Golang和JSON与结构体数组

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
", gatewayInfo)

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

Answer 1

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