没有迭代器的Java列表删除

Question

I have an event handler system in my program where plugins can register event listeners.
The events are called using this method.

/**
 * Pass an event to registered {@link EventHandler}s.
 *
 * @param event Event to pass.
 */
public void callEvent(Event event) {
    synchronized (this) {
        HandlerList handlers = event.getHandlers();
        List<RegisteredListener> listeners = handlers.getRegisteredListeners();
        for (RegisteredListener registration : listeners) {
            try {
                registration.callEvent(event);
            } catch (Throwable ex) {
                Log.e(TAG, "Could not pass event " + event.getClass().getSimpleName() + " to " + registration.getListener().getClass().getName(), ex);
            }
        }
    }
}

HandlerList is simply a container for the List<RegisteredListener> .

Some event listeners that need to be unregistered after being called and this leads to a ConcurrentModificationException in the loop. The system is performance critical and I would like to avoid doing List.toArray() or similar copies for each event; there are often chunks of 100 events. How can I allow the modification of the list from a child listener?

Answer 1

A few comments:

• you could put your listeners in a CopyOnWriteArrayList which is well suited for that type of operation (it will prevent CME) and removes the need for an explicit lock
• List.toArray is really fast (copying a list of size 100 should take less than a microsecond)
• You should try not to call alien methods from a synchronized block ( registration.callEvent(event) ) as it could lead to annoying bugs such as deadlocks for example.