Question

What am I getting a ConcurrentModificationException with this code? I have a synchronized(listeners) lock.

private void notifyListeners(MediumRendition rendition) {
    if (rendition == null) return;
    synchronized (listeners) {
        for (RenditionEventListener l : listeners) {
            if (l.renditionType.equals(rendition.getType()) && l.mediumId == rendition.getMediumId()) {
                l.listener.onRendition(rendition);
            }
        }
    }
}

public void put(String renditionType, long mediumId, MediumRendition rendition) {
    HashMap<Long, MediumRendition> l = list.get(renditionType);
    if (l == null) {
        l = new HashMap<Long, MediumRendition>();
        list.put(renditionType, l);
    }
    l.put(mediumId, rendition);
    notifyListeners(rendition);
}

public void addRenditionListener(String renditionType, long mediumId, RenditionListener listener) {
    synchronized (listeners) {
        listeners.add(new RenditionEventListener(renditionType, mediumId, listener));
    }
}

01-30 16:47:55.147 6953 6974 E AndroidRuntime: FATAL EXCEPTION: Thread 1
01-30 16:47:55.147 6953 6974 E AndroidRuntime: java.util.ConcurrentModificationException
01-30 16:47:55.147 6953 6974 E AndroidRuntime: at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
01-30 16:47:55.147 6953 6974 E AndroidRuntime: at com..vos.RenditionList.notifyListeners(RenditionList.java:79)
01-30 16:47:55.147 6953 6974 E AndroidRuntime: at com.t.vos.RenditionList.put(RenditionList.java:41)
01-30 16:47:55.147 6953 6974 E AndroidRuntime: at com.*.controllers.OmwController$6.run(OmwController.java:212)

SOLVED: Apparently I was removing. See below.

Answer 1

The exception indicates that listeners might have been modified from another thread. So you should check if it is being modified without synchronization somewhere else.

Also check if the implementation of onRendition() removes listener from list as that could be another cause if multiple threads are not involved.