java - 在迭代 ConcurrentHashMap 时添加和删除值

Question

我有这个代码:

private ConcurrentMap<String, Integer> myMap = new ConcurrentHashMap<>();

 @Scheduled(fixedDelay = 600_000)
 public void foo(){     
    myMap.values().stream().
                filter(predicate()).
                forEach(this::remove);
}

public void insert(String str, Integer value){
   myMap.put(str, value);
}

如果在迭代此映射时，有人会在其中添加新值或从中删除现有值，会发生什么情况？

Answer 1

ConcurrentHashMap 的文档有一些关于行为的细节。首先我们看看什么 ConcurrentHashMap.values() 是:

Returns a Collection view of the values contained in this map...

The view's iterators and spliterators are weakly consistent.

The view's spliterator reports Spliterator.CONCURRENT and Spliterator.NONNULL.

有趣的是术语 "weakly consistent"和 Spliterator.CONCURRENT ，其中前者描述为:

Most concurrent Collection implementations (including most Queues) also differ from the usual java.util conventions in that their Iterators and Spliterators provide weakly consistent rather than fast-fail traversal:

• they may proceed concurrently with other operations
• they will never throw ConcurrentModificationException
• they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction.

和 Spliterator.CONCURRENT 描述为:

Characteristic value signifying that the element source may be safely concurrently modified (allowing additions, replacements, and/or removals) by multiple threads without external synchronization. If so, the Spliterator is expected to have a documented policy concerning the impact of modifications during traversal.

从所有这些文档中，并与 ConcurrentHashMap 的并发模型保持一致，这意味着流管道是完全线程安全的，并且将遍历创建迭代器时存在的元素。