Java volatile并发 [英] Java volatile for concurrency

问题描述

好，所以我只读了这个问题您使用过吗Java中的volatile关键字？，我开始使用volatile变量来停止循环。我也看到了这个参考， http://www.javamex.com/tutorials/synchronization_volatile。 shtml 。现在，文章说易失性变量是非阻塞的。它还说，它不能用于读取-更新-写入序列中的并发。这是有道理的，因为它们不受阻碍。

Ok so I just read this question Do you ever use the volatile keyword in Java?, and I get using a volatile variable in order to stop a loop. Also I've seen this reference, http://www.javamex.com/tutorials/synchronization_volatile.shtml. Now the article says that volatile variables are non-blocking. Also it says that it cannot be used for concurrency in a read-update-write sequence. Which makes sense because they're non-blocking.

由于从不缓存volatile变量，仅使用同步来停止循环（从较早的链接开始）是否更快？

Since volatile variables are never cached is it faster to simply use synchronization to stop the loop (from the earlier link)?

编辑：使用同步解决方案

Using a synchronized solution

public class A{
  private boolean test;

  public A(){
    test = true;
  }

  public synchronized void stop(){
    test = false;
  }

  public synchronized boolean isTrue(){
    return test;
  }
}

public class B extends Thread {
  private A a;

  public B(A refA){
    a = refA;
  }

  public void run(){
    //Does stuff here
    try{
      sleep(1000);
    }
    catch(Exception e){}
    a.stop();
  }

  public static void main(String [] args){
    A TestA = new A();
    B TestB = new B(TestA);
    TestB.start();
    while(TestA.isTrue()){
      //stay in loop
      System.out.println("still in loop");
    }
    System.out.println("Done with loop");
  }
 }

推荐答案

否，读取 volatile 变量比读取 synchronized的非 volatile 变量要快。 块。

No, reading a volatile variable is faster than than reading an non-volatile variable in a synchronized block.

A 已同步块清除条目上的缓存值，这是相同的读取 volatile 变量。 但是，还会在退出同步块时将所有缓存的写操作刷新到主内存中，这在读取 volatile 变量时不是必需的。

A synchronized block clears the cached values on entry which is the same as reading a volatile variable. But, it also flushes any cached writes to main memory when the synchronized block is exited, which isn't necessary when reading volatile variable.

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