如何在Go中使用非必需的JSON参数？

Question

Hi I am working on a rest API in Go and I want the user to pass JSON parameters:

Offset int64  `json:"offset"`
Limit  int64  `json:"limit"`
SortBy string `json:"sortby"`
Asc    bool   `json:"asc"`
Username   string `json:"username"`
First_Name string `json:"first_name"`
Last_Name  string `json:"last_name"`
Status     string `json:"status"`

But they are not always required so for example a user can pass only Offset and ignore the others. He can even send 0 parameters. How can I do this?

Answer 1

When unmarshalling a value from JSON text, the json package does not require that all fields to be present in JSON, nor that all JSON fields have a matching Go field.

So you don't have anything special to do, just unmarshal what you have to a Go value what you want or may want.

One thing to note is that if a field is missing in the JSON text, the json package will not change the corresponding Go field, so if you start with a "fresh", zero value, the field will be left with the zero value of its type.

Most of the time this is enough to detect the presence or absence of a field (in JSON), for example if in the Go struct you have a SortBy field of type string, if this is missing in JSON, it will remain the empty string: "".

If the zero value is something useful and valid, then you may turn to use pointers. For example if in your application the empty string would be a valid SortBy value, you may declare this field to be a pointer: *string. And in this case if it's missing in the JSON text, it will remain nil, the zero value for any pointer type.

See this example:

type Data struct {
    I int
    S string
    P *string
}

func main() {
    var d Data
    var err error

    d, err = Data{}, nil
    err = json.Unmarshal([]byte(`{"I":1, "S":"sv", "P":"pv"}`), &d)
    fmt.Printf("%#v %v
", d, err)

    d, err = Data{}, nil
    err = json.Unmarshal([]byte(`{"I":1}`), &d)
    fmt.Printf("%#v %v
", d, err)

    d, err = Data{}, nil
    err = json.Unmarshal([]byte(`{"S":"abc"}`), &d)
    fmt.Printf("%#v %v
", d, err)
}

Output (try it on the Go Playground):

main.Data{I:1, S:"sv", P:(*string)(0x1050a150)} <nil>
main.Data{I:1, S:"", P:(*string)(nil)} <nil>
main.Data{I:0, S:"abc", P:(*string)(nil)} <nil>