Contexts and Dependency Injection for the Java EE Platform: Advanced Topics - The Java EE 6 Tutorial: Advanced Topics - page 267

import javax.transaction.UserTransaction;
...
public class TransactionServlet extends HttpServlet {
@Resource UserTransaction transaction;
...
}
Using Events in CDI Applications
Events allow beans to communicate without any compile-time dependency. One bean can
define an event, another bean can fire the event, and yet another bean can handle the event.
The beans can be in separate packages and even in separate tiers of the application.
Defining Events
An event consists of the following:
• The event object, a Java object
• Zero or more qualifier types, the event qualifiers
For example, in the billpayment example described in “ The billpayment
Example: Using Events and Interceptors ” on page 280 , a PaymentEvent bean defines
an event using three properties, which have setter and getter methods:
public String paymentType;
public BigDecimal value;
public Date datetime;
public PaymentEvent() {
}
The example also defines qualifiers that distinguish between two kinds of Pay-
mentEvent . Every event also has the default qualifier @Any .
Using Observer Methods to Handle Events
An event handler uses an observer method to consume events.
Each observer method takes as a parameter an event of a specific event type that is an-
notated with the @Observes annotation and with any qualifiers for that event type. The
observer method is notified of an event if the event object matches the event type and if
all the qualifiers of the event match the observer method event qualifiers.
The observer method can take other parameters in addition to the event parameter. The
additional parameters are injection points and can declare qualifiers.