Custom config loading in Go - json

Question

I'm using JSON files to store/load my config. Let's say I have the following:
type X interface
// implements interface X
type Y struct {
Value string
}
// implements interface X
type Z struct {
Value string
}
type Config struct {
interfaceInstance X `json:"X"`
}
Config file example:
{
"config1": {
"X": {
"type": "Z",
"Value": "value_1"
}
},
"config2": {
"X": {
"type": "Y",
"Value": "value_2"
}
}
}
I want to be able to define config files something like this example, and be able to dynamically load the JSON as either struct Y or struct Z. Any suggestions on how to accomplish this? I'm using a simple json.Decoder to load the JSON as a struct.
decoder := json.NewDecoder(file)
err = decoder.Decode(&config)

Answer 1

One possible strategy would be to implement json.Unmarshaler for the Config type in such a way that you first unmarshal into a generic object and inspect the "type" attribute then, switching on the type string, unmarshal the same byte array into the known type and assign to the "interfaceInstance" member of the config.
For example (Go Playground):
// Note the slightly different JSON here...
var jsonstr = `{
"config1": {
"type": "Z",
"Value": "value_1"
},
"config2": {
"type": "Y",
"Value": "value_2"
}
}`
func main() {
config := map[string]Config{}
err := json.Unmarshal([]byte(jsonstr), &config)
if err != nil {
panic(err)
}
fmt.Printf("OK: %#v\n", config)
// OK: map[string]main.Config{
// "config1": main.Config{interfaceInstance:main.Z{Value:"value_1"}},
// "config2": main.Config{interfaceInstance:main.Y{Value:"value_2"}},
// }
}
func (c *Config) UnmarshalJSON(bs []byte) error {
// Unmarshal into an object to inspect the type.
var obj map[string]interface{}
err := json.Unmarshal(bs, &obj)
if err != nil {
return err
}
// Unmarshal again into the target type.
configType := obj["type"].(string)
switch configType {
case "Y":
var y Y
if err = json.Unmarshal(bs, &y); err == nil {
c.interfaceInstance = y
}
case "Z":
var z Z
if err = json.Unmarshal(bs, &z); err == nil {
c.interfaceInstance = z
}
default:
return fmt.Errorf("unexpected type %q", configType)
}
return err
}