a file into a map

JSON parse a file into a struct

Create a file employee.json with below contents

{"Name":"John","Age":21}

Below is the code

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
)

type employee struct {
	Name string `json:"Name"`
	Age  int    `json:"Age"`
}

func main() {

	var emp employee
	file, err := ioutil.ReadFile("employee.json")
	if err != nil {
		log.Fatalf("Some error occured while reading file. Error: %s", err)
	}
	err = json.Unmarshal([]byte(file), &emp)
	if err != nil {
		log.Fatalf("Error occured during unmarshaling. Error: %s", err.Error())
	}
	fmt.Printf("emp Struct: %#v\n", emp)
}

Output

emp Struct: main.employee{Name:"John", Age:21}

In the above code, we created an employee struct.

type employee struct {
	Name string `json:"Name"`
	Age  int    `json:"Age"`
}

The employee struct has some json meta tags which are used for converting json to a struct. See this link for more details – https://golangbyexample.com/struct-field-meta-or-tags/

This is how we unmarshalled the file bytes into the employee instance.

err = json.Unmarshal([]byte(file), &emp)

JSON parse a file into a map

A very important point to note though is that map allows integer values for keys while JSON doesn’t allow integer values for keys. JSON only allows string values for keys. So a map having an integer value for the key when converted to JSON will have a string value for the key.

Let’s say we have below json

{"1":"John", "2": "Simon"}

Below is the program that will read from the above and then convert the above json to map

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
)

func main() {
	a := make(map[int]string)

	file, err := ioutil.ReadFile("temp.json")
	if err != nil {
		log.Fatalf("Error occured during unmarshaling. Error: %s", err.Error())
	}
	json.Unmarshal([]byte(file), &a)

	fmt.Println(a)
}

Output

map[1:John 2:Simon]