Go struct標(biāo)籤是附加到結(jié)構(gòu)體字段上的字符串註解，用於提供元數(shù)據(jù)以指導(dǎo)某些包如何處理這些字段，最常見的用途是編碼和解碼操作，如JSON、XML、YAML等；它們不會(huì)直接影響程序運(yùn)行時(shí)行為，除非通過反射被顯式讀取。 1. 在JSON序列化中，如json:"name"指定字段映射為指定JSON鍵，omitempty表示零值時(shí)省略，"-"表示忽略該字段；2. 在數(shù)據(jù)庫映射中，如GORM使用gorm:"column:product_id"將字段映射到指定列名；3. 在表單解析中，如form:"email"指示框架將表單字段email綁定到結(jié)構(gòu)體字段；4. 在數(shù)據(jù)驗(yàn)證中，如validate:"required,email"定義字段必須滿足的規(guī)則。標(biāo)籤通過反射機(jī)制讀取，例如使用reflect包獲取字段的Tag信息，從而實(shí)現(xiàn)與外部數(shù)據(jù)格式的靈活交互，提升代碼互操作性。

In Go, struct tags are string annotations attached to struct fields that provide metadata for how those fields should be handled by certain packages—most commonly, encoding and decoding operations like JSON, XML, YAML, and others.

They don't affect the behavior of the Go program directly during runtime unless explicitly read and interpreted by code (usually via reflection).

How Struct Tags Work

A struct tag looks like this:

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

Here, `json:"name"` is the struct tag. It's a string in backticks associated with the field, and it's typically formatted as space-separated key-value pairs:

 `key1:"value1" key2:"value2"`

The most common use is with the json package, but tags are also used by xml , yaml , db (in ORMs like GORM), validate , and others.

Common Use Cases

1. JSON Marshaling/Unmarshaling

Struct tags tell the encoding/json package how to map between Go struct fields and JSON keys.

 type User struct {
    FullName string `json:"full_name"`
    Age int `json:"age"`
    Password string `json:"-"` // Exclude from JSON output
}

• "full_name" means the JSON field will be full_name , not FullName .
• "-" means this field is ignored in JSON.
• omitempty (eg, json:"age,omitempty" ) means the field is omitted from the output if it has a zero value (eg, 0, "", nil).

2. Database Mapping (eg, GORM)

ORMs use tags to map fields to database columns.

 type Product struct {
    ID uint `gorm:"column:product_id;primary_key"`
    Name string `gorm:"column:product_name"`
    Price float64
}

Here, GORM knows to use product_id as the column name instead of id .

3. Form Parsing (eg, web frameworks)

Some web frameworks use tags to bind HTTP form data:

 type LoginForm struct {
    Email string `form:"email"`
    Password string `form:"password"`
}

This tells the framework to map form values by the keys email and password .

4. Validation

Validation libraries use tags to define rules:

 type SignupRequest struct {
    Email string `validate:"required,email"`
    Age int `validate:"min=18"`
}

This tells a validator that Email must be present and a valid email, and Age must be at least 18.

How Tags Are Read (Briefly)

Internally, packages use reflection ( reflect package) to read struct tags at runtime. For example:

 field, _ := reflect.TypeOf(User{}).FieldByName("FullName")
tag := field.Tag.Get("json") // Returns "full_name"

This allows libraries to interpret how each field should be handled without changing the Go type system.

Key Points to Remember

• Struct tags are optional metadata.
• They are ignored by default unless a package reads them.
• The format matters : use backticks, proper key-value syntax.
• Common keys: json , xml , yaml , db , form , validate , etc.
• Tags are often used for serialization, deserialization, and data binding .

Basically, struct tags help bridge Go structs with external data formats and systems—making your code more flexible and interoperable.

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