How to parse a complicated JSON with Go unmarshal?

Question

In go the standard package encoding/json exposes json.Unmarshal function to parse JSON.

It's possible to either unmarshal the JSON string in a predefined struct, or use the interface{} and iterate the result for unexpected JSON data structure.

That said, I can't parse complex JSON properly. Can someone tell me how to achieve this?

 {
     "k1" : "v1", 
     "k2" : "v2", 
     "k3" : 10, 
     "result" : [
                 [
                 ["v4", v5, {"k11" : "v11", "k22" : "v22"}]
                 , ... , 
                 ["v4", v5, {"k33" : "v33", "k44" : "v44"}
                 ]
                 ], 
                 "v3"
                ] 
}

Answer 1

Citing from JSON and Go:

Without knowing this data's structure, we can decode it into an interface{} value with Unmarshal:

b := []byte(`{
   "k1" : "v1", 
   "k3" : 10,
   result:["v4",12.3,{"k11" : "v11", "k22" : "v22"}]
}`)
var f interface{}
err := json.Unmarshal(b, &f)

At this point the Go value in f would be a map whose keys are strings and whose values are themselves stored as empty interface values:

f = map[string]interface{}{
    "k1": "v1",
    "k3":  10,
    "result": []interface{}{
       "v4",
       12.3,
       map[string]interface{}{
           "k11":"v11",
           "k22":"v22",
       },
    },
}

To access this data we can use a type assertion to access f's underlying map[string]interface{}:

m := f.(map[string]interface{})

We can then iterate through the map with a range statement and use a type switch to access its values as their concrete types:

for k, v := range m {
    switch vv := v.(type) {
    case string:
        fmt.Println(k, "is string", vv)
    case int:
        fmt.Println(k, "is int", vv)
    case []interface{}:
        fmt.Println(k, "is an array:")
        for i, u := range vv {
            fmt.Println(i, u)
        }
    default:
        fmt.Println(k, "is of a type I don't know how to handle")
    }
}

In this way you can work with unknown JSON data while still enjoying the benefits of type safety.

More information about Go and JSON can be found in the original article. I changed the code snippets slightly to be more similar to the JSON in the question.

Answer 2

More recently, gjson offers selection of properties in JSON

k1 := gjson.Get(json, "k1")
k33 : = gjson.Get(json, "result.#.#.k33")