Parsing JSON

To understand how to parse (that is, unmarshal) JSON data in Go, we will be using some data from the Citi Bike API (https://www.citibikenyc.com/system-data), a bike-sharing service operating in New York City. Citi Bike provides frequently updated operational information about its network of bike sharing stations in JSON format at https://gbfs.citibikenyc.com/gbfs/en/station_status.json:

{
  "last_updated": 1495252868,
  "ttl": 10,
  "data": {
    "stations": [
      {
        "station_id": "72",
        "num_bikes_available": 10,
        "num_bikes_disabled": 3,
        "num_docks_available": 26,
        "num_docks_disabled": 0,
        "is_installed": 1,
        "is_renting": 1,
        "is_returning": 1,
        "last_reported": 1495249679,
        "eightd_has_available_keys": false
      },
      {
        "station_id": "79",
        "num_bikes_available": 0,
        "num_bikes_disabled": 0,
        "num_docks_available": 33,
        "num_docks_disabled": 0,
        "is_installed": 1,
        "is_renting": 1,
        "is_returning": 1,
        "last_reported": 1495248017,
        "eightd_has_available_keys": false
      },

      etc...

      {
        "station_id": "3464",
        "num_bikes_available": 1,
        "num_bikes_disabled": 3,
        "num_docks_available": 53,
        "num_docks_disabled": 0,
        "is_installed": 1,
        "is_renting": 1,
        "is_returning": 1,
        "last_reported": 1495250340,
        "eightd_has_available_keys": false
      }
    ]
  }
}

To parse the import and this type of data in Go, we first need to import encoding/json (along with a couple of other things from a standard library, such as net/http, because we are going to pull this data off of the previously mentioned website). We will also define struct that mimics the structure of the JSON shown in the preceding code:

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
)

// citiBikeURL provides the station statuses of CitiBike bike sharing stations.
const citiBikeURL = "https://gbfs.citibikenyc.com/gbfs/en/station_status.json"

// stationData is used to unmarshal the JSON document returned form citiBikeURL.
type stationData struct {
    LastUpdated int `json:"last_updated"`
    TTL int `json:"ttl"`
    Data struct {
        Stations []station `json:"stations"`
    } `json:"data"`
}

// station is used to unmarshal each of the station documents in stationData.
type station struct {
    ID string `json:"station_id"`
    NumBikesAvailable int `json:"num_bikes_available"`
    NumBikesDisabled int `json:"num_bike_disabled"`
    NumDocksAvailable int `json:"num_docks_available"`
    NumDocksDisabled int `json:"num_docks_disabled"`
    IsInstalled int `json:"is_installed"`
    IsRenting int `json:"is_renting"`
    IsReturning int `json:"is_returning"`
    LastReported int `json:"last_reported"`
    HasAvailableKeys bool `json:"eightd_has_available_keys"`
}

Note a couple of things here: (i) we have followed Go idioms by avoiding the struct field name with underscores, but (ii) we have utilized the json struct tags to label the struct fields with the corresponding expected fields in the JSON data.

Now we can get the JSON data from the URL and unmarshal it into a new stationData value. This will produce a struct variable with the respective fields filled with the data in the tagged JSON data fields. We can check it by printing out some data associated with one of the stations:

// Get the JSON response from the URL.
response, err := http.Get(citiBikeURL)
if err != nil {
    log.Fatal(err)
}
defer response.Body.Close()

// Read the body of the response into []byte.
body, err := ioutil.ReadAll(response.Body)
if err != nil {
    log.Fatal(err)
}

// Declare a variable of type stationData.
var sd stationData

// Unmarshal the JSON data into the variable.
if err := json.Unmarshal(body, &sd); err != nil {
    log.Fatal(err)
}

// Print the first station.
fmt.Printf("%+v\n\n", sd.Data.Stations[0])

When we run this, we can see that our struct contains the parsed data from the URL:

$ go build
$ ./myprogram 
{ID:72 NumBikesAvailable:11 NumBikesDisabled:0 NumDocksAvailable:25 NumDocksDisabled:0 IsInstalled:1 IsRenting:1 IsReturning:1 LastReported:1495252934 HasAvailableKeys:false}