golang使用sync.Once实现懒加载的用法和坑点详解

public class Singleton {
    private volatile static Singleton uniqueSingleton;
 
    private Singleton() {
    }
 
    public Singleton getInstance() {
        if (null == uniqueSingleton) {
            synchronized (Singleton.class) {
                if (null == uniqueSingleton) {
                    uniqueSingleton = new Singleton();
                }
            }
        }
        return uniqueSingleton;
    }
}

type User struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

var user *User
var userOnce sync.Once

func initUser() {
    user = &User{}
    cfgStr := `{"name":"foobar","age":18}`
    if err := json.Unmarshal([]byte(cfgStr), user); err != nil {
        panic("load user err: " + err.Error())
    }
}

func getUser() *User {
    userOnce.Do(initUser)
    return user
}

func TestSyncOnce(t *testing.T) {
    var wg sync.WaitGroup
    for i := 1; i < 1000; i++ {
        wg.Add(1)
        go func(n int) {
            defer wg.Done()
            curUser := getUser()
            t.Logf("[%d] got user: %+v", n, curUser)
        }(i)
    }
    wg.Wait()
}

func (o *Once) Do(f func()) {
	if atomic.LoadUint32(&o.done) == 0 { // 如果是1表示已经完成了，跳过
		o.doSlow(f)
	}
}

func (o *Once) doSlow(f func()) {
	o.m.Lock() // 只有1个goroutine能拿到锁，其它的等待
	defer o.m.Unlock()
	if o.done == 0 { // 如果还是0表示第一个来的，不是0就表示已经有goroutine做完了
		defer atomic.StoreUint32(&o.done, 1)
		f()
	}
}

func initUser() {
    user = &User{} // 先给一个zero-value实例
    cfgStr := `{"name":"foobar","age":18}` // 然后加载json内容，完成初始化
    if err := json.Unmarshal([]byte(cfgStr), user); err != nil {
        panic("load user err: " + err.Error())
    }
}

func getUser() *User {
    if user == nil {
        userOnce.Do(initUser)
    }
    return user
}