How Can I Efficiently Extract Specific JSON Values in Go Without Using Structs?

JSON Value Parsing in Go

In some cases, extracting a specific value from a JSON object without the need for a struct is advantageous. This article explores how to achieve this in Go, providing alternatives to the more traditional approach of unmarshalling into a struct first.

Option 1: Map[string]interface{}

Go offers a built-in solution for storing JSON values without a predefined struct. Using the json.Unmarshal function, you can decode the JSON into a map[string]interface{}, where the keys are strings representing the JSON field names, and the values are the corresponding values as Go native types.

Example:

b := []byte(`{"ask_price": "1.0"}`)
data := make(map[string]interface{})
err := json.Unmarshal(b, &data)
if err != nil {
  panic(err)
}

if price, ok := data["ask_price"].(string); ok {
  fmt.Println(price)
} else {
  panic("wrong type")
}

This approach allows you to access the specific value you need directly by its JSON property name, without the need for a corresponding struct field. However, it requires type assertions to ensure that the retrieved value is the expected type.

Option 2: Custom Unmarshaler

Alternatively, you can implement a custom json.Unmarshaler interface for your target type to handle the decoding process more specifically. This provides more control over how the JSON data is unmarshalled into the desired Go value.

Example:

type AskPrice struct {
  price string
}

func (a *AskPrice) UnmarshalJSON(data []byte) error {
  var v string 
  if err := json.Unmarshal(data, &v); err != nil {
    return err
  }
  a.price = v
  return nil
}

// ...

With this approach, you can specify the custom unmarshaller by tagging the corresponding struct field with json:"ask_price" and using the & pointer receiver to ensure the decoded value is assigned to the struct field directly. This eliminates the need for explicit type assertions and provides a more tailored unmarshalling experience.

By carefully considering these options, you can efficiently extract specific JSON values in Go without relying on predefined structs.

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