java - Lock 是否保证 happens-before 关系？

Question

我对 Java 中的代码重新排序和竞争条件有疑问。

假设我有以下代码，有 2 个或更多线程同时执行 workForThread():

public class Job {
   private Lock lock = new ReentrantLock();
   private int sharedObject = 1;
   public void workForThread() {
       lock.lock();
       try {
           sharedObject++;
       } finally {
           lock.unlock();
       }
   }
}

JVM 是否有可能以错误的顺序执行它？例如，是否可以进行以下重新排序？:

sharedObject++;
lock.lock();
lock.unlock();

或者保证锁不会被重新排序？

Answer 1

让我们看一下 Java 文档对 Lock 接口(interface)的看法:

All Lock implementations must enforce the same memory synchronization semantics as provided by the built-in monitor lock, as described in section 17.4 of The Java™ Language Specification:

A successful lock operation has the same memory synchronization effects as a successful Lock action.

A successful unlock operation has the same memory synchronization effects as a successful Unlock action.

所以你的问题的答案是肯定的。 Lock 为您提供与常规 synchronized block /方法相同的重新排序保证。