All geek questions in one place

Java.util.ConcurrentModificationException - ArrayList

START EDIT scroll down updated code END OF EDIT

I have google and search around SO why this exception occurs, and I understand that this is caused by the object reading the list, and meanwhile the element has been removed from the list.

I changed my code according to the suggestions I found, but from time to time I still get this exception and this is a crash of my application. And it looks random, I try to replicate the exception, and 90% of the time I do not get the exception and do not always follow the same procedure, which makes debugging difficult.

I use the observer pattern. Sometimes this happens with a method unregister, some others with register, in other cases with a method from notify... this is a rather random case when this happens.

I use android asynctaskto download several bytes from my server, and the observer pattern - if necessary, update the GUI.

Here is my code:

@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);
        }
    }

Now, for the last time, it turned out that this happened according to the method notifyObserverNewRemoteBackupin the lineObserver 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)

---------------------- EDIT --------------------- ------ ----

I followed the Anubian Noob suggestion and I implemented a synchronized list, but I still get the exception.

Here is my updated code:

// 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);
        }
    }
}

Stacktrace:

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)
+4
source share
3 answers

@Rogue, , notify (notifyDownloadCompleted() ..) . , , :

1) . .

2) notify , .

3) , ConcurrentModificationException, .

, :

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

, , .

+5

, ArrayList , , . ArrayList CopyOnWriteArrayList ( ) Collections.synchronizedList().

:

List<Observer> list = Collection.synchronizedList(new ArrayList<Observer>);
+2

, , , - :

for (Observer observer : new ArrayList<>(observers)) {
  observer.notifyNewLocalBackup(backupInfo);
}

, .

CopyOnWriteArrayList, .

+1

Source: https://habr.com/ru/post/1544356/

More articles:


All Articles