如何正确覆盖UnmarshalJSON？

Question

I am trying to write a simple custom marshaler and failing. Notice I have an interface that has three functions. Both Happy and Sad structs implement this interface by embedding the emotion struct which implements all the three required functions.

The problem is UnmarshalJSON does not get invoked when I call json.Unmarshal() on the pointer to either Happy or Sad and I can't understand why. You can reproduce the exact codebase in Go Playground or just look below. You will notice that while MarshalJSON is correctly called, UnmarshalJSON isn't.

type Emotion interface {
    String() string
    MarshalJSON() ([]byte, error)
    UnmarshalJSON(data []byte) error 
}

type emotion struct {
    status string
}

func (s emotion) String() string {
    return s.status
}

func (s emotion) MarshalJSON() ([]byte, error) {
        fmt.Println("MarshalJSON is overriden: I am called fine")
    x := struct {
        Status string
    }{
        Status: s.String(),
    }

    return json.Marshal(x)
}

func (s *emotion) UnmarshalJSON(data []byte) error {
        fmt.Println("MarshalJSON is overriden: I am never called")
    y := struct {
        Status string
    }{
        Status: "",
    }

    err := json.Unmarshal(data, &y)
    if err != nil {
        return err
    }

    s.status = y.Status
    return nil
}

type Happy struct {
    *emotion
}

// Job is not in any detention
type Sad struct {
    *emotion
}


func main() {
    x := Happy{&emotion{status: "happy"}}
    jsonX, _ := json.Marshal(x)
    var y Emotion
    err := json.Unmarshal(jsonX, &y)
    fmt.Printf("%v", err)
}

Answer 1

You cannot unmarshal into an abstract interface type. An interface value is just a pointer to a type (associating that types methods) - it has no storage behind it - because an abstract type cannot know the exact size of any concrete value it may have in the future.

Using a concrete value type (that also implements that interface) will work:

y2 := emotion{}
err = json.Unmarshal(jsonX, &y2)

Playground: https://play.golang.org/p/8aCEjLgfKVQ

MarshalJSON is overriden: I am called fine
EXPECTED ERROR, Can't unmarshal into a non-concrete value: json: cannot unmarshal object into Go value of type main.Emotion

MarshalJSON is overriden: I am (fixed) and now called
SHOULD NOT ERROR: <nil>
VALUE: happy