Event Listener Registration in JavaBeans:
1. Define Event Listener Interface: First, you need to define an event listener interface. This interface should extend `java.util.EventListener` and declare methods that will be called when the event occurs.
public interface CustomEventListener extends java.util.EventListener {
void customEventOccurred(CustomEvent evt);
}
2. Create an Event Object: You need to create an event object that extends `java.util.EventObject`. This object will be passed to the listener methods when the event occurs.
public class CustomEvent extends java.util.EventObject {
public CustomEvent(Object source) {
super(source);
}
}
3. Implement Event Generation: Inside your JavaBean, implement the logic to generate events and notify registered listeners. This involves maintaining a list of listeners and providing methods to add and remove listeners.
public class CustomBean {
protected transient java.util.List<CustomEventListener> listenerList = new java.util.ArrayList<>();
public synchronized void addCustomEventListener(CustomEventListener listener) {
if (!listenerList.contains(listener)) {
listenerList.add(listener);
}
}
public synchronized void removeCustomEventListener(CustomEventListener listener) {
listenerList.remove(listener);
}
protected void fireCustomEvent() {
CustomEvent evt = new CustomEvent(this);
for (CustomEventListener listener : listenerList) {
listener.customEventOccurred(evt);
}
}
}
4. Implement Event Listener: Create a class that implements the event listener interface and provide implementations for the methods declared in the interface.
public class CustomEventListenerImpl implements CustomEventListener {
@Override
public void customEventOccurred(CustomEvent evt) {
System.out.println("Event occurred in " + evt.getSource());
}
}
5. Register and Test the Listener: Finally, create an instance of your JavaBean and an instance of your event listener. Register the event listener with the JavaBean and test the event notification mechanism.
public class Test {
public static void main(String[] args) {
CustomBean bean = new CustomBean();
CustomEventListener listener = new CustomEventListenerImpl();
bean.addCustomEventListener(listener);
bean.fireCustomEvent(); // This should trigger the event and call customEventOccurred method
}
}
JavaBeans utilize a delegation-based event model, allowing objects to delegate their event-handling responsibilities to other objects, known as listeners. This design promotes a clean separation between the source of events and the objects that handle them, facilitating a more modular and maintainable codebase. Event listeners are registered with the event source, and upon the occurrence of an event, all registered listeners are notified, ensuring a robust and responsive application design.