Class EventManager


  • public class EventManager
    extends Object
    This class is the central class for the Event Manager. Each program that wishes to use the Event Manager should construct an EventManager object and use that object to construct ListenerQueue for dispatching events. CopyOnWriteIdentityMap objects must be used to manage listener lists.

    This example uses the fictitious SomeEvent class and shows how to use this package to deliver a SomeEvent to a set of SomeEventListeners.

    
            // Create an EventManager with a name for an asynchronous event dispatch thread
            EventManager eventManager = new EventManager("SomeEvent Async Event Dispatcher Thread");
            // Create a CopyOnWriteIdentityMap to hold the list of SomeEventListeners
            Map eventListeners = new CopyOnWriteIdentityMap();
    
            // Add a SomeEventListener to the listener list
            eventListeners.put(someEventListener, null);
    
            // Asynchronously deliver a SomeEvent to registered SomeEventListeners
            // Create the listener queue for this event delivery
            ListenerQueue listenerQueue = new ListenerQueue(eventManager);
            // Add the listeners to the queue and associate them with the event dispatcher
            listenerQueue.queueListeners(eventListeners.entrySet(), new EventDispatcher() {
                    public void dispatchEvent(Object eventListener, Object listenerObject, 
                                        int eventAction, Object eventObject) {
                            try {
                                    (SomeEventListener)eventListener.someEventOccured((SomeEvent)eventObject);
                            } catch (Throwable t) {
                                    // properly log/handle any Throwable thrown by the listener
                            }
                    }
            });
            // Deliver the event to the listeners. 
            listenerQueue.dispatchEventAsynchronous(0, new SomeEvent());
                    
            // Remove the listener from the listener list
            eventListeners.remove(someEventListener);
    
            // Close EventManager to clean when done to terminate async event dispatch thread.
            // Note that closing the event manager while asynchronously delivering events 
            // may cause some events to not be delivered before the async event dispatch 
            // thread terminates
            eventManager.close();
     

    At first glance, this package may seem more complicated than necessary but it has support for some important features. The listener list supports companion objects for each listener object. This is used by the OSGi framework to create wrapper objects for a listener which are passed to the event dispatcher. The ListenerQueue class is used to build a snap shot of the listeners prior to beginning event dispatch. The OSGi framework uses a 2 level listener list for each listener type (4 types). Level one is managed per framework instance and contains the list of BundleContexts which have registered a listener. Level 2 is managed per BundleContext for the listeners in that context. This allows all the listeners of a bundle to be easily and atomically removed from the level one list. To use a "flat" list for all bundles would require the list to know which bundle registered a listener object so that the list could be traversed when stopping a bundle to remove all the bundle's listeners. When an event is fired, a snapshot list (ListenerQueue) must be made of the current listeners before delivery is attempted. The snapshot list is necessary to allow the listener list to be modified while the event is being delivered to the snapshot list. The memory cost of the snapshot list is low since the ListenerQueue object uses the copy-on-write semantics of the CopyOnWriteIdentityMap. This guarantees the snapshot list is never modified once created. The OSGi framework also uses a 2 level dispatch technique (EventDispatcher). Level one dispatch is used by the framework to add the level 2 listener list of each BundleContext to the snapshot in preparation for delivery of the event. Level 2 dispatch is used as the final event deliverer and must cast the listener and event objects to the proper type before calling the listener. Level 2 dispatch will cancel delivery of an event to a bundle that has stopped between the time the snapshot was created and the attempt was made to deliver the event.

    The highly dynamic nature of the OSGi framework had necessitated these features for proper and efficient event delivery.

    Since:
    3.1
    Restriction:
    This class is not intended to be subclassed by clients.
    • Field Detail

      • threadName

        protected final String threadName
        Thread name used for asynchronous event delivery
      • threadGroup

        protected final ThreadGroup threadGroup
        The thread group used for asynchronous event delivery
    • Constructor Detail

      • EventManager

        public EventManager()
        EventManager constructor. An EventManager object is responsible for the delivery of events to listeners via an EventDispatcher.
      • EventManager

        public EventManager​(String threadName)
        EventManager constructor. An EventManager object is responsible for the delivery of events to listeners via an EventDispatcher.
        Parameters:
        threadName - The name to give the event thread associated with this EventManager. A null value is allowed.
      • EventManager

        public EventManager​(String threadName,
                            ThreadGroup threadGroup)
        EventManager constructor. An EventManager object is responsible for the delivery of events to listeners via an EventDispatcher.
        Parameters:
        threadName - The name to give the event thread associated with this EventManager. A null value is allowed.
        threadGroup - The thread group to use for the asynchronous event thread associated with this EventManager. A null value is allowed.
        Since:
        3.4
    • Method Detail

      • close

        public void close()
        This method can be called to release any resources associated with this EventManager.

        Closing this EventManager while it is asynchronously delivering events may cause some events to not be delivered before the async event dispatch thread terminates.