[Golang] 初探之 sync.Once

Once 官方描述 Once is an object that will perform exactly one action,即 Once 是一个对象,它提供了保证某个动作只被执行一次功能,最典型的场景就是单例模式。

[Golang] 初探之 sync.Once

sync.Once 是 Golang package 中使方法只执行一次的对象实现,作用与 init 函数类似。但也有所不同。

  • init 函数是在文件包首次被加载的时候执行,且只执行一次
  • sync.Onc 是在代码运行中需要的时候执行,且只执行一次

当一个函数不希望程序在一开始的时候就被执行的时候,我们可以使用 sync.Once 。


package main

import (

func main() {
    var once sync.Once
    onceBody := func() {
        fmt.Println("Only once")
    done := make(chan bool)
    for i := 0; i < 10; i++ {
        go func() {
            done <- true
    for i := 0; i < 10; i++ {

# Output:
Only once

sync.Once 使用变量 done 来记录函数的执行状态,使用 sync.Mutex 和 sync.atomic 来保证线程安全的读取 done 。


// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

import (

// Once is an object that will perform exactly one action.
type Once struct {
    // done indicates whether the action has been performed.
    // It is first in the struct because it is used in the hot path.
    // The hot path is inlined at every call site.
    // Placing done first allows more compact instructions on some architectures (amd64/x86),
    // and fewer instructions (to calculate offset) on other architectures.
    done uint32
    m    Mutex

// Do calls the function f if and only if Do is being called for the
// first time for this instance of Once. In other words, given
//     var once Once
// if once.Do(f) is called multiple times, only the first call will invoke f,
// even if f has a different value in each invocation. A new instance of
// Once is required for each function to execute.
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
//     config.once.Do(func() { config.init(filename) })
// Because no call to Do returns until the one call to f returns, if f causes
// Do to be called, it will deadlock.
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
func (o *Once) Do(f func()) {
    // Note: Here is an incorrect implementation of Do:
    //    if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    //        f()
    //    }
    // Do guarantees that when it returns, f has finished.
    // This implementation would not implement that guarantee:
    // given two simultaneous calls, the winner of the cas would
    // call f, and the second would return immediately, without
    // waiting for the first's call to f to complete.
    // This is why the slow path falls back to a mutex, and why
    // the atomic.StoreUint32 must be delayed until after f returns.

    if atomic.LoadUint32(&o.done) == 0 {
        // Outlined slow-path to allow inlining of the fast-path.

func (o *Once) doSlow(f func()) {
    defer o.m.Unlock()
    if o.done == 0 {
        defer atomic.StoreUint32(&o.done, 1)


posted @ 2020-06-16 10:26  landv  阅读(678)  评论(0编辑  收藏  举报