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Dependency injection enables loose-coupling by allowing the implementation of the injected bean type to vary, either at deployment time or runtime. Events go one step further, allowing beans to interact with no compile time dependency at all. Event producers raise events that are delivered to event observers by the container.
This basic schema might sound like the familiar observer/observable pattern, but there are a couple of twists:
The CDI event notification facility uses more or less the same typesafe approach that we’ve already seen with the dependency injection service.
The event object carries state from producer to consumer. The event object is nothing more than an instance of a concrete Java class. (The only restriction is that an event type may not contain type variables). An event may be assigned qualifiers, which allows observers to distinguish it from other events of the same type. The qualifiers function like topic selectors, allowing an observer to narrow the set of events it observes.
An event qualifier is just a normal qualifier, defined using
@Qualifier. Here’s an example:
@Qualifier
@Target({METHOD, FIELD, PARAMETER, TYPE})
@Retention(RUNTIME)
public @interface Updated {}
An observer method is a method of a bean with a parameter annotated
@Observes.
public void onAnyDocumentEvent(@Observes Document document) { ... }
The annotated parameter is called the event parameter. The type of the
event parameter is the observed event type, in this case Document.
The event parameter may also specify qualifiers.
public void afterDocumentUpdate(@Observes @Updated Document document) { ... }
An observer method need not specify any event qualifiers—in this case it is interested in only unqualified events of a particular type. If it does specify qualifiers, it’s only interested in events which have those qualifiers.
The observer method may have additional parameters, which are injection points:
public void afterDocumentUpdate(@Observes @Updated Document document, User user) { ... }
Event producers fire events using an instance of the parameterized
Event interface. An instance of this interface is obtained by
injection:
@Inject @Any Event<Document> documentEvent;
A producer raises events by calling the fire() method of the Event
interface, passing the event object:
documentEvent.fire(document);
This particular event will be delivered to every observer method that:
Document) is
assignable, andThe container simply calls all the observer methods, passing the event
object as the value of the event parameter. If any observer method
throws an exception, the container stops calling observer methods, and
the exception is rethrown by the fire() method.
Qualifiers can be applied to an event in one of two ways:
Event injection point, orselect() of Event.Specifying the qualifiers at the injection point is far simpler:
@Inject @Updated Event<Document> documentUpdatedEvent;
Then, every event fired via this instance of Event has the event
qualifier @Updated. The event is delivered to every observer method
that:
Event injection point.The downside of annotating the injection point is that we can’t specify
the qualifier dynamically. CDI lets us obtain a qualifier instance by
subclassing the helper class AnnotationLiteral. That way, we can pass
the qualifier to the select() method of Event.
documentEvent.select(new AnnotationLiteral<Updated>(){}).fire(document);
Events can have multiple event qualifiers, assembled using any
combination of annotations at the Event injection point and qualifier
instances passed to the select() method.
By default, if there is no instance of an observer in the current context, the container will instantiate the observer in order to deliver an event to it. This behavior isn’t always desirable. We may want to deliver events only to instances of the observer that already exist in the current contexts.
A conditional observer is specified by adding receive = IF_EXISTS to
the @Observes annotation.
public void refreshOnDocumentUpdate(@Observes(receive = IF_EXISTS) @Updated Document d) { ... }
A bean with scope @Dependent cannot be a conditional observer, since
it would never be called!
An event qualifier type may have annotation members:
@Qualifier
@Target({METHOD, FIELD, PARAMETER, TYPE})
@Retention(RUNTIME)
public @interface Role {
RoleType value();
}
The member value is used to narrow the messages delivered to the observer:
public void adminLoggedIn(@Observes @Role(ADMIN) LoggedIn event) { ... }
Event qualifier type members may be specified statically by the event producer, via annotations at the event notifier injection point:
@Inject @Role(ADMIN) Event<LoggedIn> loggedInEvent;
Alternatively, the value of the event qualifier type member may be
determined dynamically by the event producer. We start by writing an
abstract subclass of AnnotationLiteral:
abstract class RoleBinding
extends AnnotationLiteral<Role>
implements Role {}
The event producer passes an instance of this class to select():
documentEvent.select(new RoleBinding() {
public void value() { return user.getRole(); }
}).fire(document);
Event qualifiers may be combined, for example:
@Inject @Blog Event<Document> blogEvent;
...
if (document.isBlog()) blogEvent.select(new AnnotationLiteral<Updated>(){}).fire(document);
An observer method is only notified if all the observed qualifiers are specified when the event is fired. Assume the following observers in this example:
public void afterBlogUpdate(@Observes @Updated @Blog Document document) { ... }
public void afterDocumentUpdate(@Observes @Updated Document document) { ... }
public void onAnyBlogEvent(@Observes @Blog Document document) { ... }
public void onAnyDocumentEvent(@Observes Document document) { ... }}}
All of these observer methods will be notified.
However, if there were also an observer method:
public void afterPersonalBlogUpdate(@Observes @Updated @Personal @Blog Document document) { ... }
It would not be notified, as @Personal is not a qualifier of the event
being fired.
Transactional observers receive their event notifications during the
before or after completion phase of the transaction in which the event
was raised. For example, the following observer method needs to refresh
a query result set that is cached in the application context, but only
when transactions that update the Category tree succeed:
public void refreshCategoryTree(@Observes(during = AFTER_SUCCESS) CategoryUpdateEvent event) { ... }
There are five kinds of transactional observers:
IN_PROGRESS observers are called immediately (default)AFTER_SUCCESS observers are called during the after completion phase
of the transaction, but only if the transaction completes successfullyAFTER_FAILURE observers are called during the after completion phase
of the transaction, but only if the transaction fails to complete
successfullyAFTER_COMPLETION observers are called during the after completion
phase of the transactionBEFORE_COMPLETION observers are called during the before completion
phase of the transactionTransactional observers are very important in a stateful object model because state is often held for longer than a single atomic transaction.
Imagine that we have cached a JPA query result set in the application scope:
import javax.ejb.Singleton;
import javax.enterprise.inject.Produces;
@ApplicationScoped @Singleton
public class Catalog {
@PersistenceContext EntityManager em;
List<Product> products;
@Produces @Catalog
List<Product> getCatalog() {
if (products==null) {
products = em.createQuery("select p from Product p where p.deleted = false")
.getResultList();
}
return products;
}
}
From time to time, a Product is created or deleted. When this occurs,
we need to refresh the Product catalog. But we should wait until
after the transaction completes successfully before performing this
refresh!
The bean that creates and deletes `Product`s could raise events, for example:
import javax.enterprise.event.Event;
@Stateless
public class ProductManager {
@PersistenceContext EntityManager em;
@Inject @Any Event<Product> productEvent;
public void delete(Product product) {
em.delete(product);
productEvent.select(new AnnotationLiteral<Deleted>(){}).fire(product);
}
public void persist(Product product) {
em.persist(product);
productEvent.select(new AnnotationLiteral<Created>(){}).fire(product);
}
...
}
And now Catalog can observe the events after successful completion of
the transaction:
import javax.ejb.Singleton;
@ApplicationScoped @Singleton
public class Catalog {
...
void addProduct(@Observes(during = AFTER_SUCCESS) @Created Product product) {
products.add(product);
}
void removeProduct(@Observes(during = AFTER_SUCCESS) @Deleted Product product) {
products.remove(product);
}
}