首页 / 更多教程 / fail-fast和modcount
fail-fast和modcount
内容导读
互联网集市收集整理的这篇技术教程文章主要介绍了fail-fast和modcount，小编现在分享给大家，供广大互联网技能从业者学习和参考。文章包含4073字，纯文字阅读大概需要6分钟。
内容图文
Arraylist,HashMap,HashSet等容器持有的的fail-fast迭代器，具体实现原理和modcount域有关.
Lets looks at following code
01 publicclass FailFastIteratorTest { 02 03 publicstaticvoid main(String[] args) { 04 05 final List<String> list = new ArrayList<String>(); 06 list.add("a"); 07 list.add("b"); 08 list.add("c"); 09 10 Iterator<String> it = list.iterator(); 11 12 Thread t = new Thread() { 13 14 publicvoid run() { 15 list.add("d"); 16 list.add("e"); 17 list.add("f"); 18 } 19 }; 20 t.start(); 21 22 // Make sure, thread gets kicked off 23 try { 24 Thread.sleep(1000); 25 } catch (InterruptedException e) { 26 } 27 28 while(it.hasNext()){ 29 System.out.println(it.next()); 30 } 31 } 32 }
What will happen when above program is run ? Hope you have guessed it right, you will get java.util.ConcurrentModificationException thrown at line it.next(). Because its not allowed for a thread to modify the collection, while another is iterating over it. Ok, lets make it single threaded and modify code (lines 06 to 30) as below:
1 list.add("a");
2 list.add("b");
3 list.add("c");
4
5 Iterator<String> it = list.iterator();
6 list.add("d");
7 while(it.hasNext()){
8 System.out.println(it.next());
9 }
Huh!.. again you will get throw ConcurrentModificationException thrown at line it.next(). After the creation of Iterator, the container cannot be modified at any time by any method other than the Iterator‘s own remove or add methods. Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future. Ok Now lets dig little deep and ascertain how fail-fast iterators are implemented. Java uses modCount variable to track number of modifications made. Here goes the javadoc comments for declaraion of modCount variable in AbstractList class.
01 /** 02 * The number of times this list has been structurally modified. 03 * Structural modifications are those that change the size of the 04 * list, or otherwise perturb it in such a fashion that iterations in 05 * progress may yield incorrect results. 06 * 07 * This field is used by the iterator and list iterator implementation 08 * returned by the iterator and listIterator methods. 09 * If the value of this field changes unexpectedly, the iterator (or list 10 * iterator) will throw a ConcurrentModificationException in 11 * response to the next, remove, previous, set or add operations. 12 * This provides fail-fast behavior, rather than non-deterministic behavior in 13 * the face of concurrent modification during iteration. 14 * 15 * Use of this field by subclasses is optional. If a subclass 16 * wishes to provide fail-fast iterators (and list iterators), then it 17 * merely has to increment this field in its add(int, Object) and 18 * remove(int) methods (and any other methods that it overrides 19 * that result in structural modifications to the list). A single call to 20 * add(int, Object) or remove(int) must add no more than 21 * one to this field, or the iterators (and list iterators) will throw 22 * bogus ConcurrentModificationExceptions. If an implementation 23 * does not wish to provide fail-fast iterators, this field may be 24 * ignored. 25 */ 26 protectedtransientint modCount = 0; 27 28 Now lets look at implementation iterator class returned by list. 29 30 privateclass Itr implements Iterator<E> { 31 int expectedModCount = modCount; 32 33 publicboolean hasNext() { 34 // implementation omitted 35 } 36 37 public E next() { 38 checkForComodification(); 39 // implementation omitted 40 41 } 42 43 privatevoid checkForComodification() { 44 if (modCount != expectedModCount) 45 thrownew ConcurrentModificationException(); 46 } 47 }
As it is clear, expectedModCount is initialized to modCount at creation of Iterator and any method that modify the list would increament modCount. Every call to next() method, checks expectedModCount and throws ConcurrentModificationException if it does not match with modCount.
原文：http://www.cnblogs.com/Guoyutian/p/5185456.html
内容总结
以上是互联网集市为您收集整理的fail-fast和modcount全部内容，希望文章能够帮你解决fail-fast和modcount所遇到的程序开发问题。 如果觉得互联网集市技术教程内容还不错，欢迎将互联网集市网站推荐给程序员好友。
内容备注
版权声明：本文内容由互联网用户自发贡献，该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容， 请发送邮件至 gblab@vip.qq.com 举报，一经查实，本站将立刻删除。
内容手机端
扫描二维码推送至手机访问。