How to get the json field names of a struct in golang?

What is the way to get the json field names of this struct ?

type example struct {
    Id 		    int `json:"id"`
    CreatedAt 	string `json:"created_at"`
    Tag 		string `json:"tag"`
    Text 		string `json:"text"`
    AuthorId 	int `json:"author_id"`
}

I try to print the fields with this function :

func (b example) PrintFields() {
    val := reflect.ValueOf(b)
    for i := 0; i < val.Type().NumField(); i++ {
	    fmt.Println(val.Type().Field(i).Name)
    }
}

Of course I get :

Id
CreatedAt
Tag
Text
AuthorId

But I would like something like :

id
created_at
tag
text
author_id

You use the StructTag type to get the tags. The documentation I linked has examples, look them up, but your code could be something like

func (b example) PrintFields() {
  val := reflect.ValueOf(b)
  for i := 0; i < val.Type().NumField(); i++ {
     fmt.Println(val.Type().Field(i).Tag.Get("json"))
  }
}

NOTE The json tag format supports more than just field names, such as omitempty or string, so if you need an approach that takes care of that too, further improvements to the PrintFields function should be made:

1. we need to check whether the json tag is - (i.e. json:"-")
2. we need to check if name exists and isolate it

Something like this:

func (b example) PrintFields() {
  val := reflect.ValueOf(b)
  for i := 0; i < val.Type().NumField(); i++ {
	t := val.Type().Field(i)
	fieldName := t.Name

	switch jsonTag := t.Tag.Get("json"); jsonTag {
	case "-":
	case "":
		fmt.Println(fieldName)
	default:
		parts := strings.Split(jsonTag, ",")
		name := parts[0]
		if name == "" {
			name = fieldName
		}
		fmt.Println(name)
	}
  }
}

Instead of using StructField's Name, you can use Tag to get a StructTag object.
See: https://golang.org/pkg/reflect/#StructTag

Then you can use StructTag's Get or Lookup methods to get the json tag:

Using Get:

func (b example) PrintFields() {
    val := reflect.ValueOf(b)
    for i := 0; i < val.Type().NumField(); i++ {
        // prints empty line if there is no json tag for the field
        fmt.Println(val.Type().Field(i).Tag.Get("json"))
    }
}

Using Lookup:

func (b example) PrintFields() {
    val := reflect.ValueOf(b)
    for i := 0; i < val.Type().NumField(); i++ {
        // skips fields without json tag
        if tag, ok := val.Type().Field(i).Tag.Lookup("json"); ok {
            fmt.Println(tag)
        }
    }
}

Use:

func (b example) PrintFields() {
    val := reflect.ValueOf(b)
    t := val.Type()
    for i := 0; i < t.NumField(); i++ {
        fmt.Println(t.Field(i).Tag.Get("json"))
    }
}

See it in playground.

an updated version with a generic interface as parameter:

func PrintFields(b interface{}) {
    val := reflect.ValueOf(b)
    for i := 0; i < val.Type().NumField(); i++ {
        t := val.Type().Field(i)
        fieldName := t.Name

        if jsonTag := t.Tag.Get("json"); jsonTag != "" && jsonTag != "-" {
			// check for possible comma as in "...,omitempty"
			var commaIdx int
			if commaIdx = strings.Index(jsonTag, ","); commaIdx < 0 {
				commaIdx = len(jsonTag)
			}
			fieldName = jsonTag[:commaIdx]
        }
        fmt.Println(fieldName)
    }
}

func (e example) GetJsonField() []string {
	b := example{}
	marshaled, _ := json.Marshal(b)
	m := make(map[string]interface{})
	_ = json.Unmarshal(marshaled, &m)
	
	result := make([]string, 0)
	for k := range m {
		result = append(result, k)
	}
	return result
}

Not the Name you are looking for. What you looking is the Tag

func (b example) PrintFields() {
  val := reflect.ValueOf(b)
  for i := 0; i < val.Type().NumField(); i++ {
     fmt.Println(val.Type().Field(i).Tag.Get("json"))
  }
}

Get Json field name Using Struct Field Name

val := reflect.ValueOf(structobject)    
field, _ := val.Type().FieldByName("Id")
fmt.Println(field.Tag.Get("json"))