JSON Decoding in Go: `json.Unmarshal` vs. `json.NewDecoder.Decode` – Which Method Should I Choose?

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

When developing API clients, the task of encoding JSON payloads for requests and decoding JSON responses arises. This inquiry explores the options available for JSON decoding: json.Unmarshal and json.NewDecoder.Decode.

Which Method Should I Use?

The second method, json.NewDecoder.Decode, appears more efficient when dealing with HTTP responses implementing io.Reader. However, it's crucial to understand the underlying differences between the two methods.

json.Unmarshal

json.Unmarshal takes the entire JSON string as input and decodes it into a Go value. While this approach is straightforward, it requires the JSON data to be fully loaded into memory before decoding.

json.NewDecoder.Decode

Conversely, json.NewDecoder.Decode uses a streaming approach. It buffers the JSON data while decoding, which can be more memory efficient when dealing with large JSON payloads. However, this method requires the JSON data to be supplied via an io.Reader.

When to Use Each Method

As a general guideline:

• Use json.Decoder when:

  • Data is coming from an io.Reader stream.
  • Decoding multiple values from a data stream.

• Use json.Unmarshal when:

  • JSON data is already in memory.

Conclusion

Both methods, json.Unmarshal and json.NewDecoder.Decode, serve different purposes and should be chosen based on the input format and requirements of your application. For decoding JSON responses from HTTP requests, json.NewDecoder.Decode is a more suitable choice due to its streaming approach.

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