Decoding JSON using json.Unmarshal vs json.NewDecoder.Decode

I’m developing an API client where I need to encode a JSON payload on request and decode a JSON body from the response.

I’ve read the source code from several libraries and from what I have seen, I have basically two possibilities for encoding and decoding a JSON string.

Use json.Unmarshal passing the entire response string

data, err := ioutil.ReadAll(resp.Body)
if err == nil && data != nil {
    err = json.Unmarshal(data, value)

or using json.NewDecoder.Decode

err = json.NewDecoder(resp.Body).Decode(value)

In my case, when dealing with HTTP responses that implements io.Reader , the second version seems to be require less code, but since I’ve seen both I wonder if there is any preference whether I should use a solution rather than the other.

Moreover, the accepted answer from this question says

Please use json.Decoder instead of json.Unmarshal .

but it didn’t mention the reason. Should I really avoid using json.Unmarshal ?

1 Like

It really depends on what your input is. If you look at the implementation of the Decode method of json.Decoder , it buffers the entire JSON value in memory before unmarshalling it into a Go value. So in most cases it won’t be any more memory efficient (although this could easily change in a future version of the language).

So a better rule of thumb is this:

  • Use json.Decoder if your data is coming from an io.Reader stream, or you need to decode multiple values from a stream of data.
  • Use json.Unmarshal if you already have the JSON data in memory.

For the case of reading from an HTTP request, I’d pick json.Decoder since you’re obviously reading from a stream.