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Java has a number of event models, differing in various subtle ways. All of these involve an object (an event source) generating an event in response to some change of state, either in the object itself (for example, if someone has changed a field) or in the external environment (such as when a user moves the mouse). At some earlier stage, a listener (or set of listeners) will have registered interest in this event. When the event source generates an event, it will call suitable methods called on the listeners with the event as parameter. The event models all have their origin in the Observer pattern from Design Patterns, by Eric Gamma et al. (Addison-Wesley, 1995), but this is modified by other pressures, such as JavaBeans. There are low-level input events, which are generated by user actions when they control an application with a graphical user interface. These events of type KeyEvent and MouseEvent are placed in an event queue. They are removed from the queue by a separate thread and dispatched to the relevant objects. In this case, the object that is responsible for generating the event is not responsible for dispatch to listeners, and creation and dispatch of events occurs in different threads. Input events are a special case caused by the need to listen to user interactions and always deal with them without losing response time. Most events are dealt with in a simpler manner: an object maintains its own list of listeners, generates its own events, and dispatches them directly to its listeners. In this category fall all the semantic events generated by the AWT and Swing toolkits, such as ActionEvent, ListSelectionEvent, and so on. There is a large range of these event types, and they all call different methods in the listeners, based on the event name. For example, an ActionEvent is used in a listener s actionPerformed() method of an ActionListener. There are naming conventions involved in this, specified by JavaBeans. JavaBeans is also the influence behind PropertyChange events, which get delivered whenever a bean changes a bound or constrained property value. These are delivered to the
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CHAPTER 16 REMOTE EVENTS
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PropertyChangeListener s propertyChange() method and to the VetoableChangeListener s vetoableChange() method. These events are usually used to signal a change in a field of an object, where this change may be of interest to the listeners either for information or for vetoing. Jini objects may also be interested in changes in other Jini objects, and would like to be listeners for such changes. The networked nature of Jini has led to a particular event model that differs slightly from the other models already in Java. The differences are caused by several factors: Network delivery is unreliable; messages may be lost. Synchronous methods requiring a reply may not work here. Network delivery is time dependent; messages may arrive at different times to different listeners. As a result, the state of an object as perceived by a listener at any time may be inconsistent with the state of that object as perceived by others. Passing complex object state across the network may be more complex to manage than passing simpler information. A remote listener may have disappeared by the time the event occurs. Listeners have to be allowed to time out, like services do. JavaBeans can require method names and event types that vary. This requires the availability of classes across the network, which is more complex than a single method on a single event type (the original Observer pattern used a single method, for simplicity).
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Remote Events
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Unlike the large number of event classes in AWT and Swing (for example), Jini typically uses events of one type, the RemoteEvent, or a small number of subclasses of RemoteEvent. The class has these public methods: package net.jini.core.event; public class RemoteEvent implements java.io.Serializable { public long getID(); public long getSequenceNumber(); public java.rmi.MarshalledObject getRegistrationObject(); } Events in JavaBeans and AWT convey complex object state information, and this is enough for the listeners to act with full knowledge of the changes that have caused the event to be generated. Jini events avoid this and convey just enough information to allow state information to be found if needed. A remote event is serializable and can be moved around the network to its listeners. The listeners then have to decide whether or not they need more detailed information than the simple information in each remote event. If they do need more information, they will have to contact the event source to get it. AWT events, such as MouseEvent, contain an id field that is set to a value such as MOUSE_PRESSED or MOUSE_RELEASED. These fields are not seen by the AWT programmer because the AWT event dispatch system uses the id field to choose an appropriate method, such as mousePressed() or mouseReleased(). Jini does not make these assumptions about event
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