[TOC] ## 示例 ### 概念示例 <details> <summary>main.go</summary> ``` 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 created1.") } } else { fmt.Println("Single instance already created2.") } return singleInstance } func main() { for i := 0; i < 5; i++ { go getInstance() } fmt.Scanln() } ``` </details> <br /> 輸出 ``` Creating single instance now. Single instance already created1. Single instance already created2. Single instance already created1. Single instance already created1. ``` ### 使用 init 函數 我們可以在`init`函數中創建單例實例。這僅適用于實例的早期初始化工作已經確定時。?`init`函數僅會在包中的每個文件里調用一次，所以我們可以確定其只會創建一個實例。 ### sync.Once [推薦] sync.Once僅會執行一次操作 <details> <summary>main.go</summary> ``` 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 } func main() { for i := 0; i < 5; i++ { go getInstance() } fmt.Scanln() } ``` </details> <br /> 輸出 ``` Creating single instance now. Single instance already created. Single instance already created. Single instance already created. ```