How to Customize JSON Unmarshaling into a Specific Struct in Go?

Custom Unmarshaling of JSON Data into a Specific Struct

In Go, unmarshalling JSON data into a predefined struct is straightforward. However, what if you want to customize the data structure even further? Suppose you have the following JSON data:

<code class="json">{
  "Asks": [
    [21, 1],
    [22, 1]
  ],
  "Bids": [
    [20, 1],
    [19, 1]
  ]
}</code>

You can easily define a struct to match this structure:

<code class="go">type Message struct {
    Asks [][]float64 `json:"Asks"`
    Bids [][]float64 `json:"Bids"`
}</code>

However, you may prefer a more specialized data structure:

<code class="go">type Message struct {
    Asks []Order `json:"Asks"`
    Bids []Order `json:"Bids"`
}

type Order struct {
    Price float64
    Volume float64
}</code>

Custom Unmarshaling Using json.Unmarshaler

To unmarshal the JSON data into this specialized structure, you can implement the json.Unmarshaler interface on your Order struct:

<code class="go">func (o *Order) UnmarshalJSON(data []byte) error {
    var v [2]float64
    if err := json.Unmarshal(data, &v); err != nil {
        return err
    }
    o.Price = v[0]
    o.Volume = v[1]
    return nil
}</code>

This implementation specifies that the Order type should be decoded from a two-element array of floats rather than the default representation for a struct (an object).

Example Usage:

<code class="go">m := new(Message)
err := json.Unmarshal(b, &m)
fmt.Println(m.Asks[0].Price) // Output: 21</code>

By implementing json.Unmarshaler, you can easily customize the unmarshalling process to suit your specific data structure requirements.

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