Golang检查接口类型是否为nil

Question

When unmarshalling a string of json, using Golang's json.Unmarshal() function, I always unmarshal the string into a map[string]interface{} - I'm not sure weather there is a more optimal way of decoding json.

To get to the point:

Sometimes the json's unmarshalled type is nil, not string (or int etc.). This always throws a panic, saying:

interface conversion: interface is nil, not int

How can I avoid this panic or check if the interface's type is nil or not?

Example:

Here is an example of my problem in action: https://play.golang.org/p/0ATzXBbdoS

Answer 1

Check is the key exist instead of letting it panic.

func keyExists(decoded map[string]interface{}, key string) {
    val, ok := decoded[key]
    return ok && val != nil
}

func main() {

jsonText := `{
        "name": "Jimmy",
        "age":  23
    }`

    var decoded map[string]interface{}

    if err := json.Unmarshal([]byte(jsonText), &decoded); err != nil {
        fmt.Println(err)
        os.Exit(0)
    }

    if keyExists(decoded, "name") {
        fmt.Println(decoded["name"].(string))
    }
    if keyExists(decoded, "age") {
        fmt.Println(decoded["age"].(float64))
    }
    if keyExists(decoded, "gender") {
        fmt.Println(decoded["gender"].(int))
    }
}

Also, this is far from being optimal if you know what your json will look like. As specified in the documentation, you can unmarshal it directly into a struct:

type Human struct {
    Name string
    Age int
    Gender int
}

func main() {
    jsonText := `{
        "name": "Jimmy",
        "age":  23
    }`

    decoded := Human{}

    if err := json.Unmarshal([]byte(jsonText), &decoded); err != nil {
        fmt.Println(err)
        os.Exit(0)
    }

    fmt.Println(decoded.Name)
    fmt.Println(decoded.Age)
    fmt.Println(decoded.Gender)
}