Java 中的 Iterator 如何知道何时抛出 ConcurrentModification 异常

问题描述

我有以下代码引发 ConcurrentModificationException，因为我在同一个列表上使用了两个不同的迭代器，其中一个正在修改列表.所以，第二个迭代器在读取列表时会抛出异常，因为其他迭代器已经修改了列表.

I have the following code which throws ConcurrentModificationException because I am using two different iterators on the same list and one of them is modifying the list. So, the second iterator throws the exception when reading the list because some other iterator has modified the list.

    List<Integer> list = new ArrayList<>();

    populate(list);//A method that adds integers to list

    ListIterator<Integer> iterator1 = list.listIterator();
    ListIterator<Integer> iterator2 = list.listIterator();

    while (iterator1.hasNext()) {
        if(iterator1.next() < 5)
            iterator1.remove();
    }

    while (iterator2.hasNext()){
        if(iterator2.next() < 5) {
         //Call handler   
        }
    }

我的问题是 iterator2 如何知道 internally 如果 list 尚未到达某个元素，则它已被其他迭代器修改iterator1 还删除了哪个?它如何确定其他一些 iterator 已经改变了 list?一种方法是跟踪大小，但这不是原因，因为其他一些迭代器可以替换任何元素.

My question is how does iterator2 know internally that the list has has been modified by some other iterator if it has not reached an element which is yet removed by iterator1? How does it figure out that some other iterator has mutated the list? One way could be keep track of size but that can't be the reason since some other iterator can just replace any element.

推荐答案

回答此类问题的一个好方法是查看源代码，例如 ArrayList 的源代码.搜索 ConcurrentModificationException.

A good way to answer questions like this is to look at the source code, for example the source code for ArrayList. Search for ConcurrentModificationException.

你应该能够说事情是这样工作的:

You should be able to tell that things work rather like this:

• 集合对象有一个修改计数，该计数从零开始，并在发生添加或删除或类似操作时增加.
• 创建迭代器对象时，我们将集合的当前修改计数存储在迭代器中.
• 每次使用迭代器时，它都会检查集合的 mod 计数与迭代器在创建时获得的 mod 计数.如果这些值不同，则会引发异常.

在您的情况下，列表中 iterator1 执行的删除操作会更改列表的结构操作计数 (modCount).当 iterator2 被要求删除时，它会看到它最初收到的 expectedModCount 为 0，与列表的当前 mod 计数不同.

In your case, remove operations performed by iterator1 on the list change the structural operation count (modCount) of the list. When iterator2 is asked to remove, it sees its expectedModCount, which it received initially as 0, differing from the current mod count of the list.

需要注意的是，it.remove 是一个特例.当迭代器自己删除时，它的 expectedModCount 会相应地调整，以与底层列表保持同步.

It should be noted that it.remove is a special case. When an iterator does a remove itself, its expectedModCount adjusts accordingly, to keep in sync with the underlying list.

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