How to Perform Type Assertions on Unmarshaled JSON Data in Go?

Type Assertions with Unmarshaled Data

In a distributed system, data is often exchanged as JSON strings. When retrieving JSON data from a message queue, you may encounter a scenario where you need to deserialize the data into an interface{} and then perform type assertions to determine the actual struct type of the data.

Problem

When unmarshaling the received JSON string into an interface{} and attempting to type assert the result, you may encounter an unexpected result. Instead of the expected struct type, you obtain a map[string]interface{}.

Solution

The default behavior of JSON unmarshaling into an interface{} yields types such as bool, float64, string, []interface{}, and map[string]interface{}. Since Something1 and Something2 are custom structs, they are not recognized by the JSON unmarshaler.

To resolve this, there are two main approaches:

1. Unmarshal Directly into Custom Structs

Code:

var input Something1
json.Unmarshal([]byte(msg), &input)
// Type assertions and processing can be performed here

var input Something2
json.Unmarshal([]byte(msg), &input)
// Type assertions and processing can be performed here

2. Unpack from Map Interface

Code:

var input interface{}
json.Unmarshal([]byte(msg), &input)

// Unpack the data from the map
switch v := input.(type) {
case map[string]interface{}:
    // Extract the data from the map and assign it to custom structs
}

Advanced Approach

For a more versatile solution, consider creating an "Unpacker" struct that handles the unmarshaling and type assertion process.

Code:

type Unpacker struct {
    Data       interface{}
}

func (u *Unpacker) UnmarshalJSON(b []byte) error {
    smth1 := &Something1{}
    err := json.Unmarshal(b, smth1)
    if err == nil && smth1.Thing != "" {
        u.Data = smth1
        return nil
    }

    smth2 := &Something2{}
    err = json.Unmarshal(b, smth2)
    if err != nil {
        return err
    }

    u.Data = smth2
    return nil
}

Conclusion

By using one of these approaches, you can successfully perform type assertions on data that is first unmarshaled into an interface{} from a JSON string. The choice of approach depends on the specific requirements of your application.

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