Common JSON handling mistakes

Type embedding

Mistake

This does not work because time.Time implements the json.Marshaler interface and will clobber any sort of JSON marshaling we are trying to accomplish.

type Event struct {
  ID int
  time.Time // embedded field
}

event := Event{
  ID: 123456,
  Time: time.Now(),
}

b, err := json.Marshal(event)
if err != nil {
  return err
}

Fix

type Event struct {
  ID: int
  Time: time.Time
}

Alternatively we could chose to implement the json.Marshaler interface

func (e *Event) MarshalJSON() ([]byte, error){
  return json.Marshal(
    // leveraging an anonymous struct
    struct {
      ID int
      Time time.Time
    }{
      ID: e.ID,
      Time: e.Time,
    },
  )
}

The monotonic clock

An OS handles two types of clocks:

Mistake

type Event struct {
  Time time.Time
}

t := time.Now()
event1 := Event{
  Time: t,
}

b, err := json.Marshal(event1)
if err != nil {
  return
}

var event2 Event
err = json.Unmarshal(b, &event2)
if err != nil {
  return
}

assert(event1 == event2)

This assertion will fail. This is because time.Time handles both monotonic and wall clocks.

2021-01-10 17:13:08.852061 +0100 CET m=+0.000338660

Wall time Monotonic time

Fix

event1.Time.Equal(event2.Time)

Alternatively we can strip away the monotonic time.

t := time.Now()
event := Event{
  Time: t.Truncate(0),
}

Map of any

This code works, however it should be made clear that any numeric field will be assigned to a float64 regardless whether it contains a decimal point or not.

b := getMessage()
var m map[string]any

err := json.Unmarshal(b, &m)
if err != nil {
  return err
}

References