Json Golang Go 如果 struct 没有字段，如何将其编组为空

struct 是编组的，即使它们的所有字段都包含zero values.

一种方法是离开Developer.ProjectRef字段nil，然后删除omitempty属性.这样，它将被封送到JSON null值中:

type Developer struct {
    Name       string `json:"name,omitempty"`
    ProjectRef *Ref   `json:"project"`
}

测试它:

developer := &Developer{
    Name:       "Charlie",
}

jsonBytes, err := json.Marshal(developer)
if err != nil {
    panic(err)
}
fmt.Println(string(jsonBytes))

developer.ProjectRef = &Ref{ID: "abc"}

jsonBytes, err = json.Marshal(developer)
if err != nil {
    panic(err)
}
fmt.Println(string(jsonBytes))

Output (try it on the Go Playground):

{"name":"Charlie","project":null}
{"name":"Charlie","project":{"id":"abc"}}

如果您总是希望Developer.ProjectRef是非nil指针，那么为Ref编写一个自定义的JSON marshaler，如果ID为空，它可以封送JSON null的值:

func (r *Ref) MarshalJSON() ([]byte, error) {
    if r.ID == "" {
        return []byte("null"), nil
    }
    type ref2 Ref
    return json.Marshal((*ref2)(r))
}

测试它:

developer := &Developer{
    Name:       "Charlie",
    ProjectRef: &Ref{ID: ""},
}

jsonBytes, err := json.Marshal(developer)
if err != nil {
    panic(err)
}
fmt.Println(string(jsonBytes))

developer.ProjectRef.ID = "abc"

jsonBytes, err = json.Marshal(developer)
if err != nil {
    panic(err)
}
fmt.Println(string(jsonBytes))

Output (try it on the Go Playground):

{"name":"Charlie","project":null}
{"name":"Charlie","project":{"id":"abc"}}