Java Unsafe.storeFence()文档错了吗？

Question

Java 8增加了三个围栏sun.misc.Unsafe.

看完他们的文档后我感到很困惑.

所以,我在网上搜索,发现了这个链接.

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,尽管技术上这些fences比volatile变量更强.所以loadFence()是一个获取栅栏,而storeFence()是一个释放栅栏,而fullFence()是一个完整的栅栏.

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();

这看起来不像是一个释放围栏.它应该如何使用？不应该

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

我之前和以后意味着先前的意思.

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"我们不会在通常的开发中使用它们".

我的意思是,即使没有这些方法,我们也可以获得这些"围栏".如果我是正确的,实际上,读取一个虚拟的volatile会产生loadFence()的效果,而写一个虚拟的volatile会产生fullFence()的效果,而unsafe.putOrderedXXX()(或者AtomicInteger.lazySet())会产生效果. storeFence().

它们可能有细微差别,但在目前的实施中,它们是可交换的.(链接暗示似乎)

这就是我所说的"他们没有添加任何新东西".

另一个编辑

这已经修复了.

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

谢谢@ john-vint

Answer 1

在JDK9中实际上存在差异.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();