问Golang -客户解封器/封送器在指针上为nil/null值

EN

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))
}

a: {B:nil}
b: {}

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