参考类型的易失性 - 它总是避免由于JMM发布参考问题？ [英] Volatile for reference type - Does it always avoid publication of references issues due to JMM?

问题描述

假设这个类：

  public class AmIThreadSafe {
 
 private int a; 
 private int b; 
 
 AmIThreadSafe（int a，int b）{
 this.a = a; 
 this.b = b; 
} 
} 
  

假设实例引用此类（声明为< 此（引用）转义时，某些线程可以访问（导致竞争条件）：code> volatile p>

  volatile AmIThreadSafe instance = new AmIThreadSafe（1,2）; 
  

在这里，我确定分配实例



但是关于 AmIThreadSafe的 c> fields？

外部 volatile 关键字也意味着发生之前关系 a 和 b 字段？
或者是可能的任何线程看到过时的值（默认值 0 在这种情况下，因为 int ）

换句话说，我应该声明 a b final 或 volatile 或者只是在实例的引用上指示 volatile ？

< ----- 更新后 - 好的答案： ----------------------------

以下文章通过其样例证实了在我的情况下， a 和 b 受到JMM优化的保护，防止永久 发生之前 关系。

http://jeremymanson.blogspot.fr/2008/11/what-volatile-means-in-java.html

解决方案

将实例声明为 volatile 不会使其字段 volatile ，但如果我正确理解你的问题，然后—是的，在你的情况下就够了。

每§ 17.4.5的规范：

• a volatile 写入一个线程 happens-before 任何后续 volatile 线程。


• 同一个线程中的语句具有您期望的 happens-before 关系。


• > 关系是可传递的。

因此，如果线程感知到 code>作为已经初始化，然后实例 发生之前的的初始化，以及实例的字段 happens-before ，因此线程会将实例的字段视为已初始化。

Assuming this class:

public class AmIThreadSafe {

    private int a;
    private int b;

    AmIThreadSafe(int a, int b) {
        this.a = a;
        this.b = b;
    }
}

Assuming that instance's reference to this class (declared as volatile) is accessible by some threads (leading to race condition) as soon as the this(reference) escapes:

volatile AmIThreadSafe instance = new AmIThreadSafe(1,2);

Here, I'm sure that the fact of assigning instance reference happens-before reading by threads.

But what about the AmIThreadSafe's fields?

Does the external volatile keyword also imply an happens-before relation concerning a and b fields? Or is it rather possible to end up with any thread seeing stale values (default values 0 in this case since int) due to a potential statements reordering during constructor?

In other word, should I declare a and b final or volatile to prevent any surprises with the JMM or is just indicating volatile on the instance's reference enough?

----------------UPDATED POST - A GOOD ANSWER:----------------------------

The following article confirms by its sample that in my case, a and b are protected from JMM optimisations that prevent a permanent happens-before relation.

http://jeremymanson.blogspot.fr/2008/11/what-volatile-means-in-java.html

解决方案

Declaring instance as volatile does not make its fields volatile, but if I understand your question correctly, then — yes, it's enough in your case.

Per §17.4.5 of the spec:

• a volatile write in one thread happens-before any subsequent volatile read in another thread.
• statements within the same thread have the happens-before relationship that you'd expect.
• happens-before relationships are transitive.

So, if a thread perceives instance as having been initialized, then the initialization of instance happened-before it, and the initialization of instance's fields happened-before that, so the thread will perceive instance's fields as having been initialized.

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