使用json.RawMessage以“拖延”解组过程决定做某件事之前要确定原始字节：

var data = []byte(`{
        "somefield1":"somevalue1",
        "somefield2": null
}`)

type Data struct {
    SomeField1 string          
    SomeField2 json.RawMessage
}

func main() {
    d := &Data{}

    _ = json.Unmarshal(data, &d)

    fmt.Println(d.SomeField1)

    if len(d.SomeField2) > 0 {
        if string(d.SomeField2) == "null" {
            fmt.Println("somefield2 is there but null")
        } else {
            fmt.Println("somefield2 is there and not null")
            // Do something with the data
        }
    } else {
        fmt.Println("somefield2 doesn't exist")
    }
}

参观游乐场https://play.golang.org/p/Wganpf4sbO