Golang 单例模式的几种形式

1.非线程安全，虽然说是单例模式，但是如果实例正在创建中，此时多个线程同时访问，就会获得多个结果

package main

var instance *Singleton

type Singleton struct {
}

func Instance() *Singleton{
    if instance == nil {
        instance = &Singleton{}
    }
    return instance
}

2. 加锁单例模式

package main

import "sync"

var (
    instance *Singleton
    mu sync.Mutex
)

type Singleton struct {
}

func Instance() *Singleton{
    mu.Lock()
    defer mu.Unlock()

    if instance == nil {
        instance = &Singleton{}
    }
    return instance
}

以下是参考他人的方法。思路很好
3.避免了每次加锁，提高了代码效率

package main

import "sync"

var (
    instance *Singleton
    mu       sync.Mutex
)

type Singleton struct {
}

func Instance() *Singleton {

    if instance == nil {
        mu.Lock()
        defer mu.Unlock()

        if instance == nil {
            instance = &Singleton{}
        }
    }
    return instance
}

4.once实现

package main

import "sync"

var (
    instance *Singleton
    once sync.Once
)

type Singleton struct {
}

func Instance() *Singleton {

    once.Do(func() {
        instance = &Singleton{}
    })
    
    return instance
}