OBSERVER PATTERN - Java EE Design Patterns - page 144

Notice that you can register many observers with the NewsAgency and receive updates. Perhaps a
TVChannel observer or an InternetNewsChannel observer can register to receive updates from
the NewsAgency . In addition, you can have other y Publishers (or any other type of object that
implements Observable ) issue updates to any observer that wants to register itself to receive news.
These observers can check the type of the Observable and process the update according to its source.
One signii cant drawback of implementing the observer pattern in this way is that you have to extend
the Observable class. This forces the use of a class hierarchy that might not be desirable. Because you
cannot extend more than one class in the single‐inheritance world of Java, this way of implementing
the observer pattern restricts the inheritance design. You can't add the Observable behavior to an
existing class that already extends another superclass, thus restricting its reuse potential.
But don't despair. You can also implement the observer pattern by “hand,” without using the
internal Observer and Observable interfaces, by following the given class diagram. However,
because this topic is focused on Java EE, this implementation is left for you to play with.

IMPLEMENTING THE OBSERVER PATTERN IN JAVA EE

Although Java had built‐in support for the observer pattern from inception, Java EE offers an easier
implementation via the @Observes annotation and javax.enterprise.event.Event<T> interface. Any
method annotated with @Observes listens for events of a certain type. When it “hears” such an event,
the parameter of the observer method receives an instance of the type and then executes the method.

The @Observes annotation lets any method listen for any event to be i red with the marked object
type. Listing 11‐4 is a simple example of a bean that i res an event of type String and another bean
that listens for events of that type from our bean.

LISTING 11‐4: The observable service bean

package com.devchronicles.observer;

import javax.ejb.Stateless;
import javax.ejb.TransactionAttribute;
import javax.ejb.TransactionAttributeType;
import javax.enterprise.event.Event;
import javax.inject.Inject;

@Stateless
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public class EventService {

@Inject
private String message;

@Inject
Event<String> event;

public void startService(){
event.fire("Starting service " + message);
}
}