Go: JSON Marshaling an error

问题:

I'm building a JSON API in Go and I'd like to return error responses as json.

Example response:

{
    "error": "Invalid request syntax"
}

I thought that I could create a wrapper struct that implements the error interface, and then use Go's json marshaler as a clean way to get the json representation of the error:

type JsonErr struct {
    Err error `json:"error"`
}
func (t JsonErr) Error() string {
    return t.Err.Error()
}

This will just marshal a JsonErr as {"error":{}}, is there a way of using the default Go json marshaler to encode this struct, or do I need to write a quick custom MarshalJson for JsonErr structs?

回答1:

Just implement the json.Marshaler interface:

func main() {
    var err error = JsonErr{errors.New("expected")}
    json.NewEncoder(os.Stdout).Encode(err)
}

type JsonErr struct {
    error
}

func (t JsonErr) MarshalJSON() ([]byte, error) {
    return []byte(`{"error": "` + t.Error() + `"}`), nil
}

The reason it doesn't work is because json.Marshal has no detection for the error interface and most error types have no exported fields so reflection can't display those fields.