java.util.ConcurrentModificationException異常排查

  java.util.ConcurrentModificationException對於這個異常我們一般會認為是在遍歷list的時候對這個list做了add,remove等修改操作造成的，最近在線上日志偶爾發現了這個異常，完整異常如下：

java.util.ConcurrentModificationException: null
	at java.util.ArrayList.sort(ArrayList.java:1462)
	at java.util.Collections.sort(Collections.java:175)
	at com.xxx.xxx.xxx.xxx(xxx.java:17)注:這是業務代碼所以用‘xxx’代替了
	at com.xxx.xxx.xxx.xxx(xxx.java:65)
	at sun.reflect.GeneratedMethodAccessor237.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
	at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
	

代碼如下：（原有代碼比較復雜，寫個類似demo代替）

public class CmeTest {
	private static Map<String,List<Integer>> map = new HashMap<>();

	public static void main(String[] args) {
		buildMap();
		Runnable run = new Runnable() {
			@Override
			public void run() {
				Collections.sort(map.get("a"));
			}
		};
		ExecutorService pool = Executors.newFixedThreadPool(10);
		for(int i=0 ;i<1000;i++) {
			pool.execute(run);
		}
		pool.shutdown();
	}

	private static void buildMap() {
		List<Integer> list1 = new ArrayList<>();
		list1.add(1);
		list1.add(2);
		list1.add(3);
		map.put("a", list1);
	}

代碼的主要邏輯就是並發地對一個list進行排序。查看Collections.sort()的源碼如下：

1    @Override
2    @SuppressWarnings("unchecked")
3    public void sort(Comparator<? super E> c) {
4        final int expectedModCount = modCount;
5        Arrays.sort((E[]) elementData, 0, size, c);
6        if (modCount != expectedModCount) {
7            throw new ConcurrentModificationException();
8        }
9        modCount++;
10    }

重點看第6，10行，當第一個線程排序完成，執行完第9行時第二個線程正好執行到第6行，這時候就會拋出ConcurrentModificationException，所以對於全局的list,做並發排序的時候需要額外注意一下。解決方法有兩個：
1.加鎖
2.創建新的list,再將全局list的數據addAll到新list中，對新list排序