json.Unmarshal是否要求您的结果结构完全匹配传入的JSON吗？

Question

I have a JSON string I want to unmarshal:

{
  "id":1720,
  "alertId":1,
  "alertName":"{stats} Test Lambda Alert",
  "dashboardId":5,
  "panelId":2,
  "userId":0,
  "newState":"alerting",
  "prevState":"ok",
  "time":1523983581000,
  "text":"",
  "regionId":0,
  "tags":[],
  "login":"",
  "email":"",
  "avatarUrl":"",
  "data":{
    "evalMatches":[
        {
         "metric":"{prod}{stats} Lambda Alert Test",
         "tags":null,
         "value":16.525333333333332
         }
     ]
   }
}

I get the raw stream via a request: bodyBytes, err := ioutil.ReadAll(resp.Body)

I was hoping I could just specify a struct that pulls the values I care about, e.g.,

type Result struct {
    ID string `json:"id"`
    Time int64 `json:"time"`
}

However, when I try this, I get errors.

type Result struct {
    ID   string `json:"id"`
    Time string `json:"time"`
}

var result Result
err2 := json.Unmarshal(bodyBytes, &result)
if err2 != nil {
    log.Fatal(fmt.Sprintf(`Error Unmarshalling: %s`, err2))
}
fmt.Println(result.ID)

Error Unmarshalling: json: cannot unmarshal array into Go value of type main.Result

I suspect this error may be due to what's actually returned from ioutil.ReadAll(), since it has the above JSON string wrapped in [ ] if I do a fmt.Println(string(bodyBytes)), but if I try to json.Unmarshal(bodyBytes[0], &result), I just get compile errors, so I'm not sure.

If I want to unmarshal a JSON string, do I have to specify the full structure in my type Result struct? Is there a way around this? I don't want to be bound to the JSON object I receive (if the API changes upstream, it requires us to modify our code to recognize that, etc.).

Answer 1

You can unmarshal into structs that represent only some fields of your JSON document, but the field types have to match, as the error clearly states:

cannot unmarshal number into Go struct field Result.id of type string

You cannot unmarshal a number into a string. If you define the ID field as any numeric type it'll work just fine:

package main

import (
    "encoding/json"
    "fmt"
)

var j = []byte(`
{
  "id":1720,
  "prevState":"ok",
  "time":1523983581000,
  "text":"",
  "regionId":0
}
`)

type Result struct {
    ID   int   `json:"id"` // or any other integer type, or float{32,64}, or json.Number
    Time int64 `json:"time"`
}

func main() {
    var r Result
    err := json.Unmarshal(j, &r)
    fmt.Println(r, err)
}

Try it on the playground: https://play.golang.org/p/lqsQwLW2dHZ

Update

You have just edited your question with the actual error you receive. You have to unmarshal JSON arrays into slices. So if the HTTP response in fact returns a JSON array, unmarshal into []Result:

var j = []byte(`
[
    {
      "id":1720,
      "prevState":"ok",
      "time":1523983581000,
      "text":"",
      "regionId":0
    }
]
`)

var r []Result
err := json.Unmarshal(j, &r)
fmt.Println(r[0], err)

https://play.golang.org/p/EbOVA8CbcFO

To generate Go types that match your JSON document pretty well, use https://mholt.github.io/json-to-go/.