Java Unsafe.storeFence() 文档错误？

Question

Java 8 为 sun.misc.Unsafe 添加了三个栅栏。

在阅读他们的文档后，我感到很困惑。

所以，我在网上搜索，找到了这个 link .

根据上面的页面，我相信这些方法在实践中几乎没有增加任何内容。如果我错了请纠正我，粗略地说，loadFence()、storeFence() 和 fullFence() 分别对应于 volatile read、lazy write 和 volatile write，尽管从技术上讲这些栅栏比 volatile 变量更强。所以loadFence()是acquire fence，storeFence()是release fence，fullFence()是full fence。

但是 storeFence() 的文档看起来很奇怪。

它说，

/**
 * Ensures lack of reordering of stores before the fence
 * with loads or stores after the fence.
 */
void storeFence();

这看起来不像是释放栅栏。应该如何使用？不应该吗

/**
 * Ensures lack of reordering of loads or stores before the fence
 * with stores after the fence.
 */
void storeFence();

我假设 before 表示较早，after 表示较晚。

编辑

当我说这些“围栏在实践中没有添加任何东西”时，我并不是说“我们不在通常的开发中使用它们”。

我的意思是，即使在 Unsafe 中没有这些方法，我们也可以获得这些“栅栏”。如果我是正确的，在实践中，读取虚拟 volatile 具有 loadFence() 的效果，写入虚拟 volatile 具有 fullFence() 的效果，而 unsafe.putOrderedXXX()(或 AtomicInteger.lazySet())具有效果storeFence() 的。

它们可能有细微的差别，但在当前的实现中，它们是可以互换的。 (似乎由链接暗示)

这就是我所说的“他们没有添加任何新内容”的意思。

另一个编辑

这已经修复了。

参见 https://bugs.openjdk.java.net/browse/JDK-8038978

谢谢@john-vint

Answer 1

在 JDK9 中实际上有一个 diff。提出并澄清了类似的问题:

http://hg.openjdk.java.net/jdk9/jdk9/jdk/rev/84e19392365e

      /**
!      * Ensures that loads before the fence will not be reordered with loads and
!      * stores after the fence; a "LoadLoad plus LoadStore barrier".
!      *
!      * Corresponds to C11 atomic_thread_fence(memory_order_acquire)
!      * (an "acquire fence").
!      *
!      * A pure LoadLoad fence is not provided, since the addition of LoadStore
!      * is almost always desired, and most current hardware instructions that
!      * provide a LoadLoad barrier also provide a LoadStore barrier for free.
       * @since 1.8
       */
      public native void loadFence();

      /**
!      * Ensures that loads and stores before the fence will not be reordered with
!      * stores after the fence; a "StoreStore plus LoadStore barrier".
!      *
!      * Corresponds to C11 atomic_thread_fence(memory_order_release)
!      * (a "release fence").
!      *
!      * A pure StoreStore fence is not provided, since the addition of LoadStore
!      * is almost always desired, and most current hardware instructions that
!      * provide a StoreStore barrier also provide a LoadStore barrier for free.
       * @since 1.8
       */
      public native void storeFence();

      /**
!      * Ensures that loads and stores before the fence will not be reordered
!      * with loads and stores after the fence.  Implies the effects of both
!      * loadFence() and storeFence(), and in addition, the effect of a StoreLoad
!      * barrier.
!      *
!      * Corresponds to C11 atomic_thread_fence(memory_order_seq_cst).
       * @since 1.8
       */
      public native void fullFence();