当给定的 JSON 值不是 JSON object 时，解组自定义结构

Question

I have a custom struct defined that I'm using to Unmarshal an API response into. Sometimes, the API decides to send an empty string ( "" ) when the object is empty or missing. I'm unclear how to handle this type mismatch. I actually have hundreds of these custom structs (nested) and the API will do this for the top-most parent if it is empty. If there is some way to do this globally without the need for a custom UnmarshalJSON method, that would be ideal. I'm using code generation, so I could add those methods easily.

When I first attempted to write my own custom UnmarshalJSON , I ended up in a loop:

func (e MyStruct) UnmarshalJSON(b []byte) error {
    t := MyStruct{}
    if string(b) == `""` {e = t} else {
        if err := json.Unmarshal(b, e); err != nil {
            return fmt.Errorf("failed to parse nested structure: %v", err)
        }
    }
    return nil
}

I understand why I ended up in the loop, but I have a feeling there is a better solution I'm missing.

I've created an example in the Go Playground.

https://play.golang.org/p/DrrFhA3TzPv

My current, generated code is here if you're looking for more context: https://github.com/mbrancato/edgeos/blob/a8af9143aa82ddece27089bf2f543f473c97d2db/sdk/config.go

Answer 1

Use the following code to unmarshal the value.

var emptyString = []byte(`""`)

func (e *MyStruct) UnmarshalJSON(b []byte) error {
    if bytes.Equal(b, emptyString) {
        *e = MyStruct{}
        return nil
    }
    type t MyStruct
    if err := json.Unmarshal(b, (*t)(e)); err != nil {
        return fmt.Errorf("failed to parse nested structure: %w", err)
    }
    return nil
}

Notes:

• UnmarshalJSON must be on pointer receiver.
• Avoid recursion by declaring new type t with same underlying type as MyStruct . Unmarshal to a value of type t .
• Compare bytes instead of string to avoid allocations.
• To allow use of errors.As in the caller, use %w as the formatting verb for the error.