我们称与被锁定的资源关联的事务持有或拥有该锁。
The transaction that a locked resource is associated with is said to hold or own the lock.
通常，资源锁应当在事务完成时被释放掉。
In general, the resource locks should be released upon transaction completion.
ConcurrentHashMap被设计用来优化检索操作；实际上，成功的get操作完成之后通常根本不会有锁着的资源。
ConcurrentHashMap is designed to optimize retrieval operations; in fact, successful get operations usually succeed with no locking at all.
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