问题
I'm fairly new to Golang. And I was looking for a way to do some custom stuff for marshaling and unmarshalling json
. I have found the solution to implement Marshaller
and Unmarshaller
interfaces.
Here is my struct
with implemented interfaces (I have also implemented Stringer):
type Data struct {
Foo string `json:"foo"`
bar string
}
func (d Data) MarshalJSON() ([]byte, error) {
return []byte("{\"foo\":\"test\",\"bar\":\"data\"}"), nil
}
func (d Data) String() string {
return fmt.Sprintf("Foo: %s, bar: %s", d.Foo, d.bar)
}
func (d Data) UnmarshalJSON(b []byte) error {
d.bar = "testtest"
d.Foo = "data"
return nil
}
For Marshaller
everything works as expected:
data := &Data{}
marshal, _ := json.Marshal(data)
fmt.Println(string(marshal))
The output as expected:
{"foo":"test","bar":"data"}
But Unmarshaller doesn't work as I was expecting:
jsonData := "{\"foo\":\"test\"}"
data := Data{}
json.Unmarshal([]byte(jsonData), data)
fmt.Println(data)
This code prints:
Foo: , bar:
Is there something that I'm missing here?
回答1:
You have a few distinct problems here.
The method receiver must be a pointer if you want to modify the receiver, otherwise you're only modifying the copy local to the method.
You always need to unmarshal into a pointer.
You're declaring a
"field"
json tag for theFoo
field, but passing in"foo"
You're calling
json.Unmarshal
inside yourUnmarshalJSON
method, which is going to recurse indefinitely.
A working example would look like
func (d *Data) UnmarshalJSON(b []byte) error {
type data Data
tmp := &data{bar: "bar"}
err := json.Unmarshal(b, tmp)
if err != nil {
return err
}
*d = Data(*tmp)
return nil
}
func main() {
jsonData := "{\"field\":\"test\"}"
data := Data{}
json.Unmarshal([]byte(jsonData), &data)
fmt.Printf("%#v\n", data)
}
https://play.golang.org/p/FnF2li63dt
回答2:
Data
methods has value receiver, but the variable data has pointer type (*Data
).
Pointer type variables cannot use functions with value receiver. Either change variable type or receiver type.
Also see https://github.com/golang/go/wiki/CodeReviewComments#receiver-type about receivers.
来源:https://stackoverflow.com/questions/46448102/go-json-unmarshaller-is-not-called