java - 为什么 Vector 方法 Iterator 和 ListIterator 快速失败

Question

根据 http://download.oracle.com/javase/1.4.2/docs/api/java/util/Vector.html

The Iterators returned by Vector's iterator and listIterator methods are fail-fast: if the Vector is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Vector's elements method are not fail-fast. 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

你能给我一个例子来验证上面的语句集吗？我仍然不清楚 vector 的方法 Iterator 和 ListIterator 的快速失败行为。 困惑:-((

Answer 1

if the Vector is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException.

这是一个例子:

import java.util.*;

public class Test {

    public static void main(String[] args) {
        List<String> strings = new Vector<String>();

        strings.add("lorem");
        strings.add("ipsum");
        strings.add("dolor");
        strings.add("sit");

        int i = 0;

        Iterator<String> iter = strings.iterator();
        while (iter.hasNext()) {
            System.out.println(iter.next());

            // Modify the list in the middle of iteration.
            if (i++ == 1)
                strings.remove(0);
        }
    }
}

输出:

lorem
ipsum
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java:343)
    at Test.main(Test.java:18)

该程序执行以下操作:

1. 创建一个 Vector 并获取一个迭代器
2. 调用 next() 两次。
3. 修改 vector (通过删除第一个元素)
4. 再次调用next()(在 vector 被修改之后)
5. 这会导致抛出 ConcurrentModificationException。

由于 Java 的 for-each 循环依赖于迭代器，这些结构也可能抛出 ConcurrentModificationExceptions。解决方案是在迭代之前制作列表的副本(因此您迭代副本)或使用例如 CopyOnWriteArrayList像这样:

import java.util.*;
import java.util.concurrent.CopyOnWriteArrayList;

public class Test {

    public static void main(String[] args) {
        List<String> strings = new CopyOnWriteArrayList<String>();

        strings.add("lorem");
        strings.add("ipsum");
        strings.add("dolor");
        strings.add("sit");

        int i = 0;

        Iterator<String> iter = strings.iterator();
        while (iter.hasNext()) {
            System.out.println(iter.next());

            // Modify the list in the middle of iteration.
            if (i++ == 1)
                strings.remove(0);
        }
    }
}

输出:

lorem
ipsum
dolor
sit