Java Reference
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with the name of the event they want to listen to. This method creates a new
instance of class DSubscription and inserts it into the list of observers. In
order to be registered as observers, remote clients must implement the
updateEvent() method of interface DObserver .
Observable servers notify events to their remote observer clients accord-
ing to the push model by invoking method notifyObservers() of class DActivity .
This method looks for registered observers in its DSubscription list and
invokes their remote method updateEvent() .
Since DActivity objects can play the roles of both observable and observer,
we extend class DActivity in order provide a standard implementation of
method updateEvent() : it creates a temporary thread that invokes the
abstract method update() , which should be implemented in every subclass of
DActivity .
Please note how important is to use a temporary thread in method
updateEvent() . This approach decouples the remote observable service from
the actual execution of the observer's update() method. In fact, the update()
method might be a complex and time-consuming procedure that must be
executed by the observer's thread of execution. If we did not follow this
approach, the update() method of every registered observer would be
executed by the observable service itself.
The implementation of the class diagram depicted in Figure 19.7 reuses part
of the code written for Prototype 1 of Chapter 9 (Manufacturing work cell)
and for Prototype 1 of Chapter 11 (Car parking).
Class DEvent represents the generic event that is notified by a distributed
control module to its observers. The broadcaster control module is the
owner of the event and is identified by its URL.
Class DSubscription records the association between an event, an observer
and an observable. The observer's URL is used to obtain an RMI reference of
the observer control module.
Class DActivity is extended in order to implement the methods defined in
interface DObserver and interface Observable .
package mmi;
import java.rmi.*;
public class DEvent implements Serializable{
public String name; // the name of the event
public Object value; // the data associated with the event
public String ownerURL;
// the process that activates this event
public DEvent(String name, String ownerURL, Object value) {
this .name # name;
this .ownerURL # ownerURL;
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