How Can I Efficiently Parse JSON in Go Without Redundant Unmarshaling?

Parsing JSON Without Redundant Unmarshaling in Go

When dealing with JSON messages that contain different types of data, a common approach may involve unmarshaling the data into a generic map[string]interface{} to inspect the key and attempt to cast the value into specific structs. However, this approach can lead to unnecessary double unmarshaling.

In the context of Go, there is a more efficient solution to partially unmarshal the JSON data into a json.RawMessage instead of interface{}. This is achieved by declaring the map as:

var myMap map[string]json.RawMessage

When it comes to casting the values to specific structs, the marshaling/unmarshaling step can be eliminated. For instance, in your case, you were casting to your "Ack" struct using:

ackjson, err := json.Marshal(v)
if err != nil {
    fmt.Println("marshal error: ", err)
}
err = json.Unmarshal(ackjson, &myAck)
if err != nil {
    fmt.Println("unmarshal error", err)
}

Instead, it can be simplified to:

err = json.Unmarshal(v, &myAck)

This approach avoids the redundant unmarshaling step, making the parsing more efficient.

Here is an updated version of your code that incorporates this optimization:

package main

import (
    "encoding/json"
    "fmt"
)

type Ping struct {
    Ping string `json:"ping"`
}

type Ack struct {
    Messages []Message `json:"messages"`
}

type Message string

func main() {
    testJSON := []byte(`{"ack":{"messages":["Hi there","Hi again"]}}`)
    var myAck = Ack{}
    var myMap map[string]json.RawMessage
    err := json.Unmarshal(testJSON, &myMap)
    if err != nil {
        fmt.Println("error unmarshalling: ", err)
    }
    for k, v := range myMap {
        fmt.Printf("key: %s, value: %s \n", k, v)

        switch k {
        case "ping":
            fmt.Println(k, " is a ping", v)
        case "ack":
            fmt.Println(k, " is an ack containing a message list")
            err = json.Unmarshal(v, &myAck)
            if err != nil {
                fmt.Println("unmarshal error", err)
            } else {
                fmt.Println("New ack object: ", myAck)
            }
        default:
            fmt.Printf("%s is of a type (%T) I don't know how to handle", k, v)
        }
    }
}

By utilizing json.RawMessage and eliminating the unnecessary unmarshaling, you can improve the performance of your JSON parsing without compromising functionality.

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