Golang在编组而不是编组时会忽略Json字段

Assume I have a password field in a User struct.

type User struct{
   UserName string `json:"username"`
   Password string `json:"-"`
}

My clients register their users by posting username and password together. So if I decode JSON to above struct, it ignores password. It's expected. But I wondered is there any way to ignore fields when only marshalling. I checked go official documentation page but couldn't find anything.

https://golang.org/pkg/encoding/json/

I can add an extra field into the struct but I need to know first is it possible to do that with JSON lib.

One common approach is to use a temporary type or variable, with same structure, but different json tags or even different structure:

type User struct {
    UserName string `json:"username"`
    Password string `json:"password"`
}

func (usr User) MarshalJSON() ([]byte, error) {
    var tmp struct {
        UserName string `json:"username"`
    }
    tmp.UserName = usr.UserName
    return json.Marshal(&tmp)
}

As with any custom marshaling/unmarshaling requirements in Go, your best bet is to look at implementing json.Marshaler/json.Unmarshaler interface on a custom type.

In this case, you can do it for the password field:

// Use an explicit password type.
type password string

type User struct{
    UserName string   `json:"username"`
    Password password `json:"password"`
}

// Marshaler ignores the field value completely.
func (password) MarshalJSON() ([]byte, error) {
    return []byte(`""`), nil
}

Note that by not implementing json.Unmarshaler, type password retains the default behavior of its underlying type (string), which allows the struct to be unmarshalled with the Password value.

Working example: https://play.golang.org/p/HZQoCKm0vN

I prefer type composition. For example:

type User struct {
    UserName string `json:"username"`
}

type UserWithPassword struct {
    *User
    Password string `json:"password"`
}

use UserWithPassword when receiving data (eg. user creation / modification) and User when returning it (authentication).