Java编译器可以重新排序synchronized语句以进行优化吗？ [英] Can the synchronized statements be re-ordered by Java compiler for optimization?

问题描述

可以重新排序同步语句。即：
可以：

Can the synchronization statements be reordered. i.e : Can :

synchronized(A) {
   synchronized(B) {
     ......
   }
}

成为：

synchronized(B) { 
    synchronized(A) { 
     ...... 
     }  
}

推荐答案

可以重新排序同步语句吗？

Can the synchronization statements be reordered?

我假设您在询问编译器是否可以重新排序 synchronized 阻止所以锁定顺序的顺序与代码不同。

I assume you are asking if the compiler can reorder the synchronized blocks so the lock order happens in a different order than the code.

答案是否定的。 synchronized 块（以及 volatile 字段访问权限）对编译器施加了排序限制。在您的情况下，您无法在另一个监视器输入之前移动监视器 - 输入，也不能在另一个监视器退出之后移动监视器退出。请参阅下面的网格。

The answer is no. A synchronized block (and a volatile field access) impose ordering restrictions on the compiler. In your case, you cannot move a monitor-enter before another monitor-enter nor a monitor-exit after another monitor-exit. See the grid below.

引用 JSR 133（Java内存模型）常见问题解答：

它例如，编译器无法在获取之前或发布之后移动代码。当我们说获取和释放对缓存有效时，我们使用速记来表示一些可能的效果。

It is not possible, for example, for the compiler to move your code before an acquire or after a release. When we say that acquires and releases act on caches, we are using shorthand for a number of possible effects.

Doug Lea的 JSR-133 Cookbook 有一个网格，显示重新排序的可能性。网格中的空白条目表示允许重新排序。在您的情况下，输入 synchronized 块是MonitorEnter（与加载 volatile 字段相同的重新排序限制）退出 synchronized 块是一个MonitorExit（与存储到 volatile 字段相同）。

Doug Lea's JSR-133 Cookbook has a grid which shows the reordering possibilities. A blank entry in the grid means that reordering is allowed. In your case, entering a synchronized block is a "MonitorEnter" (same reordering limitations as loading of a volatile field) and exiting a synchronized block is a "MonitorExit" (same as storing to a volatile field).

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