How to fix json: cannot unmarshal object into Go value of type []json.RawMessage

Question

I want to unmarshal

var j = []byte(`[{"major":1},{"minor":0}]`)

into

type Version struct {
    Major int `json:"major"`
    Minor int `json:"minor"`
}

using custom unmarshaler by looping the inner slice:

func (h *Version) UnmarshalJSON(b []byte) error {
    var wrapper []json.RawMessage

    err := json.Unmarshal(b, &wrapper)
    if err == nil {
        for _, v := range wrapper {
            if err = json.Unmarshal(v, &h); err != nil {
                break
            }
        }
    }

    return err
}

The inner UnmarshalJSON triggers

json: cannot unmarshal object into Go value of type []json.RawMessage

which is strange because the target is a *Version. What is wrong here? Play: https://play.golang.org/p/Av59IkYTioS

You call json.Unmarshal(v, h) which invokes Version.UnmarshalJSON. You simply cannot do this like this. You either need a helper type or redesign. — Volker, Feb 17 '21 at 08:52
Alternatives [1](https://play.golang.org/p/D6VfSV_UGjM), [2](https://play.golang.org/p/VKN3L6zsF8R) if you can rely on the order and number of the objects in the array. — mkopriva, Feb 17 '21 at 09:04

icza · Accepted Answer · 2021-02-17T09:03:07.623

The "inner" unmarshal call will recursively call Version.UnmarshalJSON():

json.Unmarshal(v, &h)

And your error comes from the recursive call: you try to unmarshal {"major":1} into []json.RawMessage.

You do not want to call Version.UnmarshalJSON() recursively, so create a new type that strips all methods, including the UnmarshalJSON():

    type version Version
    h2 := (*version)(h)
    for _, v := range wrapper {
        if err = json.Unmarshal(v, &h2); err != nil {
            break
        }
    }

With this change it works, and adding fmt.Println(msg) the output will be (try it on the Go Playground):

<nil>
{1 0}

Normally this would cause a stack overflow error, but since your second call errors, the recursion breaks. See related: Call json.Unmarshal inside UnmarshalJSON function without causing stack overflow

How to fix json: cannot unmarshal object into Go value of type []json.RawMessage

1 Answers1