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?
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.
io.LimitReader to prevent that.
ReadAll is almost always the wrong" I think if JSON is small and about few Kb then it might be better to read it all to release connection. And this is very often so it looks like ReadAll and then Unmarshall is better solution in most cases
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.
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.
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Bufferedmethod is there to let you see any extra data that was read into the internal buffer after the value.