Unmarshaling Into an Interface{} and Then Performing Type Assertion

Question

I get a string through a rabbitmq message system. Before sending,

I use json.Marshal, convert the outcome to string and send t

Answer 1

The default types that the json package Unmarshals into are shown in the Unmarshal function documentation

bool, for JSON booleans
float64, for JSON numbers
string, for JSON strings
[]interface{}, for JSON arrays
map[string]interface{}, for JSON objects
nil for JSON null

Since you're unmarshaling into an interface{}, the returned types will only be from that set. The json package doesn't know about Something1 and Something2. You either need to convert from the map[string]interface{} that the json object is being unmarshaled into, or unmarshal directly into the struct type you want.

If you don't want to do unpack the data from a generic interface, or somehow tag the data so you know what type to expect, you could iteratively take the json and try to unmarshal it into each type you want.

You can even pack those into a wrapper struct to do the unmarshaling for you:

type Something1 struct {
    Thing      string `json:"thing"`
    OtherThing int64  `json:"other_thing"`
}

type Something2 struct {
    Croc  int  `json:"croc"`
    Odile bool `json:"odile"`
}

type Unpacker struct {
    Data       interface{}
}

func (u *Unpacker) UnmarshalJSON(b []byte) error {
    smth1 := &Something1{}
    err := json.Unmarshal(b, smth1)

    // no error, but we also need to make sure we unmarshaled something
    if err == nil && smth1.Thing != "" {
        u.Data = smth1
        return nil
    }

    // abort if we have an error other than the wrong type
    if _, ok := err.(*json.UnmarshalTypeError); err != nil && !ok {
        return err
    }

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

    u.Data = smth2
    return nil
}

http://play.golang.org/p/Trwd6IShDW

Answer 2

You have encountered a typical json vs typed language problem! Since json is untyped and schemaless, it is not possible to infer what data is "under the string" without actually decoding it.

So your only option is to unmarshal into an interface{} which always produces a map[string]interface{}. You could do some reflection magic here to build the final struct, but that's a lot of manual work and error prone. Here are some possible solutions:

Quick 'n' dirty

Let the json package do the reflection stuff. Attempt to unmarshal into every expected type:

func typeAssert(msg string) {

 var thing1 Something1

 err := json.Unmarshal([]byte(msg), &thing1)
 if err == nil{
    // do something with thing1
    return
 }    

 var thing2 Something2

 err = json.Unmarshal([]byte(msg), &thing2)
 if err == nil{
    // do something with thing2
    return
 }    

 //handle unsupported type

}

Build your own "type system" on top of json

Defer the encoding until you know what's inside. Use this struct as an intermediate representation of your data:

type TypedJson struct{
  Type string 
  Data json.RawMessage
}

Marshal:

thing := Something1{"asd",123}
tempJson, _ := json.Marshal(thing)

typedThing := TypedJson{"something1", tempJson}
finalJson, _ := json.Marshal(typedThing)

Unmarshal:

func typeAssert(msg string) {

  var input TypedJson  
  json.Unmarshal([]byte(msg), &input)

  switch input.Type{
  case "something1":
    var thing Something1
    json.Unmarshal(input.Data, &thing)
    queueStatsRes(thing)   
   case "something2":
    var thing Something2
    json.Unmarshal(input.Data, &thing)
    queueStatsRes(thing)
  default:
    //handle unsupported type
}

Use a typed serialization format

• Go's own gob encoding
• Protocol Buffers
• and many more...