为非内置类型定义自定义解组

Question

Most of us know that a JSON object can be unmarshaled with JSON tags:

var jsonData = `{"name": "Brown Bear"}`

type Elephant struct {
    Name string `json:"name"`
}

This is because string is a built in types. But what if Name were not a built in type, and we wanted to use this type across different structs?

var jsonData = `{"name": "Brown Bear"}`

type Elephant struct {
    Name Name   `json:"name"`  // Unmarshalling fails here
}

type Feline struct {
    Name Name   `json:"name"`  // Unmarshalling fails here
}

type Bear struct {
    Name Name   `json:"name"`  // Unmarshalling fails here
}

type Name struct {
    CommonName     string
    ScientificName string  // This field should intentionally be blank
}

Is there a way we can define the Name type so that the json unmarshaller knows how to unmarshal it?

PS: The solution I want to avoid is creating UnmarshalJSON methods for Elephant, Feline, and Bear above. It would be better to create a method just for the Name type.

Answer 1

See the json.Marshaler and json.Unmarshaler types in the encoding/json package which allow you to define custom JSON en/decoding functions for arbitrary types.

package main

import (
  "encoding/json"
  "fmt"
)

type Name struct {
  CommonName     string
  ScientificName string
}

func (n *Name) UnmarshalJSON(bytes []byte) error {
  var name string
  err := json.Unmarshal(bytes, &name)
  if err != nil {
    return err
  }
  n.CommonName = name
  n.ScientificName = ""
  return nil
}

type Elephant struct {
  Name Name `json:"name"`
  Age  int  `json:"age"`
}

func main() {
  alice := Elephant{}
  aliceJson := `{"name":"Alice","age":2}`
  err := json.Unmarshal([]byte(aliceJson), &alice)
  if err != nil {
    panic(err)
  }
  fmt.Printf("%#v
", alice)
}
// main.Elephant{Name:main.Name{CommonName:"Alice", ScientificName:""}, Age:2}