Fail-Fast in Java

绝地灬酷狼 2022-05-15 04:14 126阅读 0赞

> **A fail-fast system is nothing but immediately report any failure that is likely to lead to failure. When a problem occurs, a fail-fast system fails immediately. In Java, we can find this behavior with iterators. Incase, you have called iterator on a collection object, and another thread tries to modify the collection object, then concurrent modification exception will be thrown. This is called fail-fast.**

**[Fail-Fast][]：**又称**“快速失败”**或**“早期失败”**是一个软件工程概念，是指当异常发生时，立即停止执行以防止复杂问题的发生。

在Java语言中，由集合类比如：**ArrayList, HashSet, Vector**等等所返回的迭代器对象都是**Fail-Fast**的，也就是说，当你通过迭代器**Iterator**来遍历集合内的对象，同时又对集合进行修改（增加**add**或者删除**remove**）操作时，就会抛出**ConcurrentModificationException**异常。

我们看下面这个例子：

List<Integer> ints = new ArrayList<>(asList(1, 2, 3, 4, 5, 6, 9, 15, 67, 23, 22, 3, 1, 4, 2));
            for (Integer i : ints) {
                // some code
                ints.add(57);  // throws java.util.ConcurrentModificationException
            }

当你执行时，你就会得到以下异常：

Exception in thread "main" java.util.ConcurrentModificationException
    1
        at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
        at java.util.ArrayList$Itr.next(ArrayList.java:859)
        at simulate.SetTest.main(SetTest.java:40)

在Java 8u20 发行版本中，**Collections.sort()**同样也是一个**Fail-Fast**，这也意味着你无法在对集合迭代遍历的同时执行**Collections.sort()**排序操作，比如类似下面这样：

List<Integer> ints = new ArrayList<>(asList(1, 2, 3, 4, 5, 6, 9, 15, 67));
            for (Integer i : ints) {
                // some code
                Collections.sort(ints);
            }

这么设计是有意义的，如果你在迭代遍历的同时执行排序操作，不仅违反正常的语义，而且还容易引起未知的后果，当然，你在调用**Collections.sort()**方法之后，立即跳出迭代（**break**），那就不会引起该异常，但通常这不是推荐做法。

List<Integer> ints = new ArrayList<>(asList(1, 2, 3, 4, 5, 6, 9, 15, 67));
            for (Integer i : ints) {
                // some code
                Collections.sort(ints);
                break;
            }

*  [https://dzone.com/articles/more-fail-early-java-8][https_dzone.com_articles_more-fail-early-java-8]
 *  [http://www.java2novice.com/java\_interview\_questions/fail-fast/][http_www.java2novice.com_java_interview_questions_fail-fast]

[Fail-Fast]: https://en.wikipedia.org/wiki/Fail-fast
[https_dzone.com_articles_more-fail-early-java-8]: https://dzone.com/articles/more-fail-early-java-8
[http_www.java2novice.com_java_interview_questions_fail-fast]: http://www.java2novice.com/java_interview_questions/fail-fast/