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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?

Dave C
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Simone Carletti
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  • This [pull request on GitHub](https://github.com/codegangsta/martini-contrib/pull/84) replaced a call to Unmarshal with json.NewDecoder to "remove buffer in JSON decoding." – Matt Jan 17 '14 at 22:46
  • It just depends on what input is more convenient for you to use. http://blog.golang.org/json-and-go gives examples of using both techniques. – rexposadas Jan 17 '14 at 23:00
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    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 Jan 17 '14 at 23:40
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    @Dustin You can use `io.LimitReader` to prevent that. – Inanc Gumus Apr 01 '18 at 20:17
  • @Dustin "`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 – Sergey Ponomarev Sep 07 '21 at 12:32

2 Answers2

367

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.

Seth
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James Henstridge
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    Also: by inspecting the Go 1.3 source code, we can also learn that for encoding, if you use a json.Encoder, it will reuse a global buffer pool (backed by the new sync.Pool), which should decrease buffer churn a lot if you're encoding a lot of json. There's only one global pool so different json.Encoder's share it. The reason this couldn't be done for the json.Marshal interface is because the bytes are returned to the user and the user doesn't have a way to "return" the bytes to the pool. So if you're doing a lot of encoding, json.Marshal always has quite a bit buffer churn. – Aktau Jun 23 '14 at 11:19
  • @Flimzy: are you sure? The source code still says it reads the entire value into the buffer before decoding: https://github.com/golang/go/blob/master/src/encoding/json/stream.go#L56-L62. The `Buffered` method is there to let you see any extra data that was read into the internal buffer after the value. – James Henstridge Apr 19 '17 at 01:36
  • @JamesHenstridge: No, you're probably right. I was just interpreting your statement differently than you intended. Apologies for the confusion. – Jonathan Hall Apr 19 '17 at 13:02
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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.

Samir Kape
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