如何在Go中解析纯数字和加引号的JSON数字？

Question

I'm dealing with a third-party JSON-based API. Usually it wraps all numbers in quotes, but sometimes it doesn't. Nothing I can do about it.

I'm trying to come up with a solution, which parses the numbers regardless of whether they're enquoted or not. Standard library provides a ,string field tag, which allows to map numeric fields to enquoted values, but, unfortunately, it then fails to process the value, if it's not in quotes.

func test(s string) {
  err := json.Unmarshal([]byte(s), &struct {
    F1 float64
    F2 float64 `json:",string"`
  }{})
  if err != nil {
    fmt.Println(err)
    return
  }

  fmt.Println("success")
}

func main() {
  test(`{"f1": 1.23}`)   // success
  test(`{"f1": "1.23"}`) // cannot unmarshal string into Go value of type float64
  test(`{"f2": 1.23}`)   // invalid use of ,string struct tag, trying to unmarshal unquoted value into ...
  test(`{"f2": "1.23"}`) // success
}

The Go Playground

Is there a way around this?

Answer 1

The proper solution is to "clone" float64 and define custom MarshalJSON and UnmarshalJSON for it:

type jsonFloat64 float64

func (f jsonFloat64) MarshalJSON() ([]byte, error) {
  return json.Marshal(float64(f))
}

func (f *jsonFloat64) UnmarshalJSON(data []byte) error {
  if len(data) >= 2 && data[0] == '"' && data[len(data)-1] == '"' {
    data = data[1 : len(data)-1]
  }

  var tmp float64
  err := json.Unmarshal(data, &tmp)
  if err != nil {
    return err
  }

  *f = jsonFloat64(tmp)
  return nil
}

Then you'd be able to do something like this:

func test(s string) {
  err := json.Unmarshal([]byte(s), &struct {
    F jsonFloat64
  }{})
  if err != nil {
    fmt.Println(err)
    return
  }

  fmt.Println("success")
}

func main() {
  test(`{"f": 1.23}`)   // success
  test(`{"f": "1.23"}`) // success
}

The Go Playground

Feel free to adjust UnmarshalJSON to your needs, mine is pretty strict about the spacing. Credit goes to Dave C, this was heavily inspired by his comment on another question (which also features more variations on the solution above).

Alternatively, you could pre-process the JSON data with a regular expression, but don't do this if the solution above is a viable option, it's much faster.

re := regexp.MustCompile(`(":\s*)([\d\.]+)(\s*[,}])`)
rawJsonByteArray = re.ReplaceAll(rawJsonByteArray, []byte(`$1"$2"$3`))