GoLang设计模式04 - Golang教程网

单例模式恐怕是最为人熟知的一种设计模式了。它同样也是创建型模式的一种。当某个struct只允许有一个实例的时候，我们会用到这种设计模式。这个struct的唯一的实例被称为单例对象。下面是需要创建单例对象的一些场景：

数据库实例：一般在开发中，对于一个应用，我们通常只需要一个数据库对象实例
日志实例：同样，对于一个应用来说，日志操作对象也只需要一个实例

getInstance()

getInstance()

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("Creting Single Instance Now")

            singleInstance = &single{}

        } else {

            fmt.Println("Single Instance already created-1")

        }

    } else {

        fmt.Println("Single Instance already created-2")

    }

    return singleInstance

}

single

getInstance()singleInstancegetInstance()singleInstancesingleInstancesingle

完整代码在这里：

single.go

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-1")

		}

	} else {

		fmt.Println("Single Instance already created-2")

	}

	return singleInstance

}

　　main.go

import (

	"fmt"

)

func main() {

	for i := 0; i < 100; 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()

}

　　输出内容：

Creating Single Instance Now

Single Instance already created-1

Single Instance already created-2

Single Instance already created-1

Single Instance already created-1

Single Instance already created-1

Single Instance already created-1

Single Instance already created-1

Single Instance already created-1

Single Instance already created-2

Single Instance already created-2

...

简单说明下：

singlesingleInstance

除了锁+二次校验的方式，还有其它创建单例的方法，我们来看一下：

基于init()函数

init()init()

import (

	"fmt"

	"log"

)

type single struct {

}

var singleInstance *single

func init() {

	fmt.Println("Creating Single Instance Now")

	singleInstance = &single{}

}

func GetInstance() *single {

	if singleInstance == nil {

		log.Fatal("Single Instance is nil")

	} else {

		fmt.Println("Single Instance already created-2")

	}

	return singleInstance

}

这应该就是go语言中的懒汉式单例创建方法了。如果不介意过早创建实例造成的资源占用，推荐使用这种方法创建单例。

通过sync.Once

sync.Once

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{}

			})

		fmt.Println("Single Instance already created-1")

	} else {

		fmt.Println("Single Instance already created-2")

	}

	return singleInstance

}

相比二次校验的方式，这里的代码可以说非常简洁了。这也是我非常建议使用的一种单例创建方式。

输出内容为：

Creating Single Instance Now

Single Instance already created-1

Single Instance already created-2

Single Instance already created-2

Single Instance already created-1

Single Instance already created-2

Single Instance already created-1

Single Instance already created-1

Single Instance already created-2

Single Instance already created-1

Single Instance already created-2

Single Instance already created-1

Single Instance already created-2

Single Instance already created-2

Single Instance already created-2

Single Instance already created-2

Single Instance already created-2

Single Instance already created-2

Single Instance already created-2

Single Instance already created-2

...

简单说明下：　

输出的内容和二次校验的方式差不多，仍然存在多行“Single Instance already created-1”这样的输出，说明有多个goroutine通过了if校验
输出内容中只有一行“Creating Single Instance Now”，说明只有一个goroutine能够创建实例。

End！