常看到在易变域上的同步代码，并且写的同学会很自然觉得这样是安全和正确的。

Demo说明

主线程中开启2个任务线程执行addListener。主线程最终结果检查。

问题说明

最终Listener的个数不对。

快速运行

mvn compile exec:java -Dexec.mainClass=com.oldratlee.fucking.concurrency.SynchronizationOnMutableFieldDemo

具体demo：

public class SynchronizationOnMutableFieldDemo {    static final int ADD_COUNT = 10000;    static class Listener {        // stub class    }    private volatile List
    
      listeners = new CopyOnWriteArrayList
     
      ();    public static void main(String[] args) throws Exception {        SynchronizationOnMutableFieldDemo demo = new SynchronizationOnMutableFieldDemo();        Thread thread1 = new Thread(demo.getConcurrencyCheckTask());        thread1.start();        Thread thread2 = new Thread(demo.getConcurrencyCheckTask());        thread2.start();        thread1.join();        thread2.join();        int actualSize = demo.listeners.size();        int expectedSize = ADD_COUNT * 2;        if (actualSize != expectedSize) {            // 在我的开发机上，几乎必现！（简单安全的解法：final List字段并用并发安全的List，如CopyOnWriteArrayList）            System.err.printf("Fuck! Lost update on mutable field! actual %s expected %s.\n", actualSize, expectedSize);        } else {            System.out.println("Emm... Got right answer!!");        }    }    public void addListener(Listener listener) {        synchronized (listeners) {            List
      
        results = new ArrayList
       
        (listeners);            results.add(listener);            listeners = results;        }    }    ConcurrencyCheckTask getConcurrencyCheckTask() {        return new ConcurrencyCheckTask();    }    private class ConcurrencyCheckTask implements Runnable {        @Override        public void run() {            System.out.println("ConcurrencyCheckTask started!");            for (int i = 0; i < ADD_COUNT; ++i) {                addListener(new Listener());            }            System.out.println("ConcurrencyCheckTask stopped!");        }    }}