将结构编组为json时在字段上执行操作

Question

I have the following struct:

type Message struct {
    DoubleMe    int     `json:"double_me"`
    Message     string  `json:"message"`
}

And a instance of it:

m := Message{5, "Hello, World!"}

My goal is to perform some operation to the DoubleMe field when it's marshaled to json, for example doubling it. So the output would be:

{"double_me":10,"message":"Hello, World!"}

How can I achieve this?

Answer 1

Implement json.Marshaler on your struct, which is to define a custom marshaling logic, which may double the DoubleMe field.

See this example:

type Message struct {
    DoubleMe int    `json:"double_me"`
    Message  string `json:"message"`
}

func (m Message) MarshalJSON() ([]byte, error) {
    m.DoubleMe *= 2

    type Message2 Message
    return json.Marshal(Message2(m))
}

func main() {
    m := Message{5, "Hello, World!"}
    data, err := json.Marshal(m)
    fmt.Println(string(data), err)
    fmt.Println("Original:", m)
}

Output (try it on the Go Playground):

{"double_me":10,"message":"Hello, World!"} <nil>
Original: {5 Hello, World!}

Some notes:

• If you need this to work backwards too (e.g. when unmarshaling, divide the value by 2), also implement json.Unmarshaler (left to the reader as an exercise).

• Even though we doubled the field, after the marshaling, the original value did not change. This is because the method has a non-pointer receiver, so inside the method only a copy is acquired and modified, but not the original value. If we would've used pointer receiver, we would have to take care of restoring it.

• Also note that a new type Message2 is created and used inside the MarshalJSON() method. This is because we also used the json package to do the "usual" marshaling once we applied our custom logic. But if we would pass m to json.Marshal(), that would be an endless "recursion", because the json package would again call this MarshalJSON() method. Instead we created a Message2 type having Message as its underlying type. This means Message2 has zero methods (it does not implement json.Marshaler), so passing a value of Message2 to json.Marshal() will not call this method again, and since it has Message as its underlying type, we can simply convert a value of type Message to Message2.