延迟加载,只在需要的时候初始化变量,同时需要防止重复的初始化变量,这可以节省不必要的变量初始化时间和空间(如果程序根本用不到此变量,就不会初始化它)

以下代码来自 Gin

import (
    "html/template"
    "net/http"
    "sync"

    "github.com/gin-gonic/gin"
)

// 定义一个 sync.Once 变量
var once sync.Once
var internalEngine *gin.Engine

func engine() *gin.Engine {

    //执行,并且只执行一次初始化操作
    once.Do(func() {
        internalEngine = gin.Default()
    })
    return internalEngine
}

// 后续其它要用到变量 internalEngine 的地方,都通过 engine() 方法来间接访问
func LoadHTMLGlob(pattern string) {
    engine().LoadHTMLGlob(pattern)
}

// 后续其它要用到变量 internalEngine 的地方,都通过 engine() 方法来间接访问
func LoadHTMLFiles(files ...string) {
    engine().LoadHTMLFiles(files...)
}

此方案的原理,是 sync.Once 里有一个原子性的操作,里面有个变量用来原子性的记录操作次数,
如果从未调用过,此变量为0,如果调用过一次,将其设置为1
下次再调用,因为变量是1了,下次调用将被忽略
此内部变量的操作是原子性的
以下是 sync.Once 的实现代码:


// Once is an object that will perform exactly one action.
//
// A Once must not be copied after first use.
//
// In the terminology of the Go memory model,
// the return from f “synchronizes before”
// the return from any call of once.Do(f).
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 (amd64/386),
    // 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()
    }
}

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