Golang - Customer Unmarshaler/Marshaler on pointer with nil/null value

Golang - Customer Unmarshaler/Marshaler on pointer with nil/null value

I'm trying to implement custom UnmarshalJSON and MarshalJSON on pointer type, however none of this function is called when data from json is null/nil like in example below:

package main

import (
    "encoding/json"
	"fmt"
)

type A struct {
	B *B `json:"b,omitempty"`
}

type B int

// Only for displaying value instead of
// pointer address when calling `fmt.Println`
func (b *B) String() string {
	if b == nil {
		return "nil"
	}

	return fmt.Sprintf("%d", *b)
}

// This function is not triggered when json
// data contains null instead of number value
func (b *B) UnmarshalJSON(data []byte) error {
	fmt.Println("UnmarshalJSON on B called")

	var value int
	if err := json.Unmarshal(data, &value); err != nil {
		return err
	}

	if value == 7 {
		*b = B(3)
	}

	return nil
}

// This function is not triggered when `B`
// is pointer type and has `nil` value
func (b *B) MarshalJSON() ([]byte, error) {
	fmt.Println("MarshalJSON on B called")

	if b == nil {
		return json.Marshal(0)
	}

	if *b == 3 {
		return json.Marshal(7)
	}

	return json.Marshal(*b)
}

func main() {
	var a A

	// this won't call `UnmarshalJSON`
	json.Unmarshal([]byte(`{ "b": null }`), &a)
	fmt.Printf("a: %+v\n", a)

	// this won't call `MarshalJSON`
	b, _ := json.Marshal(a)
	fmt.Printf("b: %s\n\n", string(b))

	// this would call `UnmarshalJSON`
	json.Unmarshal([]byte(`{ "b": 7 }`), &a)
	fmt.Printf("a: %+v\n", a)

	// this would call `MarshalJSON`
	b, _ = json.Marshal(a)
	fmt.Printf("b: %s\n\n", string(b))
}

output:

a: {B:nil}
b: {}

UnmarshalJSON on B called
a: {B:3}
MarshalJSON on B called
b: {"b":7}

My questions are:

• why UnmarshalJSON/MarshalJSON is not called with null/nil value on pointer type
• how we can call UnmarshalJSON/MarshalJSON everytime when data is null/nil and type is a pointer instead of implementing UnmarshalJSON/MarshalJSON on A type and modify b property from level of A

For Short

Currently, unmarshaling/marshaling a Go struct will emit only the non-zero fields, since nil pointer is one zero value in Go, the UnmarshalJSON/MarshalJSON is not called in this case.

Also, it seems there are some related proposals

• proposal: encoding/json: add omitzero option
• proposal: encoding/json: allow returning nil from MarshalJSON to omit the field

However, there is no solution to resolve it now.

Per code Unmarshalers

> Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op

// By convention, to approximate the behavior of Unmarshal itself,
// Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.
type Unmarshaler interface {
	UnmarshalJSON([]byte) error
}