How to Parse JSON in Go Without Double Unmarshaling?

Parsing JSON Without Double Unmarshaling in Go

In Go, you may encounter a situation where you want to parse JSON data of different types without unmarshaling twice. Let's explore the steps involved:

Step 1: Unmarshal into a Generic Map

As you mentioned, the first step is to unmarshal the JSON into a generic map[string]interface{}. This provides a map-like structure where keys are strings and values are dynamic interface{} types that can hold various data structures.

Step 2: Identify the Key and Value Type

Once you have the map, iterate through it to find the key you're interested in. Depending on the value type, you can proceed with the next steps.

Step 3: Handle Known Types Directly

If the value type is a known struct, you can directly cast it and use it as your desired struct. For example, if you have a Ping struct, you can check if the map value is castable to Ping. This works well if the JSON field names match the struct field names and the data types are compatible.

Step 4: Alternative Approach for Unknown Types

In cases where the value type is not known or incompatible with your predefined structs, you can employ the following approach:

1. Marshal the value from the interface{} back to JSON using json.Marshal.
2. Unmarshal the JSON into your desired struct using json.Unmarshal.

Improved Partial Unmarshaling

To avoid the need for double unmarshaling, you can partially unmarshal the data by unmarshaling values into json.RawMessage instead of interface{}. json.RawMessage represents a raw JSON value and can be unmarshaled directly into your desired struct later on.

Example with Code

In your provided example, you can modify the code as follows:

var myMap map[string]json.RawMessage

...

switch k {
    case "ping":
        // ...
    case "ack":
        err = json.Unmarshal(v, &myAck)
}

This approach avoids the need for the second unmarshaling step when the value type is known. However, you would still need to write a switch statement to handle different cases.

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