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

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.