Collections.sort 引起的线程不安全操作

测试场景

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public static void main(String [] args) throws Exception {

    //快递优先级对于所有线程来说是一个全局变量
    List<String> list = new ArrayList<>();

    list.add("A");
    list.add("D");
    list.add("C");
    list.add("B");
    list.add("F");
    list.add("E");

    Runnable runnable = new Runnable() {
      @Override
      public void run() {
        //这里对一个全局变量进行排序
        Collections.sort(list);
      }
    };

    ExecutorService executorService = Executors.newFixedThreadPool(10);
    //使用1000个线程模拟
    for (int i = 0; i < 1000; i++) {
      executorService.execute(runnable);
    }
    try {
      TimeUnit.SECONDS.sleep(100);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }

报错信息

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Exception in thread "pool-1-thread-2" java.util.ConcurrentModificationException
	at java.base/java.util.ArrayList.sort(ArrayList.java:1752)
	at java.base/java.util.Collections.sort(Collections.java:145)
	at com.example.surf.content.Demo2$1.run(Demo2.java:34)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

原因

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public void sort(Comparator<? super E> c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
    modCount++;
}

在这段源码种，多个线程同时对同一个 list 排序操作，同时修改list，抛出异常

实际场景

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private Config getConfigByRandomId(String region, Integer randomId) {
  List<Config> configs = configService.getConfigMap()
      .getOrDefault(region, configService.getConfigMap().get(DEFAULT_REGION));
  log.info("sort:{}", JSON.toJSONString(configs));
  Collections.sort(configs, Comparator.comparing(Config::getRatio));

  int index = 0;
  for (Config config : configs) {
    index += config.getRatio();
    if (index >= randomId) {
      return config;
    }
  }
  return configs.get(0);
}

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2021-07-07 04:38:13.975 ERROR 1 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.util.ConcurrentModificationException] with root cause
java.util.ConcurrentModificationException: null
        at java.base/java.util.ArrayList.sort(ArrayList.java:1751) ~[na:na]
        at java.base/java.util.Collections.sort(Collections.java:177) ~[na:na]
        at com.example.surf.content.controller.ContentController.getConfigByRandomId(ContentController.java:163) ~[classes!/:0.0.1-SNAPSHOT]
        at com.example.surf.content.controller.ContentController.layer(ContentController.java:77) ~[classes!/:0.0.1-SNAPSHOT]
        at jdk.internal.reflect.GeneratedMethodAccessor132.invoke(Unknown Source) ~[na:na]

这段代码同时从一个 cache 中获取list，然后多个线程对其进行排序。之前一直以为是缓存失效导致数据被删除，找错方向，实际问题还是 sort 的问题。