java - ConcurrentHashMap检索操作不带锁？

Question

根据 Java 文档

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

问题:假设线程 t1 正在更新一个键值对(称为 x)，然后其他线程 t2 到来并想要读取 x，是否会在 t2 开始时创建 x 的副本 c1，并且 t2 会从该副本 c1 中读取

Answer 1

Retrievals reflect the results of the most recently completed update operations holding upon their onset

完整的单词在这里很重要。现在你的场景是

 Suppose a thread t1 is updating a key-value pair(called x), and then other thread t2 comes and want to read x.

T2 将读取已完成但不在进程中的操作更新的值。在您的场景中，如果线程 t1 的更新操作未完成，则 t2 将看不到更新的值。不创建任何副本。这是简单的时间基础。如果到 t2 线程读取 x 值并且 t2 更新操作完成时，它将看到更新的值，否则不会。