代码实现

package main

import (
	"fmt"
	"sync"
)

var once sync.Once
var lock = &sync.Mutex{}

type single struct {
}

var singleInstance *single
var singleInstance2 *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
}

// 利用 once.do 实现
func getInstanceBySyncOnce() *single {
	if singleInstance2 == nil {
		once.Do(func() {
			fmt.Println("Creating single instance2 now")
			singleInstance2 = &single{}
		})
	} else {
		fmt.Println("Single instance2 already created.")
	}

	return singleInstance2
}

func main() {
	getInstance()
	getInstance()
	getInstance()

	getInstanceBySyncOnce()
	getInstanceBySyncOnce()
	getInstanceBySyncOnce()
}