フィールドタグとJSON

Tags on struct fields are metadata strings, read via reflection by libraries like encoding/json, encoding/xml, validators, ORMs. They are written between backticks (raw string) right after the field's type.

Syntax

type User struct {
    Name string `json:"nome"`
    Age  int    `json:"eta,omitempty"`
}

Format convention: key:"value" separated by spaces (NOT commas) for multiple keys:

type User struct {
    Email string `json:"email" validate:"required,email"`
}

json tag: options

The most common option is json, used by encoding/json:

type User struct {
    Name     string `json:"nome"`
    Age      int    `json:"eta,omitempty"`
    Password string `json:"-"`
}

u := User{Name: "Ada", Age: 0, Password: "segreto"}
b, _ := json.Marshal(u)
fmt.Println(string(b))   // {"nome":"Ada"}

Age is omitted (zero value 0 + omitempty); Password is excluded entirely.

Marshal and Unmarshal

// struct -> JSON
b, err := json.Marshal(u)

// JSON -> struct (needs a POINTER)
var u2 User
err = json.Unmarshal(b, &u2)

Unmarshal only accepts a pointer, because it must write into the value's fields.

Other common tags

• xml:"..." — encoding/xml
• yaml:"..." — yaml.v3
• db:"..." — sqlx / pgx
• validate:"..." — go-playground/validator
• form:"..." — gin / echo binding

Libraries read them via reflection (reflect.StructTag.Get("json")): they are just strings until someone interprets them.

Try it

u := User{Name: "Ada", Age: 36}
b, _ := json.Marshal(u)
fmt.Println(string(b))

Recap

• Tags are raw strings between backticks: `key:"value" key2:"value2"`.
• json:"nome": rename. omitempty: omit if zero value. -: exclude.
• NON-exported (lowercase) fields are ignored by encoding/json.
• json.Marshal(v) ↔ json.Unmarshal(data, &v) (pointer).
• Same mechanism for xml, yaml, db, validate, form, …