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为什么Volatile变量不用于Atomicity(Why Volatile variable isn't used for Atomicity)

 来自Javadocs  
 
  使用volatile变量可以降低内存一致性错误的风险，因为任何对volatile变量的写入都会与随后的同一个变量的读取之间建立一个happen-before关系。 这意味着对volatile变量的更改始终对其他线程可见。  
 
 对任何其他线程总是可见时，对volatile变量所做的更改，那么为什么在多线程写入该变量的情况下会使用volatile变量。 为什么volatile仅用于一个线程正在写入或正在读取的情况，而另一个线程正在读取该变量？  
 如果其他线程始终可以看到更改，那么假设线程B想要写入该变量，它将看到新值（由线程A更新）并更新它。 当线程A再次想要写入时，它会再次看到线程B的更新值并写入它。问题在哪里？  
 总之，我无法理解这一点。  
 
  如果两个线程都读取和写入共享变量，那么使用volatile关键字是不够的。 在这种情况下，您需要使用同步来确保变量的读写是原子的。  

From Javadocs 
 
 Using volatile variables reduces the risk of memory consistency errors, because any write to a volatile variable establishes a happens-before relationship with subsequent reads of that same variable. This means that changes to a volatile variable are always visible to other threads. 
 
When changes made to a volatile variable are always visible to any other thread, then why volatile variable cant be used in case of multiple threads writing to that variable. Why is volatile only used for cases when one thread is writing or reading to that while the other thread is only reading the variable? 
If changes are always visible to other threads, then suppose if thread B wants to write to that variable, it will see the new value(updated by thread A) and update it. And when the thread A again wants to write, it will again see the updated value by thread B and write to it.Where is the problem in that? 
In short, i am not able to understand this. 
 
 if two threads are both reading and writing to a shared variable, then using the volatile keyword for that is not enough. You need to use synchronization in that case to guarantee that the reading and writing of the variable is atomic. 

原文：https://stackoverflow.com/questions/30430128

更新时间：2023-06-27 22:06

最满意答案

 HTML中的空格编码仅仅是一个空格。 nbsp可能看起来像一个空间，但具有不同的语义，“不中断”意味着换行符被抑制。  
 解决方案：每当我发现功能缺失或出现意外行为时（例如，asp：Label和HyperLink不进行HTML编码），我会写我自己的实用程序类，就像我说的那样;）  
public class MyHtml
{
    public static string Encode(string s)
    {
        if (string.IsNullOrEmpty(s) || s.Trim()=="")
            return "& nbsp;";
        return Html.Encode(s);
    }
}

A space encode in HTML is just a space. nbsp may look like a space, but has a different semantics, "non-breaking" meaning that line breaks are suppressed. 
Solution: Whenever I find functionality lacking or with unexpected behavior (e.g. asp:Label and HyperLink don't HTML encode), I write my own utilities class which does as I say ;) 
public class MyHtml
{
    public static string Encode(string s)
    {
        if (string.IsNullOrEmpty(s) || s.Trim()=="")
            return "& nbsp;";
        return Html.Encode(s);
    }
}

为什么Volatile变量不用于Atomicity(Why Volatile variable isn't used for Atomicity)

最满意答案

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