golang中的sync.Once

golang中的sync.Once可以让代码只执行一次，比如：

func do() {
   fmt.Println(2)
}
func main() {
   a := sync.Once{}
   for i := 0; i < 100; i++ {
      go a.Do(do)
   }
}

输出为：

GOROOT=E:\go #gosetup
GOPATH=E:\go_program;E:\go的环境变量 #gosetup
E:\go\bin\go.exe build -o C:\Windows\Temp\GoLand\___go_build_gee_123.exe gee/123 #gosetup
C:\Windows\Temp\GoLand\___go_build_gee_123.exe
2

Process finished with the exit code 0