sync.Once.Do(f func())是一个挺有趣的东西,能保证once只执行一次,无论你是否更换once.Do(xx)这里的方法,这个sync.Once块只会执行一次
与sync.WaitGroup类型一样,sync.Once类型也属于结构体类型,同样也是开箱即用和并发安全的。由于这
个类型中包含了一个sync.Mutex类型的字段,所以,复制该类型的值也会导致功能的失效。
Once类型的Do方法只接受一个参数,这个参数的类型必须是func(),即:无参数声明和结果声明的函数。
package main
import (
"fmt"
"sync"
"time"
)
func onces() {
fmt.Println("onces")
}
func onced() {
fmt.Println("onced")
}
var once sync.Once
func main() {
for i, v := range make([]string, 10) {
once.Do(onces)
fmt.Println("count: ", v, "---", i)
}
for i := 0; i < 10; i++ {
go func() {
once.Do(onced)
fmt.Println("213")
}()
}
time.Sleep(4000)
}
输出
onces
count: --- 0
count: --- 1
count: --- 2
count: --- 3
count: --- 4
count: --- 5
count: --- 6
count: --- 7
count: --- 8
count: --- 9
213
光会用是不够的,再来看下sync.once的实现原理
源码:
// Once is an object that will perform exactly one action.
type Once struct {
m Mutex
done uint32
}
.
.
.
func (o *Once) Do(f func()) {
if atomic.LoadUint32(&o.done) == 1 {
return
}
o.m.Lock()
defer o.m.Unlock()
if o.done == 0 {
defer atomic.StoreUint32(&o.done, 1)
f()
}
}