java.util.ConcurrentModificationException - ArrayList

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我已经在谷歌上搜索并搜索了为什么会发生这个异常，我知道它是由一个对象正在读取列表而同时从列表中删除了一个项目引起的。

我已经根据我发现的建议更改了我的代码，但时不时我仍然会遇到此异常，并且它使我的应用程序崩溃。它看起来是随机的，我尝试复制异常，90% 的时间我没有得到异常，并且并不总是遵循相同的过程，这使得调试变得困难。

我正在使用观察者模式。有时它会发生在unregister方法，有时与register，其他时候使用来自notify...它发生的地方是相当随机的。

我使用的是安卓asynctask从我的服务器下载几个字节，观察者模式是在需要时更新 GUI。

这是我的代码：

@Override
    public void register(final Observer newObserver) {
        Log.d(TAG, "(Register) Observer registred: " + newObserver.toString());
        observers.add(newObserver);

        Log.d(TAG, "(Register) Number of registered observers: " + observers.size());

    }

    @Override
    public void unregister(final Observer observer) {

        int indexObersver = observers.indexOf(observer);

        // Avoid java.util.ConcurrentModificationException 
        // at java.util.ArrayList$ArrayListIterator.next(ArrayList.java)

        if(indexObersver >= 0)
        {
            observers.remove(indexObersver);
            Log.d(TAG, "(Unregister) Unregistered Observer: " + observer.toString());
            Log.d(TAG, "(Unregister) Now we have: " + observers.size() + " observers");
        }
        else
        {
            Log.d(TAG, "(Unregister) Registered Observer not found");
        }
    }

    @Override
    public void notifyObserverNewLocalBackup(BackupInfo backupInfo) {

        // Avoid java.util.ConcurrentModificationException 
        // at java.util.ArrayList$ArrayListIterator.next(ArrayList.java)

        for( Iterator< Observer > it = observers.iterator(); it.hasNext() ; )
//      for(Observer observer : observers)
        {
            Observer observer = it.next();
            observer.notifyNewLocalBackup(backupInfo);
        }

    }

    @Override
    public void notifyObserverNewRemoteBackup(ArrayList<PhoneBackup> phoneBackups) {

        // Avoid java.util.ConcurrentModificationException 
        // at java.util.ArrayList$ArrayListIterator.next(ArrayList.java)

//      for(Observer observer : observers)
        for( Iterator< Observer > it = observers.iterator(); it.hasNext() ; )
        {
            Observer observer = it.next();
            observer.notifyNewRemoteBackup(phoneBackups);
        }
    }

    @Override
    public void notifyObserverDownloadCompleted(PhoneBackup phoneBackup) {

        // Avoid java.util.ConcurrentModificationException 
        // at java.util.ArrayList$ArrayListIterator.next(ArrayList.java)

//      for(Observer observer : observers)
        for( Iterator< Observer > it = observers.iterator(); it.hasNext() ; )
        {
            Observer observer = it.next();
            observer.notifyDownloadCompleted(phoneBackup);
        }

    }

    @Override
    public void notifyObserverUploadCompleted(boolean isSucccess) {

        // Avoid java.util.ConcurrentModificationException 
        // at java.util.ArrayList$ArrayListIterator.next(ArrayList.java)

//      for(Observer observer : observers)
        for( Iterator< Observer > it = observers.iterator(); it.hasNext() ; )
        {
            Observer observer = it.next();
            observer.notifyUploadCompleteted(isSucccess);
        }
    }

现在上次我遇到异常时它发生在notifyObserverNewRemoteBackup行方法Observer observer = it.next();

06-12 04:31:58.394: W/dalvikvm(31358): threadid=1: thread exiting with uncaught exception (group=0x418fcce0)
06-12 04:31:58.629: E/AndroidRuntime(31358): FATAL EXCEPTION: main
06-12 04:31:58.629: E/AndroidRuntime(31358): Process: com.mypackage.android.design.appdesgin, PID: 31358
06-12 04:31:58.629: E/AndroidRuntime(31358): java.util.ConcurrentModificationException
06-12 04:31:58.629: E/AndroidRuntime(31358):    at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at com.mypackage.android.design.appdesgin.asynctasks.ObserverSubjectManager.notifyObserverNewRemoteBackup(ObserverSubjectManager.java:99)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at com.mypackage.android.design.appdesgin.asynctasks.BackupsHandler$1.success(BackupsHandler.java:318)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at com.mypackage.android.design.appdesgin.asynctasks.BackupsHandler$1.success(BackupsHandler.java:1)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at retrofit.CallbackRunnable$1.run(CallbackRunnable.java:45)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at android.os.Handler.handleCallback(Handler.java:733)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at android.os.Handler.dispatchMessage(Handler.java:95)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at android.os.Looper.loop(Looper.java:136)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at android.app.ActivityThread.main(ActivityThread.java:5081)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at java.lang.reflect.Method.invokeNative(Native Method)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at java.lang.reflect.Method.invoke(Method.java:515)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
06-12 04:31:58.629: E/AndroidRuntime(31358):    at dalvik.system.NativeStart.main(Native Method)

- - - - - - - - - - - 编辑 - - - - - - - - - - - - - - ----

我遵循了 Anubian Noob 的建议，并实现了一个同步列表，但我仍然遇到异常。

这是我更新的代码：

// Singleton
    public synchronized static ObserverSubjectManager getInstance()
    {
        if(instance == null)
        {
            instance = new ObserverSubjectManager();

            return instance;
        }
    return instance;
}


private ObserverSubjectManager()
{
//      observers = new ArrayList<>();  



    observers = Collections.synchronizedList(new ArrayList<Observer>());
}


@Override
public void register(final Observer newObserver) {
    Log.d(TAG, "(Register) Observer registred: " + newObserver.toString());

    synchronized (observers) {
        observers.add(newObserver);
    }


    Log.d(TAG, "(Register) Number of registered observers: " + observers.size());

}

@Override
public void unregister(final Observer observer) {

    synchronized (observers) 
    {
        int indexObersver = observers.indexOf(observer);

        if(indexObersver >= 0)
        {
            observers.remove(indexObersver);
            Log.d(TAG, "(Unregister) Unregistered Observer: " + observer.toString());
            Log.d(TAG, "(Unregister) Now we have: " + observers.size() + " observers");
        }
        else
        {
            Log.d(TAG, "(Unregister) Registered Observer not found");
        }
    }


}

@Override
public void notifyObserverNewLocalBackup(final BackupInfo backupInfo) {

    synchronized (observers) 
    {
        for(Observer observer : observers)
        {
            observer.notifyNewLocalBackup(backupInfo);
        }
    }


}

@Override
public void notifyObserverNewRemoteBackup(final ArrayList<PhoneBackup> phoneBackups) {

    synchronized (observers) 
    {
        for(Observer observer : observers)
        {
            observer.notifyNewRemoteBackup(phoneBackups);
        }
    }
}

@Override
public void notifyObserverDownloadCompleted(final PhoneBackup phoneBackup) {

    synchronized (observers) 
    {
        for(Observer observer : observers)
        {
            observer.notifyDownloadCompleted(phoneBackup);
        }
    }
}

@Override
public void notifyObserverUploadCompleted(final boolean isSucccess) {

    synchronized (observers) 
    {
        for(Observer observer : observers)
        {
            observer.notifyUploadCompleteted(isSucccess);
        }
    }
}

堆栈跟踪：

06-12 05:12:49.359: W/dalvikvm(31735): threadid=1: thread exiting with uncaught exception (group=0x418fcce0)
06-12 05:12:49.426: E/AndroidRuntime(31735): FATAL EXCEPTION: main
06-12 05:12:49.426: E/AndroidRuntime(31735): Process: com.mypackage.android.design.appdesgin, PID: 31735
06-12 05:12:49.426: E/AndroidRuntime(31735): java.util.ConcurrentModificationException
06-12 05:12:49.426: E/AndroidRuntime(31735):    at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at com.mypackage.android.design.appdesgin.asynctasks.ObserverSubjectManager.notifyObserverDownloadCompleted(ObserverSubjectManager.java:126)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at com.mypackage.android.design.appdesgin.asynctasks.BackupsHandler$2.success(BackupsHandler.java:336)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at com.mypackage.android.design.appdesgin.asynctasks.BackupsHandler$2.success(BackupsHandler.java:1)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at retrofit.CallbackRunnable$1.run(CallbackRunnable.java:45)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at android.os.Handler.handleCallback(Handler.java:733)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at android.os.Handler.dispatchMessage(Handler.java:95)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at android.os.Looper.loop(Looper.java:136)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at android.app.ActivityThread.main(ActivityThread.java:5081)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at java.lang.reflect.Method.invokeNative(Native Method)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at java.lang.reflect.Method.invoke(Method.java:515)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
06-12 05:12:49.426: E/AndroidRuntime(31735):    at dalvik.system.NativeStart.main(Native Method)

为了跟进 @Rogue 的评论，我会寻找任何您的任何实例notify (notifyDownloadCompleted()等）回调实现取消注册观察者。很容易发生的情况是：

1）你正在迭代一个集合。在该迭代中，您对注册的观察者之一调用方法。

2) 该注册观察员，在notify回调，调用到注销进一步的通知本身。

3）由于您仍然处于迭代循环中，这将导致ConcurrentModificationException因为在迭代集合时无法修改集合。

您可以通过执行反向循环来解决此问题：

for (int i = collection.size() - 1; i >= 0; i--) {
    collection.get(i).notifyDownloadCompleted();
}

尽管从技术上讲您仍然可能会遇到一些边缘情况，但也不例外。