(Java)如果不同步，为什么允许HashSet同步使用?

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我一直在阅读 HashMap ， HashSet 和 HashTable 之间的区别.我注意到的关键是，我看到 HashMap / HashSet 在 HashTable 处于同步状态时不同步.

I've been reading up on the differences between a HashMap, HashSet, and HashTable. A key thing I've been noticing is that I've seen that HashMap/HashSet are not synchronized while a HashTable is.

但是，在我之前看到的代码库中，有好几个地方使用了这样的块:

However in a code base that I've seen before there are several places where a block like this is used:

synchronized (hashSet) { //Some code involving the hashset }

如果 HashSet 不同步，怎么办?同步块是否仅允许我们使用非同步数据结构，就好像它已同步一样?

How is this possible if a HashSet isn't synchronized? Does the synchronized block simply allow us to use a non synchronous data structure as if it were synchronized?

如果 HashSet 是同步的，我们是否就不必包含synced(){}块了吗?

If HashSet were synchronized would we just not have to include the synchronized() {} block?

推荐答案

一个 Synchronized 块需要一些对象进行同步.当说 HashSet 是不同步的时，这仅意味着它的方法本身并不同步，如果您打算在多线程上下文中使用它，则应自己处理同步(例如，如代码段所示，通过对 HashSet 对象进行 synchronization ).

A synchronized block requires some object to syncronize upon. When a HashSet is said to be unsynchronized it just means that it's methods aren't synchronized in their own right, and if you intend to use it in a multi-threaded context, you should handle synchronization by yourself (e.g., by synchronizing on the HashSet object as shown in your snippet).