Simone Carletti Simone Carletti - 4 months ago 384
JSON Question

Decoding JSON in Golang 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

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 question from this answer says


Please use
json.Decoder
instead of
json.Unmarshal
.


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

Answer

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.