## 使用指针和SQL Null类型进行编码和解码 当您对Go中的对象进行编码或解码时,未显式设置的类型将被填充为其默认值。字符串将默认为空字符串,整数将默认为0。通常这没什么问题,但当0在业务上包含其他含义时,歧义就出现了。 此外,如果使用结构标记,例如json omitempty,即使它们有效,也会忽略0值。 另一个例子是从SQL返回的Null。 对于Int来说,什么值最能代表Null?本文将探讨Go开发人员处理此问题的一些方法。 ### 实践 1. 建立 base.go: ``` package nulls import ( "encoding/json" "fmt" ) const ( jsonBlob = `{"name": "Aaron"}` fulljsonBlob = `{"name":"Aaron", "age":0}` ) // Example结构体包含age和name字段 type Example struct { Age int `json:"age,omitempty"` Name string `json:"name"` } // BaseEncoding 演示了基本的编码和解码操作 func BaseEncoding() error { e := Example{} // 注意jsonBlob没有age字段 if err := json.Unmarshal([]byte(jsonBlob), &e); err != nil { return err } fmt.Printf("Regular Unmarshal, no age: %+v\n", e) value, err := json.Marshal(&e) if err != nil { return err } //由于age被设置为omitempty(为空则不输出) 所以显示 Regular Marshal, with no age: {"name":"Aaron"} fmt.Println("Regular Marshal, with no age:", string(value)) if err := json.Unmarshal([]byte(fulljsonBlob), &e); err != nil { return err } fmt.Printf("Regular Unmarshal, with age = 0: %+v\n", e) value, err = json.Marshal(&e) if err != nil { return err } //Regular Marshal, with age = 0: {"name":"Aaron"} fmt.Println("Regular Marshal, with age = 0:", string(value)) return nil } ``` 2. 建立pointer.go: ``` package nulls import ( "encoding/json" "fmt" ) // 和上一个例子类似 但是*int类型会出现奇妙的nil // uses a *Int type ExamplePointer struct { Age *int `json:"age,omitempty"` Name string `json:"name"` } func PointerEncoding() error { e := ExamplePointer{} if err := json.Unmarshal([]byte(jsonBlob), &e); err != nil { return err } //Pointer Unmarshal, no age: {Age:<nil> Name:Aaron} fmt.Printf("Pointer Unmarshal, no age: %+v\n", e) value, err := json.Marshal(&e) if err != nil { return err } //Pointer Marshal, with no age: {"name":"Aaron"} fmt.Println("Pointer Marshal, with no age:", string(value)) if err := json.Unmarshal([]byte(fulljsonBlob), &e); err != nil { return err } //Pointer Unmarshal, with age = 0: {Age:0xc04200e4b8 Name:Aaron} fmt.Printf("Pointer Unmarshal, with age = 0: %+v\n", e) value, err = json.Marshal(&e) if err != nil { return err } //Pointer Marshal, with age = 0: {"age":0,"name":"Aaron"} fmt.Println("Pointer Marshal, with age = 0:", string(value)) return nil } ``` 3. 建立nullencoding.go: ``` package nulls import ( "database/sql" "encoding/json" "fmt" ) type nullInt64 sql.NullInt64 // 和前面的例子类似 又改变了Age的类型sql.NullInt64 type ExampleNullInt struct { Age *nullInt64 `json:"age,omitempty"` Name string `json:"name"` } func (v *nullInt64) MarshalJSON() ([]byte, error) { if v.Valid { return json.Marshal(v.Int64) } return json.Marshal(nil) } func (v *nullInt64) UnmarshalJSON(b []byte) error { v.Valid = false if b != nil { v.Valid = true return json.Unmarshal(b, &v.Int64) } return nil } func NullEncoding() error { e := ExampleNullInt{} if err := json.Unmarshal([]byte(jsonBlob), &e); err != nil { return err } //nullInt64 Unmarshal, no age: {Age:<nil> Name:Aaron} fmt.Printf("nullInt64 Unmarshal, no age: %+v\n", e) value, err := json.Marshal(&e) if err != nil { return err } //nullInt64 Marshal, with no age: {"name":"Aaron"} fmt.Println("nullInt64 Marshal, with no age:", string(value)) if err := json.Unmarshal([]byte(fulljsonBlob), &e); err != nil { return err } //nullInt64 Unmarshal, with age = 0: {Age:0xc0420623f0 Name:Aaron} fmt.Printf("nullInt64 Unmarshal, with age = 0: %+v\n", e) value, err = json.Marshal(&e) if err != nil { return err } //nullInt64 Marshal, with age = 0: {"age":0,"name":"Aaron"} fmt.Println("nullInt64 Marshal, with age = 0:", string(value)) return nil } ``` 4. 建立main.go: ``` package main import ( "fmt" "github.com/agtorre/go-cookbook/chapter3/nulls" ) func main() { if err := nulls.BaseEncoding(); err != nil { panic(err) } fmt.Println() if err := nulls.PointerEncoding(); err != nil { panic(err) } fmt.Println() if err := nulls.NullEncoding(); err != nil { panic(err) } } ``` 5. 这会输出: ``` Regular Unmarshal, no age: {Age:0 Name:Aaron} Regular Marshal, with no age: {"name":"Aaron"} Regular Unmarshal, with age = 0: {Age:0 Name:Aaron} Regular Marshal, with age = 0: {"name":"Aaron"} Pointer Unmarshal, no age: {Age:<nil> Name:Aaron} Pointer Marshal, with no age: {"name":"Aaron"} Pointer Unmarshal, with age = 0: {Age:0xc42000a610 Name:Aaron} Pointer Marshal, with age = 0: {"age":0,"name":"Aaron"} nullInt64 Unmarshal, no age: {Age:<nil> Name:Aaron} nullInt64 Marshal, with no age: {"name":"Aaron"} nullInt64 Unmarshal, with age = 0: {Age:0xc42000a750 Name:Aaron} nullInt64 Marshal, with age = 0: {"age":0,"name":"Aaron"} ``` ### 说明 从值切换到指针是表示空值的快速方法。这可能会为初始化带来点麻烦,因为无法直接操作*a := 1。除此之外这是一种不错的处理方式。 本节还演示了使用sql.NullInt64类型的替代方法。这通常与SQL一起使用,如果返回Null以外的任何内容,则设置valid,否则设置为Null。我们添加了MarshalJSON和UnmarshallJSON方法以允许此类型与JSON包交互,我们选择使用指针,以便omitempty将继续按预期工作。 * * * * 学识浅薄,错误在所难免。欢迎在群中就本书提出修改意见,以飨后来者,长风拜谢。 Golang中国(211938256) beego实战(258969317) Go实践(386056972)