How is JSON work (encoding/json) implemented in Go: features of serialization, features of struct tags, common pains during marshaling/unmarshaling and workarounds?

Answer.

In Go, JSON marshaling is implemented through the encoding/json package. Only exported (uppercase) fields of structs can be serialized. Struct tags are used to manage names and processing:

type User struct {
    ID   int    `json:"id"`
    Name string `json:"name,omitempty"`
    Age  int    `json:"age,string"`
}

• omitempty excludes the field when it has a zero value;
• string serializes the value as a string, even if it's a number.

Example of serialization:

user := User{ID: 1, Name: "Oleg"}
b, _ := json.Marshal(user)
fmt.Println(string(b)) // {"id":1,"name":"Oleg","age":"0"}

Example of JSON parsing

var u User
json.Unmarshal([]byte('{"id":2,"name":"Ivan"}'), &u)

Trick question.

How are unexported (lowercase) fields of a struct serialized when marshaling to JSON?

Correct answer: They are ignored; only exported (uppercase) fields are involved in serialization. This often unexpectedly breaks APIs:

type Foo struct {
   bar int // will not be serialized!
}

Examples of real mistakes due to ignorance of the subtleties of the topic.

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Story

The backend was returning incomplete JSON objects because some needed fields were unexported (lowercase). It took several days to figure out why clients couldn't see these fields.

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Story

In the API response, a field contained the omitempty construct, but sometimes it came with an empty value because the zero value for a slice is nil, not an empty slice. Clients received null instead of an empty array [] and crashed during parsing.

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Story

In the project, dynamic fields were mixed into structs through map[string]interface{}, but a custom UnmarshalJSON was forgotten to be implemented, causing some data to be "lost" without errors. Client data was lost and had to be manually restored from backups.