Go并发——Once

// 多次调用仅执行一次指定的函数 f
func (o *Once) Do(f func())

例的fooOnce函数只执行一次打印。

func fooOnce(){
    fmt.Println("只会执行一次")
}
func main() {
    var once sync.Once
    done := make(chan int)
    for i := 0; i < 10; i++ {
        go func(index int) {
            once.Do(fooOnce)
            done <- index
        }(i)
    }
    for i := 0; i < 10; i++ {
        fmt.Println("接手到的数据",<-done)
    }
}

单例模式

package main
import (
    "fmt"
    "sync"
)
type Singleton struct {
}
var singleInstance *Singleton
var once sync.Once
func GetSingletonObj() *Singleton {
    once.Do(func() {
        fmt.Println("Create object")
        singleInstance = new(Singleton)
    })
    return singleInstance
}
func main() {
    var wg sync.WaitGroup
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go func() {
            obj := GetSingletonObj()
            fmt.Printf("%p\n", obj)
            wg.Done()
        }()
    }
    wg.Wait()
}

源码

package sync
import (
    "sync/atomic"
)
// 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.
        o.doSlow(f)
    }
}
func (o *Once) doSlow(f func()) {
    o.m.Lock()
    defer o.m.Unlock()
    if o.done == 0 {
        defer atomic.StoreUint32(&o.done, 1)
        f()
    }
}