使用指针和SQL Null类型进行编码和解码
最后更新于:2022-04-02 06:47:29
## 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: 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: 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: 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: 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。
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