Why is a ConcurrentModificationException thrown and how to debug it

This is not a synchronization problem. This will occur if the underlying collection that is being iterated over is modified by anything other than the Iterator itself.

Iterator it = map.entrySet().iterator();
while (it.hasNext()) {
    Entry item = it.next();
    map.remove(item.getKey());
}

This will throw a ConcurrentModificationException when the it.hasNext() is called the second time.

The correct approach would be

Iterator it = map.entrySet().iterator();
while (it.hasNext()) {
    Entry item = it.next();
    it.remove();
}

Assuming this iterator supports the remove() operation.