json.Unmarshal返回空白结构

Question

I have a JSON blob that looks like this

{
    "metadata":{
        "id":"2377f625-619b-4e20-90af-9a6cbfb80040",
        "from":"2014-12-30T07:23:42.000Z",
        "to":"2015-01-14T05:11:51.000Z",
        "entryCount":801,
        "size":821472,
        "deprecated":false
    },
    "status":[{
         "node_id":"de713614-be3d-4c39-a3f8-1154957e46a6",
         "status":"PUBLISHED"
    }]
}

and I have a little code to transform that back into go structs

type Status struct {
    status string
    node_id string
}

type Meta struct {
    to string
    from string
    id string
    entryCount int64
    size int64
    depricated bool
}

type Mydata struct {
    met meta
    stat []status
}

var realdata Mydata
err1 := json.Unmarshal(data, &realdata)
if err1 != nil {
    fmt.Println("error:", err1)
}
fmt.Printf("%T: %+v
", realdata, realdata)

but what I see when I run this is just a zeroed structure

main.Mydata: {met:{to: from: id: entryCount:0 size:0 depricated:false} stat:[]}

I tried allocating the struct first but that also didn't work, I'm not sure why its not producing values, and its not returning an error

Answer 1

Your struct fields are not exported. This is because they start with a lowercase letter.

EntryCount // <--- Exported
entryCount // <--- Not exported

When I say "not exported", I mean they are not visible outside of your package. Your package can happily access them because they are scoped locally to it.

As for the encoding/json package though - it cannot see them. You need to make all of your fields visible to the encoding/json package by making them all start with an uppercase letter, thereby exporting them:

type Status struct {
    Status  string
    Node_id string
}

type Meta struct {
    To         string
    From       string
    Id         string
    EntryCount int64
    Size       int64
    Depricated bool
}

type Mydata struct {
    Metadata  Meta
    Status []Status
}

<kbd>See it working on the Go Playground here</kbd>

You should also reference the Golang specification for answers. Specifically, the part that talks about Exported Identifiers.