CODEWITHSUNDEEP
jsongo
Joe
Joe

Reputation: 47729

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?

Upvotes: 20

Views: 20528

Answers (2)

Hope Young
Hope Young

Reputation: 1

Here's another way, instead of using the slice []uint8, use the array [...]uint8:

marshal1, _ := json.Marshal([]uint8{32}) //"IA=="
marshal2, _ := json.Marshal([...]uint8{32}) //[32]

Also:

data := []uint8{32}
marshal, _ := json.Marshal((*[1]uint8)(data)) //after go1.17
marshal, _ := json.Marshal(*(*[1]uint8)(unsafe.Pointer(&data[0]))) //not recommended

Because go only treats byte slices specially (at least in the current version 1.23):

func newSliceEncoder(t reflect.Type) encoderFunc {
    // Byte slices get special treatment; arrays don't.
    if t.Elem().Kind() == reflect.Uint8 {
        p := reflect.PointerTo(t.Elem())
        if !p.Implements(marshalerType) && ! p.Implements(textMarshalerType) {
            return encodeByteSlice
        }
    }
    enc := sliceEncoder{newArrayEncoder(t)}
    return enc.encode
}

Upvotes: 0

the system
the system

Reputation: 9336

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

Upvotes: 31

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