在 Go 中解组 JSON 数组

Question

I've read a few answers here and none seem to help. I keep getting the same error:

json: cannot unmarshal array into Go value of type A

type A struct {
    Arr []string
}

type MA []A

func UnmarshalJSON() (ma MA, err error) {
    jsonFile, err := os.Open(aJsonFile)
    // error handling
    defer jsonFile.Close()
    byteValue, _ := ioutil.ReadAll(jsonFile)
    err = json.Unmarshal(byteValue, &ma)
    if err != nil {
        return ma, err
    }
    return ma, nil
}

The json looks like this:

[
  ["Name", "Another Name", "Another"],
  ["A", "B"],
  ["W", "X", "Y", "Z"],
  ["Foo", "Bar"]
]

I've tried various different things from top answers when searching and as previously stated, nothing has worked.

I'm still decently new to Go and having to unmarshall stuff (I'm currently unmarshalling to [][]string but I want to use structs). What am I doing wrong?

Answer 1

A struct has fields; fields have names; the name of the field in your struct is Arr . So the json input would need to be [{ "arr": ["list", "of", "names"]}, {"arr": ["more", "names"]}] for instance, given the example you have above.

You can, however, define an UnmarshalJSON on your type named A , like this:

func (p *A) UnmarshalJSON(data []byte) error {
    var s []string
    err := json.Unmarshal(data, &s)
    if err != nil {
        return err
    }
    p.Arr = s
    return nil
}

This receiver function named UnmarshalJSON takes a pointer to an A object, plus some sort of valid json input. Its job is to unmarshal that json. In this case we attempt to unmarshal into an ordinary slice-of- string —the variable s —which works as long as the json itself is a valid initializer for slice-of-strings.

If the unmarshal succeeds we then set p.Arr , knowing that the array is meant just for the thing named Arr (which is in fact the only member of the structure type) and return nil (no error).

Note that the last few lines could be written as:

if err == nil {
    p.Arr = s
}
return err

which is shorter, but Go conventions generally handle the error case first, rather than letting it flow through. (I actually prefer the shorter code myself, slightly, but use whatever your group likes.)

(Given an UnmarshalJSON receiver on the type, there probably should be a json marshaler on the type, too, but my minimal test-and-example on the Go playground does not have one.)

Answer 2

your json content should like this

[
    {
        "Arr": [
            "a0",
            "a1"
        ]
    },
    {
        "Arr": [
            "b0",
            "b1"
        ]
    },
    {
        "Arr": [
            "c0",
            "c1"
        ]
    }
]