I currently having the following problem:
I get a []byte / string via websocket which looks like
eventname {"JSON": "data", "In": "different formats"}
I split the string by the whitespace between the eventname and the JSON data and, depending on the eventname, I want to json.Unmarshal() the JSON data into a specific type or a var of a specific type, to make sure its all type safe.
So I would probably have a map which holds all the possible eventnames and the corresponding type for the JSON data, but im not sure how I would save a type, maybe by reference or by the stringified name?
type EventTypeList map[string]*interface{}
or
type EventTypeList map[string]string
So I can lookup if the event is in the EventTypeList and then let the Unmarshal func parse the data into the type from the map.
So basicly instead of the specific type "SpecificData":
type SpecificData struct {
JSON string
In string
}
socketData := SpecificData{}
err := json.Unmarshal(jsonData, &socketData)
I want to dynamically create the socketData var by the type from the EventTypeList
socketData := [dynamically determine this Type from EventTypeList]{}
After I parsed the data into that type, I want to call all listeners for that event and give them the socketData with the right type, so they can work with the expected data.
But im really not sure how I would accomplish this, if this is even possible or if this is even the right way...
Thanks for any help!
Ok, after reasearching, I think what I wanted to achive is not possible, because the type needs to be know at compile time, as it has been mentioned here:
Golang: cast an interface to a typed variable dynamically
But I think I can just pass the raw JSON string to the listener callback and Unmarshal the data in that function where the type of the data is known.
Thanks for all contributions!
Related
I know Go has a way to json.Marshal struct and also json.Marshal map[string]interface{}.
But when it marshals a struct, it can uses its json tags'. Is there a way, in standard or with another library to json.Marshal a map[string]interface{} and to pass it the same options that would have been stored in a field tag for a struct?
Actually there is a way, a bit tricky, but still.
You have to create a struct from reflect package.
You iterate over each element in your map and for each of them you create a struct field :
fields = append(fields, reflect.StructField{
Name: ,
Type: ,
Tag: ,
})
You can set the tags to whatever you want, meaning you can set json values, like the omitempty one.
Then you have to create the struct itself :
s := reflect.New(reflect.StructOf(fields))
Then another iteration over your map will be needed, to fill the struct with values.
s.Elem().FieldByName(name).Set(WhateverValue)
Finally you can pass your struct to json.Marshall(s.Interface()).
I have a struct:
type person struct{
FirstN [10]byte
Last Name [10]byte
Address [15]byte
zip [6]byte
}
Then I have Map
xyz = [01:aaaaaaaaaabbbbbbbbbbccccccccccccccc123456]
This map is exactly the same as my struct. Basically if I overlay my structure with the string in the map it is an exact match.
I am trying to get a JSON string for this data, using Marshal. But for that (as I understand it) I need to update the data in the map into the struct and then pass the structure pointer to Marshal
But I cannot find any way to get the data from the map with key '01' which is a string and initialize my structure with it. I do not want to add a code to update each field in the structure by parsing the string from the map. Is there a way to do it or hardcoding is the only option.
Also is there a way to create a JSON string from a map string directly?
That's not json data. However, the binary.Read function will decode arbitrary fixed sized values from a binary stream following the struct layout.
data := []byte("aaaaaaaaaaaaaaaaaaaaccccccccccccccc123456")
err := binary.Read(bytes.NewReader(data), binary.LittleEndian, &p)
https://play.golang.org/p/-I_XhUCvNN
I want to store an arbitrary json object in a struct:
type C struct {
Name string `json:"name" bson:"name"`
Config map[string]interface{} `json:"config" bson:"config"`
}
This works fine when I store any deeply nested json object, but when I go retrieve it and mgo tries to Unmarshal it, I get:
Unmarshal can't deal with struct values. Use a pointer.
I'm not sure what's supposed to be a pointer. I get the same errors if I change it to:
Config *map[string]interface{}
The error occurs here: https://github.com/MG-RAST/golib/blob/master/mgo/bson/bson.go#L493
I don't know what it's reflecting on though.
So when you are unmarshaling the input argument takes a pointer to the struct, and you need to define a type in order to use a pointer to a struct.
type myMap map[string]interface{}
Then you can make a pointer to that type notice the ampersand to indicate pointer to your struct for type myMap, with json you could do something like so:
json := []Byte{`{"name": "value"}`}
c := &myMap{"value": "name"}
json.Unmarshal(c, json)
So you need *myMap to the struct not a pointer to the type. In order to explain the specific solution to this problem you need to add the context of how mongodb is unmarshaling your json.
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.
I expected below code to print an object of type struct J, however it prints a map object of type map[string]interface{}. I can feel why it acts like that, however when I run, reflect.ValueOf(i).Kind(), it returns Struct, so it kinda gives me the impression that Unmarshal method should return type J instead of a map. Could anyone enlighten me ?
type J struct {
Text string
}
func main() {
j := J{}
var i interface{} = j
js := "{\"Text\": \"lala\"}"
json.Unmarshal([]byte(js), &i)
fmt.Printf("%#v", i)
}
The type you're passing into Unmarshal is not *J, you're passing in an *interface{}.
When the json package reflects what the type is of the pointer it received, it sees interface{}, so it then uses the default types of the package to unmarshal into, which are
bool, for JSON booleans
float64, for JSON numbers
string, for JSON strings
[]interface{}, for JSON arrays
map[string]interface{}, for JSON objects
nil for JSON null
There is almost never a reason to use a pointer to an interface. If you find yourself using a pointer to an interface, and you don't know exactly why, then it's probably a mistake. If you want to unmarshal into J, then pass that in directly. If you need to assign that to an intermediary interface, make sure you use a pointer to the original value, not a pointer to its interface.
http://play.golang.org/p/uJDFKfSIxN
j := J{}
var i interface{} = &j
js := "{\"Text\": \"lala\"}"
json.Unmarshal([]byte(js), i)
fmt.Printf("%#v", i)
This is expected behavior: instead of giving json.Unmarshal a pointer to a properly typed place in memory you give it a pointer to a place in memory with type interface{}. It can essentially store anything in there under the type the JSON defines, so it does just that.
See it like this:
Unmarshal gets a place to store the data v with type interface{}
Unmarshal detects a map encoded as JSON
Unmarshal sees that the target type is of type interface{}, creates a go map from it and stores it in v
If you would have given it a different type than interface{} the process would have looked like this:
Unmarshal gets a place to store the data v with type struct main.J
Unmarshal detects a map encoded as JSON
Unmarshal sees that the target type is struct main.J and begins to recursively fit the data to the type
The main point is, that the initial assignment
var i interface{} = j
is completely ignored by json.Unmarshal.