我怎么知道该字段设置为null？

Question

I want to output an error if the field in json contains a value of null. How can I do it? I have tried "encoding/json". Maybe I need another library.

Code example:

package main

import (
    "encoding/json"
    "fmt"
    "strings"
)

type Item struct {
    Value  *int
}


func main() {
    var jsonBlob = `[
        {},
        {"Value": null},
        {"Value": 0},
        {"Value": 1}
    ]`
    var items []Item

    err := json.NewDecoder(strings.NewReader(jsonBlob)).Decode(&items)
    if err != nil {
        fmt.Println("error:", err)
    }
    for _, a := range items {
        if a.Value != nil {
            fmt.Println(*a.Value)
        } else {
            fmt.Println(a.Value)
        }
    }
}

I got:

<nil>
<nil>
0
1

I want:

<nil>
<error>
0
1

Please help. Many thanks!

Answer 1

If you want to control how a type is unmarshaled, you can implement json.Unmarshaler

Since a map allows you to tell the difference between an unset value, and a null value, unmarshaling first into a generic map[string]interface{} will allow you to inspect the values without tokenizing the JSON.

type Item struct {
    Value *int
}


func (i *Item) UnmarshalJSON(b []byte) error {
    tmp := make(map[string]interface{})

    err := json.Unmarshal(b, &tmp)
    if err != nil {
        return err
    }

    val, ok := tmp["Value"]
    if ok && val == nil {
        return errors.New("Value cannot be nil")

    }
    if !ok {
        return nil
    }

    f, ok := val.(float64)
    if !ok {
        return fmt.Errorf("unexpected type %T for Value", val)
    }

    n := int(f)
    i.Value = &n
    return nil
}

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