共享变量的陈旧值 [英] Stale value of Shared variable

问题描述

在实践中阅读并发时，我读到了

否可见性证明了
同步程序的不足之处导致令人惊讶的结果：陈旧的数据。当
读者线程检查准备就绪时，它可能会看到过时的值。除非每次访问变量时都使用
同步，否则可能会看到
该变量的陈旧值。更糟糕的是，过期是
并非全是或全无：一个线程可以看到一个变量
的最新值，但是第一个写入的另一个变量的陈旧值

  public class NoVisibility {
 private static boolean ready; 
 private static int number; 
私有静态类ReaderThread扩展线程{
 public void run（）{
 while（！ready）
 Thread.yield（）; 
 System.out.println（number）; 
} 
} 
 public static void main（String [] args）{
 new ReaderThread（）。start（）; 
 number = 42; 
 ready = true; 
} 
} 
  

我没有得到陈旧的含义。两个线程共享相同的引用，如何修改一个值永远不会被其他线程看到？

解决方案

它的主要意图是在同一时间设置number = 42和ready = true，但由于它们是按照这个特定的顺序调用的，因此你的ReaderThread存在竞争条件，它可能（可能会）打印出这个数字即使我们真的不想要它。

他们希望你同步这两个被设置在一起的变量。

While reading Concurrency in practice , i read that

No Visibility demonstrate done of the ways that insufficiently synchronized programs can cause surprising results:stale data.When the reader thread examines ready, it may see an out of date value. Unless synchronization isused every time a variable is accessed, it is possible to see a stale value for that variable. Worse, staleness is not all or nothing:a thread can see an upto date value of one variable but a stale value of another variable that was written first

public class NoVisibility {
    private static boolean ready;
    private static int number;
    private static class ReaderThread extends Thread {
    public void run() {
    while (!ready)
    Thread.yield();
    System.out.println(number);
    }
    }
    public static void main(String[] args) {
    new ReaderThread().start();
    number = 42;
    ready = true;
    }
    }

I am not getting meaning of stale. Both threads are sharing the same reference , how can value modified by one will never be or not be visible to other thread?

解决方案

It's saying that in main the intention is to set number=42 and ready=true at the same time, but since they were called in this particular order there is a race condition with your ReaderThread where it could (probably will) print out the number even though we really don't want it to.

They would like you to synchronize those 2 variables being set together.

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