并发容器

Java集合类都是快速失败的，这就意味着当集合被改变且一个线程在使用迭代器遍历集合的时候，迭代器的next()方法将抛出ConcurrentModificationException异常。如下代码：

import java.util.*;public class IteratorExample {    public static void main(String args[]){        List<String> myList = new ArrayList<String>();        myList.add("1");        myList.add("2");        myList.add("3");        myList.add("4");        myList.add("5");        Iterator<String> it = myList.iterator();        while(it.hasNext()){            String value = it.next();            System.out.println("List Value:"+value);            if(value.equals("3")) myList.remove(value);        }        Map<String,String> myMap = new HashMap<String,String>();        myMap.put("1", "1");        myMap.put("2", "2");        myMap.put("3", "3");        Iterator<String> it1 = myMap.keySet().iterator();        while(it1.hasNext()){            String key = it1.next();            System.out.println("Map Value:"+myMap.get(key));            if(key.equals("2")){                myMap.put("1","4");                //myMap.put("4", "4");            }        }    }}

当运行以上代码的时候就会抛出以下异常：

 Exception in thread "main" java.util.ConcurrentModificationExceptionList Value:2    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)List Value:3    at java.util.ArrayList$Itr.next(ArrayList.java:851)    at com.test.thread.NullHaspMap.main(NullHaspMap.java:20)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:498)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

并发容器支持并发的遍历和并发的更新。j当你用iterator遍历的同时能够更新容器里面的元素
主要的类有ConcurrentHashMap, CopyOnWriteArrayList 和CopyOnWriteArraySet，
将上面的代码改成以下代码就不会抛出异常了：

package com.journaldev.java;import java.util.Iterator;import java.util.List;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.CopyOnWriteArrayList;public class ThreadSafeIteratorExample {    public static void main(String[] args) {        List<String> myList = new CopyOnWriteArrayList<String>();        myList.add("1");        myList.add("2");        myList.add("3");        myList.add("4");        myList.add("5");        Iterator<String> it = myList.iterator();        while(it.hasNext()){            String value = it.next();            System.out.println("List Value:"+value);            if(value.equals("3")){                myList.remove("4");                myList.add("6");                myList.add("7");            }        }        System.out.println("List Size:"+myList.size());        Map<String,String> myMap = new ConcurrentHashMap<String,String>();        myMap.put("1", "1");        myMap.put("2", "2");        myMap.put("3", "3");        Iterator<String> it1 = myMap.keySet().iterator();        while(it1.hasNext()){            String key = it1.next();            System.out.println("Map Value:"+myMap.get(key));            if(key.equals("1")){                myMap.remove("3");                myMap.put("4", "4");                myMap.put("5", "5");            }        }        System.out.println("Map Size:"+myMap.size());    }}