通常而言， 单例实例会在结构体首次初始化时创建。 为了实现这一操作， 我们在结构体中定义一个 get Instance获取实例方法。 该方法将负责创建和返回单例实例。 创建后， 每次调用 get Instance时都会返回相同的单例实例。
协程方面又有什么需要注意的吗？ 每当多个协程想要访问实例时， 单例结构体就必须返回相同的实例。 正因如此， 单例设计模式的实施工作很容易出错。 下方的例子表示了创建单例的正确方式。
一些值得注意的地方：
- 最开始时会有nil检查， 确保single Instance单例实例在最开始时为空。 这是为了防止在每次调用get Instance方法时都去执行消耗巨大的锁定操作。 如果检查不通过， 则就意味着single Instance字段已被填充。
- 获取single Instance结构体将在锁定期间创建。
- 在获取到锁后还会有另一个nil检查。 这是为了确保即便是有多个协程绕过了第一次检查， 也只能有一个可以创建单例实例。 否则， 所有协程都会创建自己的单例结构体实例
single.go: 单例
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: 客户端代码
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: 执行结果
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.
