HashMap的fast-fail和ConcurrentHashMap的fail-safe实例

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Java HashMap遍历过程中如果元素被修改会导致遍历失败，ConcurrentHashMap则不会有这个问题。由此引出HashMap的fast-fail机制和ConcurrentHashMap的的fail-safe机制。

看如下实例。
首先看HashMap的fast-fail

package com;import java.util.HashMap;
import java.util.Map;public class TestHashMapFastFail
{public static void main(String[] args){System.out.println("test HashMap fast-fail");Map<Integer, String> testHashMap = new HashMap<Integer, String>();testHashMap.put(1000, "1000");testHashMap.put(2000, "2000");testHashMap.put(3000, "3000");testHashMap.put(4000, "4000");testHashMap.put(5000, "5000");System.out.println(testHashMap.size());for (Map.Entry<Integer, String> entry : testHashMap.entrySet()){int key = entry.getKey();System.out.println("key=" + key);if (key == 3000){testHashMap.remove(key);}}System.out.println(testHashMap.size());for (Map.Entry<Integer, String> entry : testHashMap.entrySet()){System.out.println(entry.getKey() + "-->" + entry.getValue());}}
}

运行结果

test HashMap
5
key=2000
key=4000
key=1000
key=3000
Exception in thread "main" java.util.ConcurrentModificationExceptionat java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)at java.util.HashMap$EntryIterator.next(HashMap.java:1471)at java.util.HashMap$EntryIterator.next(HashMap.java:1469)at com.TestHashMapFastFail.main(TestHashMapFastFail.java:18)

可以看到执行remove操作后，下一轮迭代立刻失效，并抛出异常，这就是所谓的fast-fail。

再看ConcurrentHashMap的fail-safe

package com;import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;public class TestConcurrentHashMapFailSafe
{public static void main(String[] args){System.out.println("test ConcurrentHashMap fast-fail");Map<Integer, String> testConcurrentHashMap = new ConcurrentHashMap<Integer, String>();testConcurrentHashMap.put(100, "100");testConcurrentHashMap.put(200, "200");testConcurrentHashMap.put(300, "300");testConcurrentHashMap.put(400, "400");testConcurrentHashMap.put(500, "500");System.out.println(testConcurrentHashMap.size());for (Map.Entry<Integer, String> entry : testConcurrentHashMap.entrySet()){int key = entry.getKey();System.out.println("key=" + key);if (key == 300){testConcurrentHashMap.remove(key);}}System.out.println(testConcurrentHashMap.size());for (Map.Entry<Integer, String> entry : testConcurrentHashMap.entrySet()){System.out.println(entry.getKey() + "-->" + entry.getValue());}}
}

运行结果

 test ConcurrentHashMap fast-fail
5
key=400
key=100
key=500
key=200
key=300
3
100-->100
500-->500
300-->300

可以看出，尽管在迭代过程中执行了remove操作，但是ConcurrentHashMap对外的表现仍然正常，这就是所谓的fail-safe。原因在于ConcurrentHashMap返回的迭代器是弱一致性，ConcurrentHashMap底层数据结构改变时并且不会抛出ConcurrentModificationException异常。
所以，这也是选择ConcurrentHashMap可以获得的一个额外好处，或者说是解决HashMap fast-fail的一种方法，还有一个方法就是使用迭代器的remove方法而不是使用集合的remove方法，示例如下。

package com;import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;public class TestHashMapFastFail2
{public static void main(String[] args){System.out.println("test solve HashMap fast-fail");Map<Integer, String> testHashMap = new HashMap<Integer, String>();testHashMap.put(1000, "1000");testHashMap.put(2000, "2000");testHashMap.put(3000, "3000");testHashMap.put(4000, "4000");testHashMap.put(5000, "5000");System.out.println(testHashMap.size());Iterator iterator = testHashMap.entrySet().iterator();while (iterator.hasNext()){int key = (int)((Map.Entry)iterator.next()).getKey();System.out.println("key=" + key);if (key == 2000 || key == 4000){iterator.remove();}}System.out.println(testHashMap.size());for (Map.Entry<Integer, String> entry : testHashMap.entrySet()){System.out.println(entry.getKey() + "-->" + entry.getValue());}}
}

运行结果

test solve HashMap fast-fail
5
key=2000
key=4000
key=1000
key=3000
key=5000
3
1000-->1000
3000-->3000
5000-->5000

集合的 fast-fail 问题是初学者很容易犯的错误。
说说fast-fail机制和fail-safe机制设计的原因。有人可能会问，既然fast-fail有这么多弊端，为什么还要设计呢，以HashMap为例，因为HashMap本身就是设计成线程不安全的，不支持多个线程同时安全修改，但这也意味着HashMap有较快的速度。fail-safe机制设计的初衷就是保证多线程并发安全地修改集合或Map类。当然，本文的用例都是单线程中的修改操作，主要是为了引出这2个概念。至于内部实现机制，看源码吧。