Collections.sort 的多线程安全问题

背景

在实际的工作中，遇到了多线程操作 Collections.sort 导致了 ConcurrentModificationException。多线程操作了同一个共享变量导致。

解决办法可以加锁。

package com.test.wyf.springprofile;import java.util.ArrayList;
import java.util.Collections;
import java.util.List;public class MultiThreadCollectionsSort {public static void main(String[] args) {System.out.println("main start......");List<String> list = new ArrayList<>();list.add("one");list.add("two");list.add("three");for (int i = 0; i < 3; i++) {new Thread(() -> {Collections.sort(list);System.out.println("thread done...");}).start();}System.out.println("main done......");}
}

有时报错

main start......
main done......
thread done...
Exception in thread "Thread-1" Exception in thread "Thread-2" java.util.ConcurrentModificationExceptionat java.util.ArrayList.sort(ArrayList.java:1466)at java.util.Collections.sort(Collections.java:143)at com.test.wyf.springprofile.MultiThreadCollectionsSort.lambda$main$0(MultiThreadCollectionsSort.java:18)at java.lang.Thread.run(Thread.java:748)
java.util.ConcurrentModificationExceptionat java.util.ArrayList.sort(ArrayList.java:1466)at java.util.Collections.sort(Collections.java:143)at com.test.wyf.springprofile.MultiThreadCollectionsSort.lambda$main$0(MultiThreadCollectionsSort.java:18)at java.lang.Thread.run(Thread.java:748)

有时候不报错（较少时候，试了很多次才出现如下）

main start......
main done......
thread done...
thread done...
thread done...

代码改进，可以将list对象作为锁对象

public static void main(String[] args) {System.out.println("main start......");List<String> list = new ArrayList<>();list.add("one");list.add("two");list.add("three");for (int i = 0; i < 3; i++) {new Thread(() -> {synchronized (list) {Collections.sort(list);}System.out.println("thread done...");}).start();}System.out.println("main done......");}