Golang and JSON with array of struct

Question

I would like to create a JSON of a GatewayInfo where the type are defined like this:

type SpanInfo struct {
    imsi string
    network string
    network_status string
    signal_quality int
    slot int
    state string
}

type GatewayInfo []SpanInfo

The gateway information is created with:

var gatewayInfo = make(GatewayInfo, nb_spans)

To create the JSON, I use the json.Marshal function:

gatewayInfo := getGatewayInfo(spans)
log.Printf("Polling content: %s\n", gatewayInfo)

jsonInfo, _ := json.Marshal(gatewayInfo)
log.Printf("jsonInfo: %s\n", jsonInfo)

Unfortunately the result is not what I was expecting:

2015/02/09 13:48:26 Polling content: [{652020105829193 20801 Registered (Roaming) %!s(int=17) %!s(int=2) } {652020105829194 20801 Registered (Roaming) %!s(int=16) %!s(int=3) } {652020105829192 20801 Registered (Roaming) %!s(int=19) %!s(int=1) } {652020105829197 20801 Registered (Roaming) %!s(int=19) %!s(int=4) }]
2015/02/09 13:48:26 jsonInfo: [{},{},{},{}]

As we can see, the GatewayInfo instance has the SpanInfo, but in the JSON I have empty SpanInfo.

Arjan · Accepted Answer · 2017-07-25T16:55:27.290

46

Your struct fields must be exported (field is exported if it begins with a capital letter) or they won't be encoded:

Struct values encode as JSON objects. Each exported struct field becomes a member of the object

To get the JSON representation as probably expected change the code to this:

type SpanInfo struct {
    IMSI string `json:"imsi"`
    Network string `json:"network"`
    NetworkStatus string `json:"network_status"`
    SignalQuality int `json:"signal_quality"`
    Slot int `json:slot"`
    State string `json:"state"`
}

type GatewayInfo []SpanInfo

edited Jul 25 '17 at 16:55

answered Feb 09 '15 at 14:06

Arjan

19,957
2
55
48

4

While code this answer works, Arjan should probably note that the fields' names should start in upper case. (see icza answer below), – Věroš K. Jul 25 '17 at 10:31
@VěrošK.Added clarification – Arjan Jul 25 '17 at 16:55
2

it took me an hour to found this solution. Thanks – Gujarat Santana Nov 30 '17 at 08:55

score 12 · Answer 2 · answered Feb 09 '15 at 14:06

The json package can only serialize exported fields of your struct. Change your struct to start all fields with an uppercase letter so they can be included in the output:

type SpanInfo struct {
    Imsi string
    Network string
    Network_status string
    Signal_quality int
    Slot int
    State string
}

Read the documentation of json.Marshal() for details and more information.

score 0 · Answer 3 · answered Nov 13 '19 at 03:56

This is not a new answer. It is just consolidation of comments on the accepted answer.

From ORIGINAL Query

type SpanInfo struct {
    imsi string
    network string
    network_status string
    signal_quality int
    slot int
    state string
}

From Answer and comments - Please note that the first char of each field in struct is now in UPPER case along with json representation added to each field

type SpanInfo struct {
    IMSI string `json:"imsi"`
    Network string `json:"network"`
    NetworkStatus string `json:"network_status"`
    SignalQuality int `json:"signal_quality"`
    Slot int `json:slot"`
    State string `json:"state"`
}

Golang and JSON with array of struct

3 Answers3