java - ConcurrentHashMap中 'thread-safe'的含义

Question

由于桶级锁定，当使用 CHM 代替 Collections.synchronizedMap(...)/hashtable 时可以提供性能提升。

当我们不希望我们的迭代器抛出 ConcurrentModificationException 时，我们也可以使用 CHM。

但我对 CHM 上下文中 thread-safe 的含义感到困惑，因为它们不会阻止更改在检索和写入重叠之间得到反射(reflect)？

Answer 1

线程安全意味着您可以在多个线程之间共享一个ConcurrentHashMap 对象，并且可以在没有外部锁定的情况下并发访问/修改该对象。

确切的语义在 documentation 中有解释。 :

Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset.

For aggregate operations such as putAll and clear, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw ConcurrentModificationException. However, iterators are designed to be used by only one thread at a time.