I'm new to Golang and this could be so basic. Below is an example demonstrating my need. Junk should accept only id and name. Even though unmarshal eliminates extras, i need a way to hard stop execution stating json contain invalid key value pair. Could you please let me know if there's any.
package main
import (
"fmt"
"encoding/json"
)
type Junk struct{
ID int `json:"id"`
Name string `json:"name"`
Area string `json:"area"`
}
func main() {
a:=Junk{}
data:= `{"id":1,"name":"gg","junk":"Junk value"}`
err := json.Unmarshal([]byte(data),&a)
if err!=nil{
fmt.Println(err)
}
fmt.Println(a)
}
Use func (*Decoder) DisallowUnknownFields():
DisallowUnknownFields causes the Decoder to return an error when the
destination is a struct and the input contains object keys which do
not match any non-ignored, exported fields in the destination.
package main
import (
"encoding/json"
"fmt"
"strings"
)
type Junk struct {
ID int `json:"id"`
Name string `json:"name"`
Area string `json:"area"`
}
func main() {
a := Junk{}
data := `{"id":1,"name":"gg","junk":"Junk value"}`
d := json.NewDecoder(strings.NewReader(data))
d.DisallowUnknownFields()
if err := d.Decode(&a); err != nil {
fmt.Println(err)
}
fmt.Println(a)
}
Try it out: https://play.golang.org/p/aTj2C-AAuZ7
Related
I am new to Golang and I am trying to get a number of attributes from a structure
For example:
type Client struct{
name string//1
lastName string//2
age uint//3
}
func main(){
client := Client{name:"Facundo",lastName:"Veronelli",age:23}
fmt.println(client.getLengthAttibutes())//It would print "3"
}
Using the reflect package's ValueOf() function returns a value struct. This has a method called NumFields that returns the number of fields.
import (
"fmt"
"reflect"
)
type Client struct{
name string//1
lastName string//2
age uint//3
}
func main(){
client := Client{name:"Facundo",lastName:"Veronelli",age:23}
v := reflect.ValueOf(client)
fmt.Printf("Struct has %d fields", v.NumField())
}
You can use the reflect package for this:
import (
"fmt"
"reflect"
)
type Client struct {
name string //1
lastName string //2
age uint //3
}
func main() {
client := Client{name: "Facundo", lastName: "Veronelli", age: 23}
fmt.Println(reflect.TypeOf(client).NumField())
}
This is not, however, the size of that struct, only the number of fields. Use reflect.TypeOf(client).Size() to get how many bytes the struct occupies in memory.
I am sending the data to the API like following:
{"after": {"amount": 811,"id":123,"status":"Hi"}, "key": [70]}
and i am getting following while printing :
map[after:map[amount:811 id:123 status:Hi ] key:[70]]
Is there any way to print individual field like this??
amount::800
id:123
status:Hi
The code:
package main
import (
"encoding/json"
"fmt"
"io"
"io/ioutil"
"log"
"net/http"
"strings"
)
var (
PORT = ":8080"
)
func main() {
fmt.Println("In Main")
http.HandleFunc("/", changedData)
http.ListenAndServe(PORT, nil)
}
type Data struct {
Id int64 `json:"id"`
Amount float64 `json:"amount"`
Status string `json:"status"`
}
type mark map[string]interface{}
func changedData(w http.ResponseWriter, r *http.Request) {
fmt.Println("Coming From API")
reqBody, _ := ioutil.ReadAll(r.Body)
fmt.Println("Data coming from API ", string(reqBody))
digit := json.NewDecoder(strings.NewReader(string(reqBody)))
for digit.More() {
var result mark
err := digit.Decode(&result)
if err != nil {
if err != io.EOF {
log.Fatal(err)
}
break
}
fmt.Println("final_data ", result)
}
}
Decode to a Go type that matches the structure of the JSON document. You declared a type for the "after" field. Wrap that type with a struct to match the document.
func changedData(w http.ResponseWriter, r *http.Request) {
var v struct{ After Data }
err := json.NewDecoder(r.Body).Decode(&v)
if err != nil {
http.Error(w, "bad request", 400)
return
}
fmt.Printf("final_data: %#v", v.After)
}
Playground example.
I think you can define a struct type if you know the JSON file format or if the JSON format is predefined. As far as I know that mostly using interface{} is a way when you don't know the JSON format or there is no predefined format of the JSON. If you define a struct type and use it while unmarshaling the JSON to struct, you can access the variables by typing like data.Id or data.Status.
Here's an example code:
package main
import (
"encoding/json"
"fmt"
)
type Data struct {
AfterData After `json:"after"`
Key []int `json:"key"`
}
type After struct {
Id int64 `json:"id"`
Amount float64 `json:"amount"`
Status string `json:"status"`
}
func main() {
j := []byte(`{"after": {"amount": 811,"id":123,"status":"Hi"}, "key": [70]}`)
var data *Data
err := json.Unmarshal(j, &data)
if err != nil {
fmt.Println(err.Error())
return
}
fmt.Println(data.AfterData)
fmt.Println(data.AfterData.Id)
fmt.Println(data.AfterData.Amount)
fmt.Println(data.AfterData.Status)
}
Output will be
{123 811 Hi}
123
811
Hi
Go Playground
In the below program I'm extracting some data from an API.
It outputs a rather complex data.
When I ioutil.ReadAll(resp.Body), the result is of type []uint8.
If I try to read the results, its just a random array of integers.
However, I'm able to read it if I convert it to string using string(diskinfo)
But I want to use this in a Struct and having trouble unmarshalling.
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"net/http"
"net/url"
"reflect"
)
type ApiResults struct {
results []struct {
statement_id int `json.statement_id`
series []struct {
name string `json.name`
tags struct {
host string `json.host`
}
columns []string `json.columns`
values []interface{} `json.values`
}
}
}
func main() {
my_url := "my_url"
my_qry := fmt.Sprintf("my_query")
resp, err := http.Get(my_url + url.QueryEscape(my_qry))
if err != nil {
fmt.Printf("ERROR: %v\n", err)
} else {
fmt.Println(reflect.TypeOf(resp))
diskinfo, _ := ioutil.ReadAll(resp.Body)
fmt.Println(reflect.TypeOf((diskinfo)))
fmt.Println(diskinfo)
fmt.Println(string(diskinfo))
diskinfo_string := string(diskinfo)
data := ApiResults{}
json.Unmarshal([]byte(diskinfo_string), &data)
//fmt.Printf("Values = %v\n", data.results.series.values)
//fmt.Printf("Server = %v\n", data.results.series.tags.host)
}
}
If I view the data as a string, I get this (formatted):
{"results":[
{"statement_id":0,
"series":[
{"name":"disk",
"tags":{"host":"myServer1"},
"columns":["time","disk_size"],
"values":[["2021-07-07T07:53:32.291490387Z",1044]]},
{"name":"disk",
"tags":{"host":"myServer2"},
"columns":["time","disk_size"],
"values":[["2021-07-07T07:53:32.291490387Z",1046]]}
]}
]}
I think my Apireturn struct is also structured incorrectly because the API results have info for multiple hosts.
But first, I doubt if the data has to be sent in a different format to the struct. Once I do this, I can probably try to figure out how to read from the Struct next.
The ioutil.ReadAll already provides you the data in the type byte[]. Therefore you can just call json.Unmarshal passing it as a parameter.
import (
"encoding/json"
"io/ioutil"
"net/http"
)
func toStruct(res *http.Response) (*ApiResults, error) {
body, err := ioutil.ReadAll(res.Body)
if err != nil {
return nil, err
}
defer res.Body.Close()
data := ApiResults{}
if err := json.Unmarshal(body, &data); err != nil {
return nil, err
}
return data, nil
}
There also seems to be an issue with your struct. The correct way to use struct tags is as follows. Plus, fields need to be exported for the json tag (used by json.Umarshal) to work – starting with uppercase will do it.
type ApiResults struct {
Results []struct {
StatementId int `json:"statement_id"`
Series []struct {
Name string `json:"name"`
Tags struct {
Host string `json:"host"`
} `json:"tags"`
Columns []string `json:"columns"`
Values []interface{} `json:"values"`
} `json:"series"`
} `json:"results"`
}
In Go, Is it possible to get the tags from a struct field while I'm unmarshaling JSON content to it? Here's my failed attempt at doing so:
package main
import (
"log"
"encoding/json"
)
type Person struct {
ProfileName AltField `json:"profile_name"`
}
type AltField struct {
Val string
}
func (af *AltField) UnmarshalJSON(b []byte) error {
log.Println("Show tags")
//log.Println(af.Tag) // I want to see `json:"profile_name"`
if e := json.Unmarshal(b,&af.Val); e != nil {
return e
}
return nil
}
func main() {
p := Person{}
_ = json.Unmarshal([]byte(`{"profile_name":"Af"}`),&p)
}
I commented out the line log.Println(af.Tag) because it causes compilation errors. If I can get a handle on the tags from the Person struct, that will allow me to develop some other conditional logic.
Is this possible?
Use reflection to get the value of struct field tag. The reflect package provides functions to work with tags including to get the value of tag
package main
import (
"log"
"encoding/json"
"reflect"
)
type Person struct {
ProfileName AltField `json:"profile_name"`
}
type AltField struct {
Val string `json:"val"`
}
func (af *AltField) UnmarshalJSON(b []byte) error {
field, ok := reflect.TypeOf(*af).FieldByName("Val")
if !ok {
panic("Field not found")
}
log.Println(string(field.Tag))
if e := json.Unmarshal(b,&af.Val); e != nil {
return e
}
return nil
}
func main() {
p := Person{}
_ = json.Unmarshal([]byte(`{"profile_name":"Af"}`),&p)
}
You can only get the value of those field tags which has them. The struct field reflect object should be created for fetching the tags of its fields.
Working Code on Playground
I'm writing a websocket client in Go. I'm receiving the following JSON from the server:
{"args":[{"time":"2013-05-21 16:57:17"}],"name":"send:time"}
I'm trying to access the time parameter, but just can't grasp how to reach deep into an interface type:
package main;
import "encoding/json"
import "log"
func main() {
msg := `{"args":[{"time":"2013-05-21 16:56:16", "tzs":[{"name":"GMT"}]}],"name":"send:time"}`
u := map[string]interface{}{}
err := json.Unmarshal([]byte(msg), &u)
if err != nil {
panic(err)
}
args := u["args"]
log.Println( args[0]["time"] ) // invalid notation...
}
Which obviously errors, since the notation is not right:
invalid operation: args[0] (index of type interface {})
I just can't find a way to dig into the map to grab deeply nested keys and values.
Once I can get over grabbing dynamic values, I'd like to declare these messages. How would I write a type struct to represent such complex data structs?
You may like to consider the package github.com/bitly/go-simplejson
See the doc： http://godoc.org/github.com/bitly/go-simplejson
Example:
time, err := json.Get("args").GetIndex(0).String("time")
if err != nil {
panic(err)
}
log.Println(time)
The interface{} part of the map[string]interface{} you decode into will match the type of that field. So in this case:
args.([]interface{})[0].(map[string]interface{})["time"].(string)
should return "2013-05-21 16:56:16"
However, if you know the structure of the JSON, you should try defining a struct that matches that structure and unmarshal into that. Ex:
type Time struct {
Time time.Time `json:"time"`
Timezone []TZStruct `json:"tzs"` // obv. you need to define TZStruct as well
Name string `json:"name"`
}
type TimeResponse struct {
Args []Time `json:"args"`
}
var t TimeResponse
json.Unmarshal(msg, &t)
That may not be perfect, but should give you the idea
I'm extremely new to Golang coming from Python, and have always struggled with encode/decoding json. I found gjson at https://github.com/tidwall/gjson, and it helped me immensely:
package main
import "github.com/tidwall/gjson"
func main() {
msg := (`{"args":[{"time":"2013-05-21 16:56:16", "tzs":[{"name":"GMT"}]}],"name":"send:time"}`)
value := gjson.Get(msg, "args.#.time")
println(value.String())
}
-----------------------
["2013-05-21 16:56:16"]
Additionally, I noticed the comment of how to convert into Struct
package main
import (
"encoding/json"
"fmt"
)
type msgFormat struct {
Time string `json:"time"`
Tzs msgFormatTzs `json:"tzs"`
Name string `json:"name"`
}
type msgFormatTzs struct {
TzsName string `json:"name"`
}
func main() {
msg := (`{"args":[{"time":"2013-05-21 16:56:16", "tzs":[{"name":"GMT"}]}],"name":"send:time"}`)
r, err := json.Marshal(msgFormatTzs{msg})
if err != nil {
panic(err)
}
fmt.Printf("%v", r)
}
Try on Go playground