Golang json decoding fails to decode interface{}

Question

I'm using a library (go-kit) which requires I specify functions to encode / decode my request and response types to/from JSON. For encoding, it is simple:

func EncodeResponse(_ context.Context, w http.ResponseWriter, response interface{}) error {
    return json.NewEncoder(w).Encode(response)
}

I pass this function to create the HTTP server and it works fine. However, their proposed approach for requests is to make a separate function of the form:

func decodeUppercaseRequest(_ context.Context, r *http.Request) (interface{}, error) {
    var req UppercaseRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        return nil, err
    }
    return req, nil
}

For every single RPC in my application. I would really like to keep my code DRY and avoid having hundreds of almost identical methods. As such, I attempted to write a function to generate closures that decode the given request type:

func DecodeRequest(req interface{}) httptransport.DecodeRequestFunc {
    return func(_ context.Context, r *http.Request) (interface{}, error) {
        if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
            return nil, err
        }
        return req, nil
    }
}

This function can be called like so:

DecodeRequest(UppercaseRequest{}}

Unfortunately, when I do so, the JSON decoding fails, even though the type of req is in fact mypackage.UppercaseRequest. I'm not sure where to go from here. Is there a way I can avoid having to write a method per request type? Is there some way I can help the Decode function understand what this type is at runtime? Thanks in advance!

Here is a go playground demonstrating the issue: https://play.golang.org/p/GgHsLffp1G

What's the json decode errror? A type switch may help or the reflect package has lots of tools for runtime type idenfication. — Sridhar
– Sridhar, Commented Aug 20, 2016 at 8:45
@Sridhar If I call DecodeRequest(CountRequest{}) I get "http: panic serving [::1]:55992: interface conversion: interface is map[string]interface {}, not user.CountRequest" and if i change the function to use ...Decode(req) and call DecodeRequest(&CountRequest{}) I get "http: panic serving [::1]:56375: interface conversion: interface is *user.CountRequest, not user.CountRequest" — Logan Shire
– Logan Shire, Commented Aug 20, 2016 at 8:57
What's CountRequest ? A map[string]interface{} ? You really should abide by this . Maybe try Decode(Req) and call DecodeRequest(CountRequest{}) ? — Sridhar
– Sridhar, Commented Aug 20, 2016 at 9:19
@Sridhar it's just a struct: type CountRequest struct { S string json:"s" } I am not sure where the map[string]interface{} is coming from. When I try Decode(req) and call DecodeRequest(CountRequest{}) I get: interface conversion: interface is *user.CountRequest, not user.CountRequest — Logan Shire
– Logan Shire, Commented Aug 20, 2016 at 9:34

jnmoal · Accepted Answer · 2016-08-20 13:53:31Z

3

According to the piece of code you are showing us, I think you are facing a type assertion issue. I created a playground to show you what I explain below.

You're passing a UpperCaseRequest to DecodeRequest func. In this func, the argument is of type interface{}, and it passes a pointer of this argument to the json Decoder. So, The decoder sees a pointer to interface, and not a pointer to UpperCaseRequest.

This is why it's not correctly decoded. And then, trying a type assertion on it fails because asserting two different type is not possible.

So, in your code, I'd suggest:

func DecodeRequest(req interface{}) httptransport.DecodeRequestFunc {
    return func(_ context.Context, r *http.Request) (interface{}, error) {
        // Note the '&' is removed here
        if err := json.NewDecoder(r.Body).Decode(req); err != nil {
            return nil, err
        }
        return req, nil
    }
}

And call this function like this:

// Note the & is placed here.
DecodeRequest(&UppercaseRequest{}}

answered Aug 20, 2016 at 13:53

jnmoal

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4 Comments

Logan Shire Over a year ago

Unfortunately, this doesn't work for me. When I try it, the error I get is: http: panic serving [::1]:58709: interface conversion: interface is *user.UppercaseRequest, not user.UppercaseRequest

Logan Shire Over a year ago

Oh! I think i have an idea - maybe the issue isn't with the json decoder, but rather that i'm returning a pointer instead of the actual struct. I think I need to return *req, nil. But then how can I cast an interface to a pointer to an interface?

Logan Shire Over a year ago

Unfortunately neither worked - check the go playground I added which replicates the issue.

jnmoal Over a year ago

I updated your playground with a working solution (still based on what I am saying in my answer). I added the type assertion so you can see how to use it. play.golang.org/p/xjbfaoqwLg

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