Using Events in CDI Applications (original) (raw)

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, 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 PaymentEvent. 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 annotated 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.

The event handler for the billpayment example, PaymentHandler, defines two observer methods, one for each type of PaymentEvent:

public void creditPayment(@Observes @Credit PaymentEvent event) { ... }

public void debitPayment(@Observes @Debit PaymentEvent event) { ... }

Observer methods can also be conditional or transactional:

A conditional observer method is notified of an event only if an instance of the bean that defines the observer method already exists in the current context. To declare a conditional observer method, specify notifyObserver=IF_EXISTS as an argument to @Observes:
@Observes(notifyObserver=IF_EXISTS)
To obtain the default unconditional behavior, you can specify @Observes(notifyObserver=ALWAYS).
A transactional observer method is notified of an event during the before-completion or after-completion phase of the transaction in which the event was fired. You can also specify that the notification is to occur only after the transaction has completed successfully or unsuccessfully. To specify a transactional observer method, use any of the following arguments to @Observes:
@Observes(during=BEFORE_COMPLETION)
@Observes(during=AFTER_COMPLETION)
@Observes(during=AFTER_SUCCESS)
@Observes(during=AFTER_FAILURE)
To obtain the default non-transactional behavior, specify @Observes(during=IN_PROGRESS).
An observer method that is called before completion of a transaction may call the setRollbackOnly method on the transaction instance to force a transaction rollback.

Observer methods may throw exceptions. If a transactional observer method throws an exception, the exception is caught by the container. If the observer method is non-transactional, the exception terminates processing of the event, and no other observer methods for the event are called.

Firing Events

To activate an event, call the javax.enterprise.event.Event.fire method. This method fires an event and notifies any observer methods.

In the billpayment example, a managed bean called PaymentBean fires the appropriate event by using information it receives from the user interface. There are actually four event beans, two for the event object and two for the payload. The managed bean injects the two event beans. The pay method uses aswitch statement to choose which event to fire, using new to create the payload.

@Inject
@Credit
Event<PaymentEvent> creditEvent;

@Inject
@Debit
Event<PaymentEvent> debitEvent;

private static final int DEBIT = 1;
private static final int CREDIT = 2;
private int paymentOption = DEBIT;
...

@Logged
public String pay() {
    ...
    switch (paymentOption) {
        case DEBIT:
            PaymentEvent debitPayload = new PaymentEvent();
            // populate payload ... 
            debitEvent.fire(debitPayload);
            break;
        case CREDIT:
            PaymentEvent creditPayload = new PaymentEvent();
            // populate payload ... 
            creditEvent.fire(creditPayload);
            break;
        default:
            logger.severe("Invalid payment option!");
    }
    ...
}

The argument to the fire method is a PaymentEvent that contains the payload. The fired event is then consumed by the observer methods.