java - ConcurrentMap 中的内存一致性影响

Question

根据ConcurrentMap Javadoc :

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread.

上面的语句是什么意思？它是如何工作的，因为 ConcurrentHashMap 中的 get() 方法没有阻塞(例如与 BlockingQueue 相比)？

Answer 1

意思很简单。假设您有两段代码:

 a = new A();
 b = ...
 someConcurrentHashMap.put(b, whatever);

然后:

Whatever value = someConcurrentHashMap.get(b);
c = new C();

当b是同一个对象，并且这两段代码分别由两个不同的线程执行时，那么可以保证a = new A() 发生在 c = new C()之前。

有关“发生在之前”的进一步阅读 - 参见 here .

关于实现细节，我建议你研究源码code - 包含大量(非 javadoc!)解释此类内部工作原理的注释。