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As we noted previously, distributed notification differs from local notification (such as the notification used in user interface programming) in a number of ways In particular, a distributed notification may be delayed, dropped, or otherwise fail between the object in which the event occurred and the object that is the ultimate recipient of the notification of that event Additionally, a distributed event notification may require handling by a number of third party objects between the object that is interested in the notification and the object that generates the notification These third party objects need to be able to handle arbitrary events, and so from the point of view of the type system, all of the events must be delivered in the same fashion Although this model differs from the event model used for user interface tools such as the AWT or Java Foundation Classes (JFC), such a difference in model is to be expected The event model for such user interface toolkits was never meant to allow the components that communicate using these local event notifications to be distributed across virtual or physical machines; indeed, such systems assume that the event delivery will be fast, reliable, and not open to the kinds of partial failures or delays that are common in the distributed case In between the requirements of a local event model and the distributed event model presented here is the event model used by software components to communicate changes in state The delegation event model, which is the event model for JavaBeans components, written in the Java programming language, is built as an extension of the event model used for AWT and JFC This is completely appropriate, as most JavaBeans components will be located in a single address space and can assume that the communication of events between components will meet the reliability and promptness requirements of that model However, it is also possible that JavaBeans components will be distributed across virtual, and even physical, machines The assumption that the event propagation will be either fast or reliable can lead to subtle program errors that will not be found until the components are deployed (perhaps on a slow or unreliable network) In such case, an event and notification model such as that found in this specification is more appropriate One approach would be to add a second event model to the JavaBeans component specification that dealt only with distributed events While this would have the advantage of exporting the difference between local and remote components to the component builder, it would also complicate the JavaBeans component model unnecessarily We will show how the current distributed event model can be fit into the existing Java platform's event model While the mapping is not perfect (nor can it be, since there are essential differences between the two models), it will allow the current tools used to assemble JavaBeans components to be used when those components are distributed EV41 Differences with the JavaBeans Component Event Model The JavaBeans component event model is derived from the event model used in the AWT in the Java platform The model is characterized by: Propagation of event notifications from sources to listeners by Java technology method invocations on the target listener objects Identification of the kind of event notification by using a different method in the listener being called for each kind of event
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O'Reilly Network Safari Bookshelf Jini Specifications, The, Second Edition Encapsulation of any state associated with an event notification in an object that inherits from javautilEventObject and that is passed as the sole argument of the notification method Identification of event sources by the convention of those sources defining registration methods, one for each kind of event in which interest can be registered, that follow a particular design pattern The distributed event and notification model that we have defined is similar in a number of ways: Distributed event propagation is accomplished by the use of Remote methods State passed as part of the notification is encapsulated in an object that is derived from javautilEventObject and is passed as the sole argument of the notification method The RemoteEventListener interface extends the more basic interface javautilEventListener However, there are also differences between the JavaBeans component event model and the distributed event model proposed here: Identification of the kind of event is accomplished by passing an identifier from the source of the notification to the listener; the combination of the object in which the event occurred and the identifier uniquely identifies the kind of event Notifications are accomplished through a single method, notify, defined in the RemoteEventListener interface rather than by a different method for each kind of event Registration of interest in a kind of event is for a (perhaps renewable) period of time, rather than being for a period of time bound by the active cancellation of interest Objects registering interest in an event can, as part of that registration, include an object that will be passed back to the recipient of the notification when an event of the appropriate type occurs Most of these differences in the two models can be directly traced to the distributed nature of the events and notifications defined in this specification For example, as you have seen, reliability and recovery of the distributed notification model is based on the ability to create third party objects that can provide those guarantees However, for those third party objects to be able to work in general cases, the signature for a notification must be the same for all of the event notifications that are to be handled by that third party If we were to follow the JavaBeans component model of having a different method for each kind of event notification, third party objects would need to support every possible notification method, including those that had not yet been defined when the third party object was implemented This is clearly impossible Note that this is not a weakness in the JavaBeans component event model, merely a difference required by the different environments in which the event models are assumed to be used The JavaBeans component event model, like the AWT model on which it is based, assumes that the event notification is being passed between objects in the same address space Such notifications do not need various delivery and reliability guarantees delivery can be considered to be (virtually) instantaneous and can be assumed to be fully reliable Being able to send event notifications through a single Remote method also requires that the events be identified in some way other than the signature of the notification delivery method This leads to the inclusion of an event identifier in the event object Since the generation of these event identifiers cannot be guaranteed to be globally unique across all of the objects in a distributed system, they must be made relative to the object in which they are generated, thus requiring the combination of the object of origin and the event identifier to completely identify the kind of event The sequence number being included in the event object is also an outgrowth of the distributed nature of the interfaces Since no distributed mechanism can guarantee reliability, there is always the possibility that a particular notification will not be delivered, or could be delivered more than once by some notification agent This is not a problem in the single address space environment of AWT and JavaBeans components, but requires the inclusion of a sequence number in the distributed case EV4 Integration with JavaBeans Components 119
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O'Reilly Network Safari Bookshelf Jini Specifications, The, Second Edition EV42 Converting Distributed Events to JavaBeans Component Events Translating between the event models is fairly straightforward All that is required is: Allow an event listener to map from a distributed event listener to the appropriate call to a notification method Allow creation of a RemoteEvent from the event object passed in the JavaBeans component event notification method Allow creation of a JavaBeans component event object from a RemoteEvent object without loss of information Each of these is fairly straightforward and can be accomplished in a number of ways More complex matings of the two systems could be undertaken, including third party objects that keep track of the interest registrations made by remote objects and implement the corresponding JavaBeans component event notification methods by making the remote calls to the RemoteEventListener notify method with properly constructed RemoteEvent objects Such objects would need to keep track of the event sequence numbers and would need to deal with the additional failure modes that are inherent in distributed calls However, their implementation would be fairly straightforward and would fit into the JavaBeans component model of event adapters
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