Event Listeners and Triggers in Java

There are a lot of mistakes I’ve seen people make when coding event listeners (and more commonly the classes that trigger events) in Java. One of the most common mistakes is:

List listeners = new ArrayList();

So whats the problem here? Well for one thing: ArrayList (like it says in the JavaDoc) is not synchronized, so it’s great for a single-thread environment. However, GUI’s are very often multi-threaded environments (unless you do everything on the EventDispatchThread, which is also a bad idea), so you may end up with a corrupt list of listeners.

Synchronization and threading issues are not massively likely in a GUI environment, but GUI applications are not the only ones with events, the problem can be solved in a few simple ways:

List listeners = new Vector();

or a more modern approach:

List listeners = Collections.synchronizedList(new ArrayList());

There is also the lovely javax.swing.EventListenerList which also happens to be a much more memory efficient solution. Bare in mind that it’s not bound to Swing in any way (other than the package it’s in). ArrayList and Vector both allocate memory for 10 entires when they are first initialized, whats more they grow exponentially in size. This is great if you are working with rapidly growing lists, but event listener lists tend to grow slowly and very seldom. EventListenerList lazy-creates it’s internal list, and so pre-allocates no “real” memory (space for a pointer, thats about it).

My preference for lists of event listeners is the java.util.concurrent.CopyOnWriteArrayList:

List<MouseListener> listeners = new CopyOnWriteArrayList<MouseListener>();

The CopyOnWriteArrayList is expensive to write to (add or remove), but wonderfully memory efficient, and inherently thread-safe. So unlike other List types, when firing an event off, you don’t have to synchronize on the List object, since if a write happens at the same time, the iterator you use points to a different array to the edited one.

MouseEvent e = new MouseEvent(...);
    for(MouseListener l : listeners) {
    l.mousePressed(e);
}

vs.

MouseEvent e = new MouseEvent(...):
synchronized(listeners) {
    for(MouseListener l : listeners) {
        l.mousePressed(e);
    }
}

One last note: When dealing with PropertyChangeListeners and Events (which is probably 60% of all events I work with), be sure to use a java.beans.PropertyChangeSupport to handle things for you.

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• GWT’s new Event Model – Handlers in GWT 1.6
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• Javent
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