ConcurrentModificationException出现原因：

1.在Java集合类中，当我们在使用iterator遍历集合对象时，当集合被改变且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。
2.如果subList()生成一个新的List对象，那么当修改原始List对象的结构时，则将抛出ConcurrentModificationException

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;public class ConcurrentModificationExceptionExample {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")) {//由于集合被改变，所以接下来的it.next()会抛出异常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")) {//下面一条语句，由于我们正在更新myMap中的现有键值，因此其大小没有更改，所以没有获取ConcurrentModificationException//通过查看源码，HashMap的next方法是调用nextNode().key;获得的，而nextNode()判断修改的标志的是modCount != expectedModCount，即数量不变就为意味着没有修改//modCount 的定义：The number of times this HashMap has been structurally modifiedmyMap.put("1", "4");//如果放开注释，接下来的 it1.next()就会抛出异常// myMap.put("4", "4");}}}
}

结果：

List Value:1
List Value:2
List Value:3
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)atjava.util.ArrayList$Itr.next(ArrayList.java:851)
at ConcurrentModificationExceptionExample.main(ConcurrentModificationExceptionExample.java:20)

如何避免这个异常呢？

1.在单线程下，所以iterator的remove()方法从原集合中删除一个元素

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;public class ConcurrentModificationExceptionExample {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")) {it.remove();}}System.out.println("List Value:" + myList);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");it1.remove();}}System.out.println("myMap:" + myMap);}
}

执行结果：
List Value:1
List Value:2
List Value:3
List Value:4
List Value:5
List Value:[1, 2, 4, 5]
Map Value:1
Map Value:2
Map Value:3
myMap:{1=4, 3=3}

在多线程环境中避免ConcurrentModificationException

1.您可以将列表转换为数组，然后在数组上进行迭代。这种方法适用于中小型列表，但是如果列表很大，则对性能的影响很大。
2.如果使用的是JDK1.5或更高版本，则可以使用ConcurrentHashMap和CopyOnWriteArrayList类。建议使用此方法来避免并发修改异常。

import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;public class AvoidConcurrentModificationException {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");myList.remove("3");}}System.out.println("List Size:" + myList);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);}}

结果：
List Value:1
List Value:2
List Value:3
List Value:4
List Value:5
List Size:[1, 2, 5, 6, 7]
Map Value:1
Map Value:2
Map Value:4
Map Value:5
Map Size:{1=1, 2=2, 4=4, 5=5}

为什么CopyOnWriteArrayList使用iterator时修改原数据就不会报错呢？

   public Iterator<E> iterator() {return new COWIterator<E>(getArray(), 0);}

CopyOnWriteArrayList内部静态类static final class COWIterator<E> implements ListIterator<E> {/** Snapshot of the array */private final Object[] snapshot;/** Index of element to be returned by subsequent call to next.  */private int cursor;
//传入的是对象，所以原数据修改并不影响它private COWIterator(Object[] elements, int initialCursor) {cursor = initialCursor;snapshot = elements;}public boolean hasNext() {return cursor < snapshot.length;}public boolean hasPrevious() {return cursor > 0;}..........

如果subList()生成一个新的List对象，那么当修改原始List对象的结构时，则将抛出ConcurrentModificationException

import java.util.ArrayList;
import java.util.List;public class ConcurrentModificationExceptionWithArrayListSubList {public static void main(String[] args) {List<String> names = new ArrayList<>();names.add("Java");names.add("PHP");names.add("SQL");names.add("Angular 2");List<String> first2Names = names.subList(0, 2);System.out.println(names + " , " + first2Names);//		names.set(1, "JavaScript");// names.add("Oracle");// check the output below. :)System.out.println(names);System.out.println(first2Names);// Let's modify the list size and get ConcurrentModificationException// names.add("NodeJS");System.out.println(names); // this line throws exceptionfirst2Names.set(1, "RabbitMQ");
//		System.out.println(first2Names);System.out.println(names);System.out.println(first2Names);names.add(1, "RabbitMQ2");//下一行出现异常first2Names.set(1, "RabbitMQ3");}}

结果：
[Java, PHP, SQL, Angular 2] , [Java, PHP]
[Java, PHP, SQL, Angular 2]
[Java, PHP]
[Java, PHP, SQL, Angular 2]
[Java, RabbitMQ, SQL, Angular 2]
[Java, RabbitMQ]
Exception in thread “main” java.util.ConcurrentModificationException
at java.util.ArrayList$SubList.checkForComodification(ArrayList.java:1231)atjava.util.ArrayList$SubList.set(ArrayList.java:1027)
at ConcurrentModificationExceptionWithArrayListSubList.main(ConcurrentModificationExceptionWithArrayListSubList.java:34)

参考文章
CopyOnWriteArrayList

java.util.ConcurrentModificationException