用Java如何实现同步块？

Question

在某些时候线程将争夺监视器，在这一点上应该有一个线程获胜，Java是否使用内置在CPU中的原子CAS操作来实现对这些监视器的获取，如果不行的话？

Answer 1

I don't think so since in the concurrent packages you can find the Atomic* classes which use CAS internally.

Another thing is that it depends on what kind of jvm you use. So in its current form your question is not really answerable apart from telling you that CAS is used elswhere.

Answer 2

CAS is what makes all concurrency work at the hardware level. If you want to change one value in memory across all threads, CAS is the fastest way to do it; any other technique is going to use CAS also. So for quick changes, CAS is the way to go. But if you have 100, or even 5 values to change, you're likely better off using synchronization. It'll do one CAS to lock the monitor and another to unlock it, but the rest is normal memory reads and writes which are much faster than CAS. Of course, you do have the monitor locked, which may hang other threads, slowing your program and possibly wasting CPU.

A bigger concern is that in Java any CAS (or reading/writing volatiles and synching/unsynching) is accompanied by bringing other threads' views of memory up-to-date. When you write a volatile, the threads that read it see all the memory changes made by writing the thread. This involves dumping register values to memory, flushing caches, updating caches, and putting data back into registers. But these costs parallel CAS, so if you've got one figured out, you've got the other figured out too.

The basic idea, I think, from the programmers' point of view, is to use volatile or atomic operations for single reads and writes and synchronization for multiples-- if there's no other compelling reason to chose one over the other.