它不会抛出异常ConcurrentModificationException [英] It does not throw exception ConcurrentModificationException

问题描述

我有以下代码，我希望它抛出一个 ConcurrentModificationException ，但它运行成功。为什么会这样？

I have the below code and I would expect it to throw a ConcurrentModificationException, but it runs successfully. Why does this happen?

public void fun(){
    List <Integer>lis = new ArrayList<Integer>();
    lis.add(1);
    lis.add(2);

    for(Integer st:lis){
        lis.remove(1);
        System.out.println(lis.size());
    }
}

public static void main(String[] args) {
    test t = new test();
    t.fun();
}

推荐答案

remove（int） List 上的方法删除指定位置的元素。在开始循环之前，您的列表如下所示：

The remove(int) method on List removes the element at the specified position. Before you start your loop, your list looks like this:

[1, 2]

然后在列表中启动迭代器：

Then you start an iterator on the list:

[1, 2]
 ^

您的 循环然后删除位置1的元素，这是数字2：

Your for loop then removes the element at position 1, which is the number 2:

[1]
 ^

迭代器，在下一个隐含的 hasNext（）调用，返回 false ，循环终止。

The iterator, on the next implied hasNext() call, returns false, and the loop terminates.

如果向列表中添加更多元素，您将获得 ConcurrentModificationException 。然后隐式 next（）将抛出。

You will get a ConcurrentModificationException if you add more elements to the list. Then the implicit next() will throw.

作为一个注释，来自Javadoc for ArrayList ：

As a note, from the Javadoc for ArrayList from the JCF:

请注意，迭代器的失败快速行为无法保证，因为它一般来说，在存在不同步的并发修改时，不可能做出任何硬性保证。失败快速迭代器会尽最大努力抛出 ConcurrentModificationException 。因此，编写依赖于此异常的程序以确保其正确性是错误的：迭代器的故障快速行为应仅用于检测错误。

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

这实际上可能是Oracle ArrayList 迭代器实现中的一个错误; hasNext（） 不检查修改：

This is probably actually a bug in the Oracle ArrayList iterator implementation; hasNext() does not check for modification:

public boolean hasNext() {
    return cursor != size;
}

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