解码器无法解组时如何获取JSON？

Question

I am using a json.Decoder to decode JSON delivered over a network stream. It works fine, but whenever someone sends data that doesn't fit the schema (e.g. sending a negative integer when the struct's field type is unsigned) it returns an error with a vague message that doesn't pinpoint the offending property. This makes debugging more difficult.

Is there any way to extract the JSON that the decoder was trying to unmarshal when it errored? Here's a small reproducable snippet:

package main

import (
    "bytes"
    "fmt"
    "encoding/json"
)

func main() {
    buff := bytes.NewBufferString("{\"bar\": -123}")
    decoder := json.NewDecoder(buff)

    var foo struct{
        Bar uint32
    }
    if err := decoder.Decode(&foo); err != nil {
        fmt.Println(err)
        fmt.Println("TODO: how to get JSON that caused this error?")
    } else {
        fmt.Println(foo.Bar)
    }
}

Or on playground: https://play.golang.org/p/-WdYBkYEzJ

Answer 1

As of Go 1.8 this is now possible. The UnmarshalTypeError type now contains Struct and Field values which provide the name of the struct and field which caused a type mismatch.

package main

import (
    "bytes"
    "fmt"
    "encoding/json"
)

func main() {
    buff := bytes.NewBufferString("{\"bar\": -123}")
    decoder := json.NewDecoder(buff)

    var foo struct{
        Bar uint32
    }
    if err := decoder.Decode(&foo); err != nil {
        if terr, ok := err.(*json.UnmarshalTypeError); ok {
                fmt.Printf("Failed to unmarshal field %s 
", terr.Field)
        } else {
            fmt.Println(err)
        }
    } else {
        fmt.Println(foo.Bar)
    }
}

The error message string also was changed to contain this new information.

Go 1.8:

json: cannot unmarshal number -123 into Go struct field .Bar of type uint32

Go 1.7 and earlier:

json: cannot unmarshal number -123 into Go value of type uint32