How to marshal a byte/uint8 array as json array in Go?

I've got a struct with a []uint8 member and I'm writing it with json.Marshal. Trouble is, it's interpreting the uint8s as chars and it outputs a string rather than an array of numbers.

I can get this to work if it's a []int, but I don't want to have to allocate and copy over the items if I can avoid it. Can I?

According to the docs, a []byte will be encoded as a Base64 string.

>"Array and slice values encode as JSON arrays, except that []byte encodes as a base64-encoded string, and a nil slice encodes as the null JSON object."

So I think that you may need to make your struct implement the Marshaler interface by implementing your own MarshalJSON method that makes a more desirable JSON array encoding out of your []uint8.

Take this example:

import "fmt"
import "encoding/json"
import "strings"

type Test struct {
	Name  string
	Array []uint8
}

func (t *Test) MarshalJSON() ([]byte, error) {
	var array string
	if t.Array == nil {
		array = "null"
	} else {
		array = strings.Join(strings.Fields(fmt.Sprintf("%d", t.Array)), ",")
	}
	jsonResult := fmt.Sprintf(`{"Name":%q,"Array":%s}`, t.Name, array)
	return []byte(jsonResult), nil
}

func main() {
	t := &Test{"Go", []uint8{'h', 'e', 'l', 'l', 'o'}}

	m, err := json.Marshal(t)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Printf("%s", m) // {"Name":"Go","Array":[104,101,108,108,111]}
}

http://play.golang.org/p/Tip59Z9gqs

Or maybe a better idea would be to make a new type that has []uint8 as its underlying type, make that type a Marshaler, and use that type in your struct.

import "fmt"
import "encoding/json"
import "strings"

type JSONableSlice []uint8

func (u JSONableSlice) MarshalJSON() ([]byte, error) {
	var result string
	if u == nil {
		result = "null"
	} else {
		result = strings.Join(strings.Fields(fmt.Sprintf("%d", u)), ",")
	}
	return []byte(result), nil
}

type Test struct {
	Name  string
	Array JSONableSlice
}

func main() {
	t := &Test{"Go", []uint8{'h', 'e', 'l', 'l', 'o'}}

	m, err := json.Marshal(t)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Printf("%s", m) // {"Name":"Go","Array":[104,101,108,108,111]}
}

http://play.golang.org/p/6aURXw8P5d