Golang - 具有零/空值的指針上的客戶解組器/編組器我正在嘗試實現(xiàn)自定義UnmarshalJSON和MarshalJSON指針類型，但是當(dāng)來自 json 的數(shù)據(jù)null/nil如下例所示時，不會調(diào)用此函數(shù)：package mainimport (    "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 valuefunc (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` valuefunc (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))}輸出：a: {B:nil}b: {}UnmarshalJSON on B calleda: {B:3}MarshalJSON on B calledb: {"b":7}我的問題是：為什么UnmarshalJSON/MarshalJSON不使用null/nil指針類型的值調(diào)用我們?nèi)绾蜺nmarshalJSON/MarshalJSON每次調(diào)用數(shù)據(jù)null/nil和類型是指針而不是UnmarshalJSON/MarshalJSON在A類型上實現(xiàn)并b從級別修改屬性A