Golang and JSON with array of struct [duplicate] - json

Question

This question already has answers here:
My structures are not marshalling into json [duplicate]
(3 answers)
Closed 7 months ago.
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.

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

Answer 2

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.

Answer 3

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"`
}