How to use embedding with the json package for custom JSON marshaling and unmarshaling in Go?

To use embedding with the json package for custom JSON marshaling and unmarshaling in Go, you can leverage Go's struct embedding feature along with the custom MarshalJSON and UnmarshalJSON methods. Here's an example:

package main

import (
    "encoding/json"
    "fmt"
)

type Person struct {
    Name string
}

type Employee struct {
    Person
    EmployeeID int
}

func (p Person) MarshalJSON() ([]byte, error) {
    // Custom JSON marshaling logic for Person struct
    return json.Marshal(struct {
        Name string `json:"person_name"`
    }{
        Name: p.Name,
    })
}

func (p *Person) UnmarshalJSON(data []byte) error {
    // Custom JSON unmarshaling logic for Person struct
    tmp := struct {
        Name string `json:"person_name"`
    }{}

    if err := json.Unmarshal(data, &tmp); err != nil {
        return err
    }

    p.Name = tmp.Name
    return nil
}

func main() {
    emp := Employee{
        Person: Person{Name: "John"},
        EmployeeID: 1234,
    }

    // Marshaling the Employee struct
    jsonBytes, _ := json.Marshal(emp)
    fmt.Println(string(jsonBytes))

    // Unmarshaling the Employee struct
    jsonString := `{"person_name":"Jane","EmployeeID":5678}`
    emp2 := &Employee{}

    if err := json.Unmarshal([]byte(jsonString), emp2); err != nil {
        fmt.Println(err)
    }

    fmt.Println(emp2.Name)         // Output: Jane
    fmt.Println(emp2.EmployeeID)   // Output: 5678
}

In this example, the Person struct is embedded within the Employee struct. By implementing the MarshalJSON and UnmarshalJSON methods for the Person struct, you can customize the JSON marshaling and unmarshaling behavior for the embedded struct.

The MarshalJSON method is called during JSON marshaling. In this example, it defines a new anonymous struct with the desired field names and tags, and then marshals this struct instead of the Person struct.

The UnmarshalJSON method is called during JSON unmarshaling. It unmarshals the data into an anonymous struct with the desired field names and tags, and then assigns the values to the Person struct.

You can then test the custom marshaling and unmarshaling by marshaling an instance of the Employee struct, and unmarshaling a JSON string into an empty Employee struct to verify the values.