java - Java java.util.concurrent.locks.Lock 如何保证线程之间的内存同步？

Question

查看这个java代码片段:

 Lock l = ReentrantLock();
 l.lock();
 try {
     counter++;
 } finally {
     l.unlock();
 }

这段代码保证在特定时间内只有一个线程会执行try block 中的代码。

我的问题是:锁定/解锁机制如何保证线程之间的内存可见性？(确保计数器结果由线程 X 刷新到主内存中的机制是什么？ ，并且会被随后到来的线程Y从主存加载？)

Answer 1

http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5

17.4.5. Happens-before Order

Two actions can be ordered by a happens-before relationship. If one action happens-before another, then the first is visible to and ordered before the second.

...

It follows from the above definitions that:

An unlock on a monitor happens-before every subsequent lock on that monitor.

...

In a happens-before consistent set of actions, each read sees a write that it is allowed to see by the happens-before ordering.

看到暗示是从不同的线程看到。