双重检查锁定文章

2022-01-22 00:00:00 synchronization locking java double-checked-locking

我正在阅读关于Double-已检查锁定"并且超出了本文的主要主题，我想知道为什么在文章的某些地方作者使用了下一个成语:

I was reading this article about "Double-Checked locking" and out of the main topic of the article I was wondering why at some point of the article the author uses the next Idiom:

清单 7. 尝试解决乱序写入问题

Listing 7. Attempting to solve the out-of-order write problem 

public static Singleton getInstance()  
{
    if (instance == null)
    {
        synchronized(Singleton.class) {      //1
            Singleton inst = instance;         //2
            if (inst == null)
            {
                synchronized(Singleton.class) {  //3
                    inst = new Singleton();        //4
                }
                instance = inst;                 //5
            }
        }
    }
    return instance;
}

我的问题是:有什么理由用同一个锁同步两次代码吗?这有什么目的吗?

And my question is: Is there any reason to synchronize twice some code with the same lock? Have this any purpose it?

非常感谢.

推荐答案

两次锁定的目的是尝试防止乱序写入.内存模型指定了可能发生重新排序的位置，部分是根据锁.锁确保在instance = inst;"之后不会出现任何写入(包括单例构造函数中的任何写入).行.

The point of locking twice was to attempt to prevent out-of-order writes. The memory model specifies where reorderings can occur, partly in terms of locks. The lock ensures that no writes (including any within the singleton constructor) appear to happen after the "instance = inst;" line.

但是，要深入了解该主题，我建议 Bill Pugh 的文章.然后永远不要尝试它:)

However, to go deeper into the subject I'd recommend Bill Pugh's article. And then never attempt it :)

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