This question already has answers here:
Lowercase JSON key names with JSON Marshal in Go
(4 answers)
Closed 5 years ago.
What I am trying to do
I am parsing a JSON HTTP response based on this answer to a similar question. My code is able to parse the JSON without any error but is unable to read the values and store them in the provided variable.
This has been puzzling me for the last 2 hours and it might be due to a trivial reason that I am overlooking here.
CODE
type ImporterResponse struct {
results []packagemeta `json:"results"`
}
type packagemeta struct {
path string `json:"path"`
synopsis string `json:"synopsis,omitempty"`
count int `json:"import_count,omitempty`
}
func main() {
res := []byte(`{"results":[{"path":"4d63.com/randstr/lib/randstr","import_count":0,"synopsis":"Package randstr generates random strings (e.g."},{"path":"bitbucket.org/pcas/tool/mathutil","import_count":0}]}`)
fmt.Println("Decoding the JSON")
r := bytes.NewReader(res)
decoder := json.NewDecoder(r)
packageimporters := &ImporterResponse{}
err := decoder.Decode(packageimporters)
if err != nil {
log.Fatal(err)
}
fmt.Printf("Packageimporters: %+v", packageimporters)
fmt.Println(len(packageimporters.results))
}
Link to Playground: https://play.golang.org/p/NzLl7Ujo2IJ
What I want:
How to fix this?
Why is no error message raised if JSON is not parsed properly?
P.S: I understand that this question has been asked before and there are possible solutions available but none of them work for me. Hence, I have made this post.
You need to make your struct fields exported, otherwise the json package cannot access them.
Please read JSON and go for more details, specifically this paragraph:
The json package only accesses the exported fields of struct types
(those that begin with an uppercase letter). Therefore only the the
exported fields of a struct will be present in the JSON output.
And this one for more details:
How does Unmarshal identify the fields in which to store the decoded
data? For a given JSON key "Foo", Unmarshal will look through the
destination struct's fields to find (in order of preference):
An exported field with a tag of "Foo" (see the Go spec for more on
struct tags),
An exported field named "Foo", or
An exported field
named "FOO" or "FoO" or some other case-insensitive match of "Foo".
So your struct should really be:
type Packagemeta struct {
Path string `json:"path"`
Synopsis string `json:"synopsis,omitempty"`
Count int `json:"import_count,omitempty`
}
Related
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Closed 9 months ago.
I'm trying to represent some data in JSON where there is a list of things where each thing has some common features (ex name) and another field who's value can either be a string or an integer. For example:
{
"items": [
{
"name": "thing1",
"type": "string",
"value": "foo"
},
{
"name": "thing2",
"type": "int",
"value": 42
}
]
}
That JSON looks reasonable to me, but trying to create a data structure to deserialize (unmarshal) it into in Golang is proving difficult. I think I could do it in Java with class polymorphism, but in Go I feel trapped. I've tried many things but haven't got it. Ultimately, it comes down to lack of struct type polymorphism.
In Go, I can have a slice (list) of interfaces, but I need actual structs of different types as far as I can tell.
Any suggestions on how to represent this in Golang, and be able to unmarshal into?
Or alternatively, should I structure the JSON itself differently?
Thanks!
You can create a structure like this
type items struct {
name string
type_1 string
value interface{}
}
Interface can hold any data type, also as type is reserved keyword i have used type_1
You can do it in Go 1.18 like this:
type Data struct {
Items []struct {
Name string `json:"name"`
Type string `json:"type"`
Value any `json:"value"`
} `json:"items"`
}
func main() {
data := Data{}
// it's your json bytes
bytesData := []byte()
if err := json.Unmarshal(byteData, &data); err != nil {
log.Fatal(err)
}
}
// use data here
P.S. if you are using older versions use interface{} instead of any.
This data structure properly represents your JSON:
type Data struct {
Items []struct {
Name string `json:"name"`
Type string `json:"type"`
Value interface{} `json:"value"`
} `json:"items"`
}
Then, you can use json.Unmarshal.
If you use Go 1.18, you can use any alias of interface{}.
In addition, in Go you don't even need a type field. You can use Type assertions to determine the value type.
This question already has answers here:
Serialize a mixed type JSON array in Go
(3 answers)
Golang decoding JSON into slice with one string and one float64
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Fixed-size array that contains multiple specific types?
(1 answer)
Closed 3 years ago.
I just started developing in Golang, and am up against this problem:
I have json data to be accepted by a Golang server. A sample of one of the fields of this json data is below:
"DATA":[["(610, 658)",1573824148],["(594, 675)",1573824148],["(578,710)",1573824148],["(571, 728)",1573824148],["(552, 769)",1573824148],["(549, 788)",1573824148],["(549, 796)",1573824148]]
Note that it is an array of lists, each list being a string and an integer.
I have the following data structures declared:
type POINT struct {
p string
t int
}
type POINTS struct {
A int
B int
C string
DATA []FIDGET
}
The relevant field is POINTS.DATA which is an array of POINTs. A POINT is a string and an int. To convert from json to Go variables, I use the encoding/json package.
My problem is, when I call
err = json.Unmarshal(body, &m)
I get this error:
json: cannot unmarshal array into Go struct field POINTS.DATA of type main.POINT
Can anyone help me? Thank you in advance!
This question already has answers here:
Lowercase JSON key names with JSON Marshal in Go
(4 answers)
Closed 5 years ago.
I'm a new user converting an application to Go. I have something like the following which is working:
type Network struct {
Ssid string
Security string
Bitrate string
}
func Scan(w http.ResponseWriter, r *http.Request) {
output := runcmd(scripts+"scan.sh", true)
bytes := []byte(output)
var networks []Network
json.Unmarshal(bytes, &networks)
w.Header().Set("Content-Type", "application/json")
json.NewEncoder(w).Encode(networks)
}
The problem is the old version didn't use capitals on the json variables returned.
I want the front-end to see ssid not Ssid. If I make the attributes in the struct lowercase the code no longer works as they become unexported variables.
When the field names in your struct do not match the json field names, you can use field tags.
eg:
Ssid string `json:"myOtherFieldName"`
Please read the json docs for more details.
This tool very convenient for learning:
https://mholt.github.io/json-to-go/
Give it example of JSON that you want it will recommend golang.
e.g. JSON
{
"ssid": "some very long ssid",
"security": "big secret",
"bitrate": 1024
}
will suggest golang:
type AutoGenerated struct {
Ssid string `json:"ssid"`
Security string `json:"security"`
Bitrate int `json:"bitrate"`
}
now you can change AutogGenerated, Ssid, Security, Bitrate to whatever you want.
I have JSON (that I cannot control) like this:
{
"foo1":{
"a":{
"up":10,
"down":5
}
},
"foo2":{
"a":{
"up":1,
"down":1
}
},
"bar":{
"up":11,
"down":6
}
}
"foo1" and "foo2" are dynamic.
How can I properly unmarshal this structure in go?
It would be okay if I could just tell go to not try to deserialize "bar" (the inconsistent property).
Go will by default ignore fields unspecified in the struct you unmarshal into.
In this case, your structure would be set up like this:
type NestedProp2 struct {
Up int
Down int
}
type NestedProp struct {
A NestedProp2
}
type Prop struct {
Foo1 NestedProp
Foo2 NestedProp
}
When you call the the json.Unmarshal function, the extra property will not be deserialized:
var prop Prop
err := json.Unmarshal(jsonBlob, &prop)
if err != nil {
fmt.Println("error:", err)
}
fmt.Printf("%+v", prop)
So you get the following output:
{Foo1:{A:{Up:10 Down:5}} Foo2:{A:{Up:1 Down:1}}}
You can see it in action here.
You said:
I have JSON (that I cannot control)
So to what extent you could control? Here I could provide you with some scenario, and hope some of them match your purpose :)
Remember the general rule first:
In Golang, if a JSON key failed to find a matched field in struct, it will not be unmarshalled.
This means, for a key name in a JSON struct, when unmarshalling, it will look for a field in a golang struct at the same level with the same name case-insensitively. If this search failed, this key won't be unmarshalled.
For example, a key named foo1 will look for a field name foo1 in a golang struct at the same indent level. However it also matches with Foo1 or FoO1, since this matching is case-insensitive.
Remember, you could use field tag to specify the field name as well. Please take a look at the official page.
The value of some of the JSON fields are not consistent, and they could be ignored.
This is the case #gnalck solved in his answer. According to the general rule, if those inconsistent field failed to find a match, they will not be unmarshalled. Therefore, just don't put those inconsistent fields in the struct and you will be fine.
The value of some of the JSON fields are not consistent, but they could not be ignored.
In this case, #gnalck failed since those fields could not be ignored. Now a better way is to unmarshal bar into a json.RawMessage, so that you could unmarshal later.
The keys of the JSON object is undetermined, and their value is undetermined as well.
In this case, we could unmarshal the whole JSON object into a map[string]json.RawMessage, and unmarshal each fields later. When unmarshalling to a map, you could iterate through the map to get all the fields, and unmarshal them into a proper struct later.
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JSON and dealing with unexported fields
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(un)marshalling json golang not working
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json.Marshal(struct) returns "{}"
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Closed 10 months ago.
I'm a new Go programmer (From Java) and I would like to reproduce a generic way which is esay to use in Java.
I want to create some function which allow me to do an Unmarshal on a JSON string in order to avoid code duplicity.
This is my current code which is not working :
type myStruct1 struct {
id string
name string
}
func (obj myStruct1) toString() string {
var result bytes.Buffer
result.WriteString("id : ")
result.WriteString(obj.id)
result.WriteString("\n")
result.WriteString("name : ")
result.WriteString(obj.name)
return result.String()
}
func main() {
content := `{id:"id1",name="myName"}`
object := myStruct1{}
parseJSON(content, object)
fmt.Println(object.toString())
}
func parseJSON(content string, object interface{}) {
var parsed interface{}
json.Unmarshal([]byte(content), &parsed)
}
This code, on run, returns me this :
id :
name :
Do you have any idea ?
Thanks
The issue is you want to write to a generic type? You probably want a string map. This works with BSON anyways:
var anyJson map[string]interface{}
json.Unmarshal(bytes, &anyJson)
You'll be able to access the fields like so:
anyJson["id"].(string)
Don't forget to type assert your values, and they must be the correct type or they'll panic. (You can read more about type assertions on the golang site)
To parse "generic JSON" when you have no idea what schema it has:
var parsed any
err := json.Unmarshal(jsonText, &parsed)
The returned any in parsed will be a map[string]any or []any or nil or single values float64, bool, string.
You can test the type and react accordingly.
import (
"encoding/json"
"fmt"
)
func test(jsonText []byte) {
// parsing
var parsed any
err := json.Unmarshal(jsonText, &parsed)
if err != nil {
panic(err) // malformed input
}
// type-specific logic
switch val := parsed.(type) {
case nil:
fmt.Println("json specifies null")
case map[string]any:
fmt.Printf("id:%s name:%s\n", val["id"], val["name"])
case []any:
fmt.Printf("list of %d items\n", len(val))
case float64:
fmt.Printf("single number %f\n", val)
case bool:
fmt.Printf("single bool %v\n", val)
case string:
fmt.Printf("single string %s\n", val)
default:
panic(fmt.Errorf("type %T unexpected", parsed))
}
}
Unmarshal will only set exported fields of the struct.
Which means you need to modify the json struct to use capital case letters:
type myStruct1 struct {
Id string
Name string
}
The reason behind this is that the json library does not have the ability to view fields using reflect unless they are exported.
You have to export your fields:
type myStruct1 struct {
Id string
Name string
}
See Exported Identifiers from documentation.
There are a few changes you need to make in your code to make it work:
The function json.Unmarshal can only set variables inside your struct which are exported, that is, which start with capital letters. Use something like ID and Name for your variable names inside myStruct1.
Your content is invalid JSON. What you actually want is {"ID":"id1","Name":"myName"}.
You're passing object to parseJSON but you're using parsed instead, not object. Make parseJSON receive a *myStruct (instead of an interface{}), and use that variable instead of parsed when unmarshalling the string. Also, always handle the error returns, like err := json.Unmarshal(content, object), and check err.
I'd suggest you to do the Golang Tour ;)
You can also set the file as an Object with dynamic properties inside another struct. This will let you add metadata and you read it the same way.
type MyFile struct {
Version string
Data map[string]interface{}
}