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

Question

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?

This pull request on GitHub replaced a call to Unmarshal with json.NewDecoder to "remove buffer in JSON decoding." — Matt
It just depends on what input is more convenient for you to use. blog.golang.org/json-and-go gives examples of using both techniques. — rexposadas
IMO, ioutil.ReadAll is almost always the wrong thing to do. It's not related to your goal, but requires you to have enough contiguous memory to store whatever might be coming down the pipe, even if the last 20TB of response is after the last } in your JSON. — Dustin

James Henstridge James Henstridge · Accepted Answer · 2014-01-18T00:40:59

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.

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

1 Answers