How to Use Custom JSON Marshaling and Unmarshaling in Go

Custom JSON marshaling and unmarshaling in Go are achieved by implementing the MarshalJSON and UnmarshalJSON methods on a type, which override the default behavior of encoding/json. When json.Marshal is called, it checks for MarshalJSON on the type; if present, it executes that method instead of reflecting over fields. Similarly, json.Unmarshal calls UnmarshalJSON if the target type implements it, allowing you to define exactly how data is parsed.

package main

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

type MyTime struct {
	time.Time
}

func (m MyTime) MarshalJSON() ([]byte, error) {
	// Custom format: "2023-10-27"
	return []byte(`"` + m.Time.Format("2006-01-02") + `"`), nil
}

func (m *MyTime) UnmarshalJSON(data []byte) error {
	// Remove quotes
	var s string
	if err := json.Unmarshal(data, &s); err != nil {
		return err
	}
	// Parse custom format
	t, err := time.Parse("2006-01-02", s)
	if err != nil {
		return err
	}
	m.Time = t
	return nil
}

func main() {
	t := MyTime{time.Now()}
	b, _ := json.Marshal(t)
	fmt.Println(string(b)) // Output: "2023-10-27"

	var t2 MyTime
	json.Unmarshal([]byte(`"2023-10-27"`), &t2)
	fmt.Println(t2.Time) // Output: 2023-10-27 00:00:00 +0000 UTC
}

Custom marshaling lets you control exactly how your Go data structures are converted to and from JSON text. Instead of Go automatically guessing how to format your data, you write specific rules to handle special cases, like custom date formats or nested structures. Think of it as writing a custom translator for your specific data type instead of using a generic one.