How to validate JSON input using Golang?

Question

I'm using Beego framework to build a web application in Go. I have to validate the incoming JSON in an API request.

I can unmarshal the JSON into a struct which works fine, but I want to validate the data as well. For example, if the type doesn't match with the type in struct json.Unmarshal will reutrn an error on the first occurence. I want to validate and get all the errors at once for JSON.

I've tried https://github.com/thedevsaddam/govalidator but the package needs a reference to request object which is not available in the controller of Beego. There are other validators which can validate a struct but i want the json validation as well.

EDIT:

A reference to the request object in beego can be found from the controller's context object as:

func (this *MainController) Post() {
    fmt.Println(this.Ctx.Request)
}

But the problem remains same with json unmarshal. If there's any slight mismatch in the type, json.unmarshal will immediately panic. I want to be able to validate the type as well.

This isn't supported by the standard library, so it seems you're asking for a 3rd party lib? That is off-topic for SO. — icza, May 30 '19 at 12:28
[`json.Decoder.Token()`](https://golang.org/pkg/encoding/json/#Decoder.Token) might help, but it will require quite a bit of code. — Mihai Todor, May 30 '19 at 12:54

score 11 · Answer 1 · answered Nov 19 '20 at 17:54

11

Since Golang v1.9 json.Valid([]byte) has been a valid method available from the "encoding/json" package.

Reference: https://golang.org/pkg/encoding/json/#Valid

answered Nov 19 '20 at 17:54

danielkalen

111
1
2

score 6 · Answer 2 · answered May 30 '19 at 12:49

We've used go-playground/validator.v8 for a similar purpose. You can define the data validations with the tags that come out of the box (basic stuff like equality, min/max and even has somthing of an expression lang). On top of that you can add your custom validations. It's all in the docs, hope it helps.

score 1 · Answer 3 · answered Sep 18 '19 at 11:17

1

There is a method checkValid (in encoding/json package) which can do that. This method, however, is not exposed. So, you can use Unmarshal (in same package) to check if JSON is valid.

answered Sep 18 '19 at 11:17

Aakash Goyal

1,051
4
12
44

score 1 · Answer 4 · answered May 19 '20 at 21:40

I had to do this today so I will share my solution. I created a function using unmarshal that returns true or false based on whether the text returns valid json:

// isJSON checks that the string value of the field can unmarshall into valid json
func isJSON(s string) bool {
    var j map[string]interface{}
    if err := json.Unmarshal([]byte(s), &j); err != nil {
        return false
    }
    return true
}

Then I used that logic with the validator package to make and register a custom validation tag that I can use in any struct to validate that a field contains json. You can use go playground to check the full solution and experiment:

// isJSON checks that the string value of the field can unmarshall into valid json
func isJSON(fl validator.FieldLevel) bool {
    var j map[string]interface{}
    if err := json.Unmarshal([]byte(fl.Field().String()), &j); err != nil {
        return false
    }
    return true
}

// register the function with json tag:
validate.RegisterValidation("json", isJSON)

How to validate JSON input using Golang?

4 Answers4