Summary - encoding/json

Combine Marshal, Unmarshal and tags to write a record.

encoding/json summary

In memory: json.Marshal / json.Unmarshal(&v). In streams: json.NewEncoder(w).Encode / json.NewDecoder(r).Decode. Tags: json:"ad", omitempty, -. Data of unknown shape: map[string]any (numbers are float64) or json.RawMessage. Pointer fields to tell missing from zero.

Custom MarshalJSON

Keeping a money amount in cents but showing decimal text in JSON:

type Para int64  // cents

func (p Para) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf(`"%d.%02d"`, p/100, p%100)), nil
}

json.Marshal(map[string]Para{"tutar": 1250})  // {"tutar":"12.50"}

A MarshalJSON written with a value receiver works for both values and pointers.

Exercise Money type

Write the Para type and its MarshalJSON method above. Encode map[string]Para{"tutar": 1250} and print it.

Show example solution

package main

import (
    "encoding/json"
    "fmt"
)

type Para int64

func (p Para) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf(`"%d.%02d"`, p/100, p%100)), nil
}

func main() {
    b, err := json.Marshal(map[string]Para{"tutar": 1250})
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(b))
}

Test yourself

How does a type define its own JSON format?
- Only with tags
- It isn't possible
- By implementing MarshalJSON / UnmarshalJSON methods
- By writing String()
Show answer

By implementing MarshalJSON / UnmarshalJSON methods — Types implementing the json.Marshaler and json.Unmarshaler interfaces control encoding entirely themselves.

Readable output

b, _ := json.____(v, "", "  ")

Show answer

b, _ := json.MarshalIndent(v, "", "  ")

Group summary

Marshal

Go → JSON
Unmarshal

JSON → Go
omitempty

Skip zero
MarshalJSON

Custom format

Next lesson: Client with net/http

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