为什么在这个双检查锁的例子中使用volatile？

本文是小编为大家收集整理的关于为什么在这个双检查锁的例子中使用volatile？的处理/解决方法，可以参考本文帮助大家快速定位并解决问题，中文翻译不准确的可切换到English标签页查看源文。

中文
English

问题描述

从 Head First 设计模式一书中，具有双重检查锁定的单例模式已实现如下:

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

我不明白为什么要使用 volatile.volatile 的使用不会破坏使用双重检查锁定的目的，即性能吗?

推荐答案

了解为什么需要 volatile 的好资源来自 JCIP 书.Wikipedia 对此材料也有体面的解释.

真正的问题是 Thread A 可能在 instance 完成构造 instance 之前为其分配内存空间.Thread B 将看到该分配并尝试使用它.这会导致 Thread B 失败，因为它使用的是部分构造的 instance 版本.

本文地址：https://www.itbaoku.cn/post/627464.html

百度翻译此文有道翻译此文

问题描述

From Head First design patterns book, the singleton pattern with double checked locking has been implemented as below:

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

I don't understand why volatile is being used. Doesn't volatile usage defeat the purpose of using double checked locking i.e performance?

推荐答案

A good resource for understanding why volatile is needed comes from the JCIP book. Wikipedia has a decent explanation of that material as well.

The real problem is that Thread A may assign a memory space for instance before it is finished constructing instance. Thread B will see that assignment and try to use it. This results in Thread B failing because it is using a partially constructed version of instance.

上一篇：我们能否提高这种面向密钥的访问保护模式的可重用性？

下一篇：消除代码中开关的方法