Parse JSON HTTP response using golang

Question

I am trying to get the value of say "ip" from my following curl output:

{  
  "type":"example",
  "data":{  
    "name":"abc",
    "labels":{  
      "key":"value"
    }
  },
  "subsets":[  
    {  
      "addresses":[  
        {  
          "ip":"192.168.103.178"
        }
      ],
      "ports":[  
        {  
          "port":80
        }
      ]
    }
  ]
}

I have found many examples in the internet to parse json output of curl requests and I have written the following code, but that doesn't seem to return me the value of say "ip"

package main

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

type svc struct {
    Ip string `json:"ip"`
}

func main() {

url := "http://myurl.com"

testClient := http.Client{
    Timeout: time.Second * 2, // Maximum of 2 secs
}

req, err := http.NewRequest(http.MethodGet, url, nil)
if err != nil {
    log.Fatal(err)
}


res, getErr := testClient.Do(req)
if getErr != nil {
    log.Fatal(getErr)
}

body, readErr := ioutil.ReadAll(res.Body)
if readErr != nil {
    log.Fatal(readErr)
}

svc1 := svc{}
jsonErr := json.Unmarshal(body, &svc1)
if jsonErr != nil {
    log.Fatal(jsonErr)
}

fmt.Println(svc1.Ip)
}

I would appreciate if anyone could provide me hints on what I need to add to my code to get the value of say "ip".

Answer 1

You can create structs which reflect your json structure and then decode your json.

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "log"
)

type Example struct {
    Type    string   `json:"type,omitempty"`
    Subsets []Subset `json:"subsets,omitempty"`
}

type Subset struct {
    Addresses []Address `json:"addresses,omitempty"`
}

type Address struct {
    IP string `json:"IP,omitempty"`
}

    func main() {

    m := []byte(`{"type":"example","data": {"name": "abc","labels": {"key": "value"}},"subsets": [{"addresses": [{"ip": "192.168.103.178"}],"ports": [{"port": 80}]}]}`)

    r := bytes.NewReader(m)
    decoder := json.NewDecoder(r)

    val := &Example{}
    err := decoder.Decode(val)

    if err != nil {
        log.Fatal(err)
    }

    // If you want to read a response body
    // decoder := json.NewDecoder(res.Body)
    // err := decoder.Decode(val)

    // Subsets is a slice so you must loop over it
    for _, s := range val.Subsets {
        // within Subsets, address is also a slice
        // then you can access each IP from type Address
        for _, a := range s.Addresses {
            fmt.Println(a.IP)
        }
    }

}

The output would be: 192.168.103.178

By decoding this to a struct, you can loop over any slice and not limit yourself to one IP

Example here:

https://play.golang.org/p/sWA9qBWljA

Answer 2

One approach is to unmarshal the JSON to a map , eg (assumes jsData contains JSON string)

obj := map[string]interface{}{}
if err := json.Unmarshal([]byte(jsData), &obj); err != nil {
    log.Fatal(err)
}

Next, implement a function for searching the value associated with a key from the map recursively, eg

func find(obj interface{}, key string) (interface{}, bool) {
    //if the argument is not a map, ignore it
    mobj, ok := obj.(map[string]interface{})
    if !ok {
        return nil, false
    }

    for k, v := range mobj {
        //key match, return value
        if k == key {
            return v, true
        }

        //if the value is a map, search recursively
        if m, ok := v.(map[string]interface{}); ok {
            if res, ok := find(m, key); ok {
                return res, true
            }
        }
        //if the value is an array, search recursively 
        //from each element
        if va, ok := v.([]interface{}); ok {
            for _, a := range va {
                if res, ok := find(a, key); ok {
                    return res,true
                }
            }
        }
    }

    //element not found
    return nil,false
}

Note, that the above function return an interface{} . You need to convert it to appropriate type, eg using type switch:

if ip, ok := find(obj, "ip"); ok {
    switch v := ip.(type) {
    case string:
        fmt.Printf("IP is a string -> %s\n", v)
    case fmt.Stringer:
        fmt.Printf("IP implements stringer interface -> %s\n", v.String())
    case int:

    default:
        fmt.Printf("IP = %v, ok = %v\n", ip, ok)
    }
}

A working example can be found at https://play.golang.org/p/O5NUi4J0iR

Answer 3

You can write your own decoder or use existing third-party decoders. For instance, github.com/buger/jsonparser could solve your problem by iterating throw array (two times).

package main

import (
    "github.com/buger/jsonparser"
    "fmt"
)

var data =[]byte(`{
  "type":"example",
  "data":{
    "name":"abc",
    "labels":{
      "key":"value"
    }
  },
  "subsets":[
    {
      "addresses":[
        {
          "ip":"192.168.103.178"
        }
      ],
      "ports":[
        {
          "port":80
        }
      ]
    }
  ]
}`)

func main() {
    jsonparser.ArrayEach(data, func(value []byte, dataType jsonparser.ValueType, offset int, err error) {
        jsonparser.ArrayEach(value, func(value []byte, dataType jsonparser.ValueType, offset int, err error) {
            v, _, _, err := jsonparser.Get(value, "ip")
            if err != nil {
                return
            }
            fmt.Println("ip: ", string(v[:]))
        }, "addresses")
    }, "subsets")
}

Output: ip: 192.168.103.178

Answer 4

Typically in these situations you will see people describe all of these sub struct types. If you dont actually need to reuse the definition of any sub struct s (like as a type for a function argument), then you dont need to define them. You can just use one definition for the whole response. In addition, in some cases you dont need to define a type at all, you can just do it at the time of declaration:

package main
import "encoding/json"

const s = `
{  
   "subsets": [
      {  
         "addresses": [ 
            {"ip": "192.168.103.178"}
         ]
      }
   ]
}
`

func main() {
   var svc struct {
      Subsets []struct {
         Addresses []struct { Ip string }
      }
   }
   json.Unmarshal([]byte(s), &svc)
   ip := svc.Subsets[0].Addresses[0].Ip
   println(ip == "192.168.103.178")
}

Answer 5

You can use the NewDecoder feature of the encoding/json package.

Like this:

decoder := json.NewDecoder(req.Body)

err := decoder.Decode(&svc1)
if err != nil {
    log.Fatal(err)
}