在Golang中解编json：必填字段？

Question

Is it possible to generate an error if a field was not found in json while unmarshaling in Golang? I could not find it in documentation. Is there any tag that specifies the field as required?

Answer 1

There is no tag in the encoding/json package that sets a field to "required". You will either have to write your own MarshalJSON() method, or do a post check for missing fields.

To check for missing fields, you will have to use pointers in order to distinguish between missing/null and zero values:

type JsonStruct struct {
    String *string
    Number *float64
}

Full working example:

package main

import (
    "fmt"
    "encoding/json"
)

type JsonStruct struct {
    String *string
    Number *float64
}

var rawJson = []byte(`{
    "string":"We do not provide a number"
}`)


func main() {
    var s *JsonStruct
    err := json.Unmarshal(rawJson, &s)
    if err != nil {
        panic(err)
    }

    if s.String == nil {
        panic("String is missing or null!")
    }

    if s.Number == nil {
        panic("Number is missing or null!")
    }

    fmt.Printf("String: %s  Number: %f
", *s.String, *s.Number)
}

Playground