如何在Go json marshal中显示空对象而不是空结构或nil

Question

I need to show json's empty object {} when do json.Marshal() for a struct pointer. I can only output either null value or empty struct value.

If the person key is filled with &Person{} or new(Person) , it will show empty struct like below:

{
    "data": {
        "person": {
            "name": "",
            "age": 0
        },
        "created_date": "2009-11-10T23:00:00Z"
    }
}

And if we don't initialize it at all, it will show null .

{
    "data": {
        "person": null,
        "created_date": "2009-11-10T23:00:00Z"
    }
}

I want to show "person": {} . Is it possible?

Go Playground for the complete code: https://play.golang.org/p/tT15G2ESPVc

Answer 1

Option A , use the omitempty tag option on all of the Person 's fields and make sure the response's field is allocated before marshaling.

type Person struct {
    Name string `json:"name,omitempty"`
    Age  int    `json:"age,omitempty"`
}

// ...

resp.Person = new(Person)

https://play.golang.org/p/o3jWdru_8bC

---

Option B , use a non-pointer wrapper type that embeds the Person pointer type.

type PersonJSON struct {
    *Person
}

type Response struct {
    Person      PersonJSON `json:"person"`
    CreatedDate time.Time   `json:"created_date"`
}

https://play.golang.org/p/EKQc7uf1_Vk

---

Option C , have the Reponse type implement the json.Marshaler interface.

func (r *Response) MarshalJSON() ([]byte, error) {
    type tmp Response
    resp := (*tmp)(r)

    var data struct {
        Wrapper struct {
            *Person
        } `json:"person"`
        *tmp
    }
    data.Wrapper.Person = resp.Person
    data.tmp = resp
    return json.Marshal(data)
}

https://play.golang.org/p/1qkSCWZ225j

---

There may be other options...

Answer 2

In Go, an empty struct by definition assigns zero values to field elements. Eg: for int 0, "" for string, etc.

For your case, simply comparing to null would work out. Or, you could define an emptyPerson as:

 var BAD_AGE = -1
 emptyPerson := &Person{"", BAD_AGE} // BAD_AGE indicates no person
 if person[age] == BAD_AGE {
    // handle case for emptyPerson}