一、简介

       once 方法用于保证指定函数只执行一次。例如配置懒加载，客户端获取密钥等场景，都可以用到once。

二、技术实现

2.1 Once.go

type Once struct {done atomic.Uint32m    Mutex
}func (o *Once) Do(f func()) {if o.done.Load() == 0 {o.doSlow(f)}
}func (o *Once) doSlow(f func()) {o.m.Lock()defer o.m.Unlock()if o.done.Load() == 0 {defer o.done.Store(1)f()}
}

type DemoClient struct {secretKey stringonce      sync.Once
}func (c *DemoClient) getSecretKey() string {c.once.Do(func() {c.secretKey = string(rand.Int63())})return c.secretKey
}

2.2 Once.java

public class Once {private volatile  boolean done = false;private Runnable func;public Once(Runnable func) {this.func = func;}public void exec(){if (done){return;}synchronized (this){if (!done){func.run();done = true;}}}
}

/**
* 使用样例
*/
public class DemoClient {private String secretKey;private Once once = new Once(()->{secretKey = RandUtil.randAlpha(32);});public String getSecretKey(){once.exec();return secretKey;}
}