轻松写，这是什么意思？(Relaxed write, what does it mean?)

轻松写，这是什么意思？(Relaxed write, what does it mean?)

private static class Node<E> { volatile E item; volatile Node<E> next; /** * Constructs a new node. Uses relaxed write because item can * only be seen after publication via casNext. */ Node(E item) { UNSAFE.putObject(this, itemOffset, item); }

它来自java.util.concurrent.ConcurrentLinkedQueue.java

什么意思，轻松写？

private static class Node<E> { volatile E item; volatile Node<E> next; /** * Constructs a new node. Uses relaxed write because item can * only be seen after publication via casNext. */ Node(E item) { UNSAFE.putObject(this, itemOffset, item); }

It comes from java.util.concurrent.ConcurrentLinkedQueue.java

What does it mean, relaxed write?

最满意答案

这是一个很好的问题。 但理解这个术语的线索在于...... JDK 9，其中sun.misc.Unsafe已成为（或将成为 - 更适合说，见此处 ）公共API。

在JDK 9中，您引用的相应位置实现如下：

static final class Node<E> { volatile E item; volatile Node<E> next; /** * Constructs a node holding item. Uses relaxed write because * item can only be seen after piggy-backing publication via CAS. */ Node(E item) { ITEM.set(this, item); } ... }

其中ITEM是VarHandle类的实例，它实现与sun.misc.Unsafe类似的功能。 现在，我们可以查看此方法的JavaDoc描述并找到以下内容：

将变量的值设置为newValue，设置的内存语义就像变量被声明为非易失性和非最终变量一样。 通常称为普通写访问。

换句话说，我们可以得出结论， 宽松写入与普通写入访问相同。 换句话说，我相信迈克尔上面的评论是正确的：

...它就像volatile的对立面 - 一个不能保证在线程中看到的写入。

（参见相反的方法setVolatile ，就好像变量被声明为volatile ）。

It is a good question. But the clue to understanding this term lies in ... JDK 9, where sun.misc.Unsafe has become (or will become - what is more appropriate to say, see here) public API.

In JDK 9 the corresponding place, which you refers to, is implemented as follows:

static final class Node<E> { volatile E item; volatile Node<E> next; /** * Constructs a node holding item. Uses relaxed write because * item can only be seen after piggy-backing publication via CAS. */ Node(E item) { ITEM.set(this, item); } ... }

Where ITEM is instance of VarHandle class which implements similar functionality as sun.misc.Unsafe. Now, we can look at this method's JavaDoc description and find the following:

Sets the value of a variable to the newValue, with memory semantics of setting as if the variable was declared non-volatile and non-final. Commonly referred to as plain write access.

In other words, we can conclude, that relaxed write is the same as plain write access. In other words, I believe that Michael's commentary above is right:

... its like the opposite of volatile - a write that is not guaranteed to be seen across threads.

(see the opposite method setVolatile which works as if the variable was declared volatile).