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Go設(shè)計(jì)模式之單例模式講解和代碼示例

更新時(shí)間：2023年07月26日 08:18:32 作者：demo007x

單例是一種創(chuàng)建型設(shè)計(jì)模式，讓你能夠保證一個(gè)類只有一個(gè)實(shí)例，并提供一個(gè)訪問(wèn)該實(shí)例的全局節(jié)點(diǎn)，本文就通過(guò)代碼示例給大家講講Go單例模式,需要的朋友可以參考下

Go 單例模式講解和代碼示例.

單例是一種創(chuàng)建型設(shè)計(jì)模式，讓你能夠保證一個(gè)類只有一個(gè)實(shí)例，并提供一個(gè)訪問(wèn)該實(shí)例的全局節(jié)點(diǎn)。

單例擁有與全局變量相同的優(yōu)缺點(diǎn)。盡管它們非常有用，但卻會(huì)破壞代碼的模塊化特性。

在某些其他上下文中，你不能使用依賴于單例的類。你也將必須使用單例類。絕大多數(shù)情況下，該限制會(huì)在創(chuàng)建單元測(cè)試時(shí)出現(xiàn)。

概念示例

通常而言，單例實(shí)例會(huì)在結(jié)構(gòu)體首次初始化時(shí)創(chuàng)建。為了實(shí)現(xiàn)這一操作，我們?cè)诮Y(jié)構(gòu)體中定義一個(gè) getInstance獲取實(shí)例方法。該方法將負(fù)責(zé)創(chuàng)建和返回單例實(shí)例。創(chuàng)建后，每次調(diào)用 getInstance時(shí)都會(huì)返回相同的單例實(shí)例。

協(xié)程方面又有什么需要注意的嗎？每當(dāng)多個(gè)協(xié)程想要訪問(wèn)實(shí)例時(shí)，單例結(jié)構(gòu)體就必須返回相同的實(shí)例。正因如此，單例設(shè)計(jì)模式的實(shí)施工作很容易出錯(cuò)。下方的例子表示了創(chuàng)建單例的正確方式。

一些值得注意的地方：

最開(kāi)始時(shí)會(huì)有 nil檢查，確保 singleInstance單例實(shí)例在最開(kāi)始時(shí)為空。這是為了防止在每次調(diào)用 getInstance方法時(shí)都去執(zhí)行消耗巨大的鎖定操作。如果檢查不通過(guò)，則就意味著 singleInstance字段已被填充。
singleInstance結(jié)構(gòu)體將在鎖定期間創(chuàng)建。
在獲取到鎖后還會(huì)有另一個(gè) nil檢查。這是為了確保即便是有多個(gè)協(xié)程繞過(guò)了第一次檢查，也只能有一個(gè)可以創(chuàng)建單例實(shí)例。否則，所有協(xié)程都會(huì)創(chuàng)建自己的單例結(jié)構(gòu)體實(shí)例。

single.go: 單例

package main
import (
    "fmt"
    "sync"
)
var lock = &sync.Mutex{}
type single struct {
}
var singleInstance *single
func getInstance() *single {
    if singleInstance == nil {
        lock.Lock()
        defer lock.Unlock()
        if singleInstance == nil {
            fmt.Println("Creating single instance now.")
            singleInstance = &single{}
        } else {
            fmt.Println("Single instance already created.")
        }
    } else {
        fmt.Println("Single instance already created.")
    }
    return singleInstance
}

main.go: 客戶端代碼

package main
import (
    "fmt"
)
func main() {
    for i := 0; i < 30; i++ {
        go getInstance()
    }
    // Scanln is similar to Scan, but stops scanning at a newline and
    // after the final item there must be a newline or EOF.
    fmt.Scanln()
}

output.txt: 執(zhí)行結(jié)果

Creating single instance now.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.
Single instance already created.

另一個(gè)例子

init函數(shù)

我們可以在 init函數(shù)中創(chuàng)建單例實(shí)例。這僅適用于實(shí)例的早期初始化工作已經(jīng)確定時(shí)。 init函數(shù)僅會(huì)在包中的每個(gè)文件里調(diào)用一次，所以我們可以確定其只會(huì)創(chuàng)建一個(gè)實(shí)例。

sync.Once

sync.Once僅會(huì)執(zhí)行一次操作。可查看下面的代碼：

syncOnce.go: 單例

package main
import (
    "fmt"
    "sync"
)
var once sync.Once
type single struct {
}
var singleInstance *single
func getInstance() *single {
    if singleInstance == nil {
        once.Do(
            func() {
                fmt.Println("Creating single instance now.")
                singleInstance = &single{}
            })
    } else {
        fmt.Println("Single instance already created.")
    }
    return singleInstance
}

main.go: 客戶端代碼

package main
import (
    "fmt"
)
func main() {
    for i := 0; i < 30; i++ {
        go getInstance()
    }
    // Scanln is similar to Scan, but stops scanning at a newline and
    // after the final item there must be a newline or EOF.
    fmt.Scanln()
}