错误处理中间件-将err转换为字符串以进行响应

Question

I have this middleware func:

func errorMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            if err := recover(); err != nil {
                log.Error("Caught error in defer/recover middleware: ", err)
                w.WriteHeader(http.StatusInternalServerError)
                json.NewEncoder(w).Encode(struct {
                    ID string
                }{
                    err.Error(),
                })
            }
        }()
        next.ServeHTTP(w, r)
    })
}

I use it like so:

router := mux.NewRouter()
router.Use(errorMiddleware)

however I am getting a compilation error, it says:

Anybody know what that's about? I am just trying to convert err to a string, ultimately, serialize it for the client etc.

Answer 1

recover() returns an interface with no methods to proxy any value sent by panic(). In the defer block, you're trying to access the Error() method of a pure, has-no-method interface. If you want to distinguish the built-in error type, you'd have to assert its type like:

realErr, ok := err.(error)
if ok {
    // here you can use realErr.Error().
}

So that it'll give you a real value of type error. If you check out the built-in types, you'll see that error is to implement an Error() string method.

Type assertions: https://tour.golang.org/methods/15