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interface MyCustomListener +method1()
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MyEventListener +method1()
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Figure 5-7. JavaBeans event notifications sent from an event source to an event listener
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2. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software (Boston: Addison-Wesley Professional, 1995).
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CHAPTER 5 A SURVEY OF COMMERCIAL SYSTEMS
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The notification procedure call can carry an arbitrary number of parameters. By convention, a single parameter is used, derived directly or indirectly from class EventObject, which has a field that references the event source. Figure 5-7 shows a custom event type called MyCustomEvent. JavaBeans event sources publish events by using a special naming convention. For example, given an event of type MyCustomEvent, the source must have two public methods named addMyCustomListener and removeMyCustomListener to add and remove listeners, respectively. JavaBeans development tools use reflection on classes to find events, based on the naming convention. Listing 5-10 shows how you might implement class MyEventSource. Listing 5-10. A Simple JavaBeans Event Source public class MyEventSource{ MyCustomListener listener; public synchronized void removeMyCustomListener(MyCustomListener l) { if (listener == l) listener = null; } public synchronized void addMyCustomListener(MyCustomListener l) { listener = l; } protected void fireMethod1(MyCustomEvent e) { if (listener != null) listener.method1(e); } public void testMyCustomEvent() { MyCustomEvent e = new MyCustomEvent(this); fireMethod1(e); } } In order to support multicasting, an event source must use a collection to store references to multiple listeners. When firing an event, the event source must iterate over the listeners in the collection. Listing 5-11 shows how MyEventSource might look when supporting multicast MyCustomEvents. Listing 5-11. A JavaBeans Event Source with a Multicast Event public class MyEventSource { private Vector myCustomListeners = new Vector( ); public synchronized void removeMyCustomListener(MyCustomListener l) { if (myCustomListeners.contains(l)) myCustomListeners.removeElement(l); }
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public synchronized void addMyCustomListener(MyCustomListener l) { if (!myCustomListeners.contains(l)) myCustomListeners.addElement(l); } protected void fireMethod1(MyCustomEvent e) { int count = myCustomListeners.size(); for (int i = 0; i < count; i++) { MyCustomListener l = (MyCustomListener) myCustomListeners.elementAt(i); l.method1(e); } } public void testMyCustomEvent( ) { MyCustomEvent e = new MyCustomEvent(this); fireMethod1(e); } } Although there is no requirement for listener classes to use a naming convention, most Java programs name listeners after the events they relate to, so a listener interface for MyCustomEvent would be named MyCustomListener. Listing 5-12 shows how the listener implementation might look. Listing 5-12. A JavaBeans Event Listener public class MyEventListener implements MyCustomListener { public void method1(MyCustomEvent e) { // handle event... } } When using the JavaBeans event model with listeners that only provide a single method, the work required to set up the event source and listener seems excessive. Although calls through interfaces allow you to use interface contracts between the caller and callee, interactions consisting of a single method call can sometimes be handled without all the interface overhead.
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Untyped Object Calls
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Although the standard JavaBeans event model delivers notifications by calling through a listener interface, Java also makes it possible to support untyped object calls, bypassing interfaces. The technique requires the use of reflection to obtain a java.reflect.Method object referencing the target method. The class java.reflect.Method encapsulates method references. Figure 5-8 shows how an event source would use a java.reflect.Method object to invoke a target method without using the target object s interface.
CHAPTER 5 A SURVEY OF COMMERCIAL SYSTEMS
EventSource -onEvent1 : Method
EventListener
Classes
+method1()
object1: EventSource onEvent1: Method
object2: EventListener
Objects
method1()
Figure 5-8. Using java.lang.Method objects to invoke a target method directly Using untyped object calls with java.reflect.Method, the target object is no longer required to implement any special interfaces to satisfy the event model. The event source has a java.reflect. Method field that can hold a reference to any method of any class. At run time, the field is set to reference a specific method of a specific target. Listing 5-13 shows a simple implementation of an event source using a java.reflect.Method. Listing 5-13. A JavaBeans Event Source Supporting Untyped Object Calls public class EventSource { private Object onMyCustomEventTarget; private java.lang.reflect.Method onMyCustomEvent; public void setOnMyCustomEvent(Object theTarget, java.lang.reflect.Method theMethod) { onMyCustomEventTarget = theTarget; onMyCustomEvent = theMethod; } public void fireEvent1( ) { if (onMyCustomEvent == null) return; if (onMyCustomEventTarget == null) return; MyCustomEvent e = new MyCustomEvent(this); Object[ ] arguments = new Object[ ] {e}; try { onMyCustomEvent.invoke(onMyCustomEventTarget, arguments); } catch (Exception ex) { } } } The event source class contains an event called OnMyCustomEvent, which you can set via the method setOnMyCustomEvent. For each event, the class requires two fields: one to store the method to call, and one to store the target object to invoke the method on. You can improve the implementation by encapsulating the management of the target method and object in a separate class that I ll call Delegate. Listing 5-14 shows a possible implementation.
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