英文:
Unmarshalling a JSON that may or may not return an array?
问题
我正在从第三方网站检索JSON数据(家庭用电情况),根据我从网站请求的内容,返回的JSON可能是一个数组,也可能不是。例如,如果我请求我的智能电表列表,返回的结果可能是这样的(由于大小原因,结果被截断):
{"gwrcmds":{"gwrcmd":{"gcmd":"SPA_UserGetSmartMeterList","gdata":{"gip":{"version":"1"...
其中gwrcmd是一个单独的元素。
但是,如果我请求过去半小时的用电情况,返回的结果可能是这样的:
{"gwrcmds":{"gwrcmd":[{"gcmd":"DeviceGetChart","gdata":{"gip":{"version":"1"...
注意,gwrcmd现在是一个数组。
在我的Go应用程序中,我有一个结构体,看起来像这样(同样被截断,因为它还有更多的子结构体和属性在"Version"下面):
type Response struct {
Gwrcmds struct {
Gwrcmd struct {
Gcmd string
Gdata struct {
Gip struct {
Version string
如果gwrcmd
是一个数组,Gwrcmd
需要是[]struct { }
,但如果它不是,它只是一个普通的struct { }
。
问题是,如果JSON是一个数组,而结构体没有切片(或反之亦然),json.Unmarshal
会返回一个错误。
我是否需要创建一个第二个结构体,它与第一个结构体相同(除了使用[]struct { }
),或者有更好的方法来解决这个问题?我想到了使用接口的方法,但我还没有真正接触过它们,所以对它们不是100%确定。
英文:
I'm retrieving JSON from a third party website (home electricity usage), and depending on what I've requested from the site, the JSON returned may or may not be an array. For example, if I request a list of my smart meters, I get this (results truncated, due to large size):
{"gwrcmds":{"gwrcmd":{"gcmd":"SPA_UserGetSmartMeterList","gdata":{"gip":{"version":"1"...
Where gwrcmd is a single element.
But if I request electricity usage for the last half hour, I get this:
{"gwrcmds":{"gwrcmd":[{"gcmd":"DeviceGetChart","gdata":{"gip":{"version":"1" ...
See how gwrcmd is now an array?
Within my Go app, I have a struct that looks like this (again, truncated, as it goes on for a while. There's more sub-structs and properties beneath "Version":
type Response struct {
Gwrcmds struct {
Gwrcmd struct {
Gcmd string
Gdata struct {
Gip struct {
Version string
If gwrcmd
is an array, Gwrcmd
needs to be a []struct { }
, but if it's not, it's just a regular old struct { }
The problem is that json.Unmarshal
just returns an error if the JSON has an array and the struct does not have a slice (or vice versa).
Would I need to create a second struct that duplicates the first one (except with a []struct { }
instead), or is there a better way to do it? I thought of something with interfaces, but I haven't really touched those yet, so I'm not 100% sure on them.
答案1
得分: 10
通常,当你有一个未知类型的 JSON 值时,你会使用 json.RawMessage
来获取它,查看它,并将其正确地解组为相应的类型。下面是一个简化的示例:
// 这里的 A 可以是一个对象,也可以是一个对象的 JSON 数组。
type Response struct {
RawAWrapper struct {
RawA json.RawMessage `json:"a"`
}
A A `json:"-"`
As []A `json:"-"`
}
type A struct {
B string
}
func (r *Response) UnmarshalJSON(b []byte) error {
if err := json.Unmarshal(b, &r.RawAWrapper); err != nil {
return err
}
if r.RawAWrapper.RawA[0] == '[' {
return json.Unmarshal(r.RawAWrapper.RawA, &r.As)
}
return json.Unmarshal(r.RawAWrapper.RawA, &r.A)
}
Playground: http://play.golang.org/p/2d_OrGltDu.
然而,根据第一个字节猜测内容似乎不太可靠。通常,你的 JSON 中会有一些线索(例如与动态字段处于同一级别的 length
或 type
字段),告诉你是一个对象还是一个数组。
另请参阅:
英文:
Usually, whenever you have a JSON value of unknown type, you will use json.RawMessage
to get it, peek into it, and unmarshal it correctly into the corresponding type. A simplified example:
// The A here can be either an object, or a JSON array of objects.
type Response struct {
RawAWrapper struct {
RawA json.RawMessage `json:"a"`
}
A A `json:"-"`
As []A `json:"-"`
}
type A struct {
B string
}
func (r *Response) UnmarshalJSON(b []byte) error {
if err := json.Unmarshal(b, &r.RawAWrapper); err != nil {
return err
}
if r.RawAWrapper.RawA[0] == '[' {
return json.Unmarshal(r.RawAWrapper.RawA, &r.As)
}
return json.Unmarshal(r.RawAWrapper.RawA, &r.A)
}
Playground: http://play.golang.org/p/2d_OrGltDu.
Guessing the content based on the first byte doesn't seem too robust to me though. Usually you'll have some sort of a clue in your JSON (like a length
or type
field on the same level as the dynamic one) that tells you whether you have an object or an array.
See also:
答案2
得分: 5
你可以尝试创建自定义的JSON解析方法,像这样:
func (a *GwrcmCustom) UnmarshalJSON(b []byte) (err error) {
g, ga := Gwrcmd{}, []Gwrcmd{}
if err = json.Unmarshal(b, &g); err == nil {
*a = make([]Gwrcmd, 1)
(*a)[0] = Gwrcmd(g)
return
}
if err = json.Unmarshal(b, &ga); err == nil {
*a = GwrcmCustom(ga)
return
}
return
}
GwrcmCustom
是一个自定义类型,是Gwrcm
的切片:
type GwrcmCustom []Gwrcmd
所以我们将始终得到一个切片。
你可以在Go playground上尝试这个方法。
希望这能帮到你。
英文:
You can try to make custom json unmarshal method, like
func (a *GwrcmCustom) UnmarshalJSON(b []byte) (err error) {
g, ga := Gwrcmd{}, []Gwrcmd{}
if err = json.Unmarshal(b, &g); err == nil {
*a = make([]Gwrcmd, 1)
[]Gwrcmd(*a)[0] = Gwrcmd(g)
return
}
if err = json.Unmarshal(b, &ga); err == nil {
*a = GwrcmCustom(ga)
return
}
return
}
GwrcmCustom
is a custom type, slice of Gwrcm
type GwrcmCustom []Gwrcmd
So we will get slice always
Try this on <kbd>Go playground</kbd>
I hope this will help
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