Go: unexpected end of JSON input and json.Unmarshal returns nil values

Question

I'm testing out how to unmarshal a json response from an API I'm using.

The json looks something like the following;

body := []byte(`[
    {"name":"Name1", "value":100.00},
    {"name":"Name2", "value":200.00}
]`)

I've searched around for various ways to do this, but fail to get this to work. The unmarshal returns zero values. I also get an error "unexpected end of JSON input" (I've removed the error handling in the example).

Full code example - https://play.golang.org/p/VMdWuAm6HS

Reference:

1. https://godoc.org/encoding/json#RawMessage
2. Golang json Unmarshal "unexpected end of JSON input"
3. How to unmarshal json into interface{} in golang?

Answer 1

Your input JSON can be modeled with a simple []Obj where Obj is your type:

type Obj struct {
    Name  string  `json:"name"`
    Value float32 `json:"value"`
}

Nothing else is required, really:

body := []byte(`[
    {"name":"Name1", "value":100.00},
    {"name":"Name2", "value":200.00}]`)

var res []Obj
err := json.Unmarshal(body, &res)
fmt.Printf("%#v\n%v\n", res, err)

Output contains the data from the input JSON (try it on the Go Playground):

[]main.Obj{main.Obj{Name:"Name1", Value:100}, main.Obj{Name:"Name2", Value:200}}
<nil>

Back to your code:

Where you're going wrong is that you use this model:

type Obj struct {
    Name string `json:"name"`
    Value float32 `json:"value"`
}

type Result struct {
    Data json.RawMessage
}

var res []Result

But this res variable would model the following JSON:

[
    {"Data":{"name":"Name1", "value":100.00}},
    {"Data":{"name":"Name2", "value":200.00}}
]

I think you can see the difference: the elements of the array here are JSON Objects with a "Data" field, which then are modeled with your Obj. This input JSON is then properly parsed with your original parsing code, you can try it on the Go Playground:

[{{"name":"Name1", "value":100.00}} {{"name":"Name2", "value":200.00}}] 
&main.Obj{Name:"Name1", Value:100} 
&main.Obj{Name:"Name2", Value:200}