Decoding JSON using json.Unmarshal vs json.NewDecoder.Decode
JsonGoJson Problem Overview
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
?
Json Solutions
Solution 1 - Json
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 anio.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.
Solution 2 - Json
I found this paragraph in the Go web programming book. But there is no explanation given
> So when do we use Decoder versus Unmarshal?
That depends on the input. If your data is coming from an io.Reader stream, like the Body of an http.Request, use Decoder. If you have the data in a string or somewhere in memory, use Unmarshal.