How Can I Successfully Unmarshal JSON Data to Interface Types in Go?

Unmarshalling to Interface Types

One of the challenges in Go's RPC and JSON packages is unmarshalling to interface types. Let's dissect this issue and explore its solution.

The Problem

When attempting to unmarshal to an interface type, we encounter errors like "json: cannot unmarshal object into Go value of type main.Foo." This occurs because the reflector cannot determine the appropriate concrete type to instantiate for the incoming marshaled data.

The Solution

The issue arises from the lack of type information available to the reflector during unmarshalling. For example, while marshalling is possible from interface type variables, unmarshalling is not due to the reflector's inability to identify the corresponding concrete type.

To solve this problem, frameworks like Java's Json (jackson) employ annotations to provide additional type information. However, in Go, we can implement the Unmarshaler interface for custom types to handle the unmarshalling process ourselves.

Implementing Unmarshaler

Here's an example of implementing the Unmarshaler interface for our custom type RawString:

type RawString string

func (m *RawString) MarshalJSON() ([]byte, error) {
    return []byte(*m), nil
}

func (m *RawString) UnmarshalJSON(data []byte) error {
    *m += RawString(data)
    return nil
}

Now, we can unmarshal JSON data into a RawString instance:

const data = `{"i":3, "S":{"phone": {"sales": "2223334444"}}}`

type A struct {
    I int64
    S RawString `sql:"type:json"`
}

func main() {
    a := A{}
    if err := json.Unmarshal([]byte(data), &a); err != nil {
        log.Fatal("Unmarshal failed", err)
    }
    fmt.Println("Done", a)
}

By providing the Unmarshaler implementation for our custom type, we can unmarshal JSON data without encountering the aforementioned error.

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