Seam makes it very easy to perform work asynchronously from a web request. When most people think of asynchronicity in Java EE, they think of using JMS. This is certainly one way to approach the problem in Seam, and is the right way when you have strict and well-defined quality of service requirements. Seam makes it easy to send and recieve JMS messages using Seam components.
But for many usecases, JMS is overkill. Seam layers a simple asynchronous method and event facility over your choice of dispatchers:
java.util.concurrent.ScheduledThreadPoolExecutor (by default)
the EJB timer service (for EJB 3.0 environments)
Quartz
Asynchronous events and method calls have the same quality of service expectations as the underlying dispatcher mechanism. The default dispatcher, based upon a ScheduledThreadPoolExecutor performs efficiently but provides no support for persistent asynchronous tasks, and hence no guarantee that a task will ever actually be executed. If you're working in an environment that supports EJB 3.0, and add the following line to components.xml:
<async:timer-service-dispatcher/>
then your asynchronous tasks will be processed by the container's EJB timer service. If you're not familiar with the Timer service, don't worry, you don't need to interact with it directly if you want to use asynchronous methods in Seam. The important thing to know is that any good EJB 3.0 implementation will have the option of using persistent timers, which gives some guarantee that the tasks will eventually be processed.
Another alternative is to use the open source Quartz library to manage asynchronous method. You need to bundle the Quartz library JAR (found in the lib directory) in your EAR and declare it as a Java module in application.xml. In addition, you need to add the following line to components.xml to install the Quartz dispatcher.
<async:quartz-dispatcher/>
The Seam API for the default ScheduledThreadPoolExecutor, the EJB3 Timer, and the Quartz Scheduler are largely the same. They can just "plug and play" by adding a line to components.xml.
In simplest form, an asynchronous call just lets a method call be processed asynchronously (in a different thread) from the caller. We usually use an asynchronous call when we want to return an immediate response to the client, and let some expensive work be processed in the background. This pattern works very well in applications which use AJAX, where the client can automatically poll the server for the result of the work.
For EJB components, we annotate the local interface to specify that a method is processed asynchronously.
@Local
public interface PaymentHandler
{
@Asynchronous
public void processPayment(Payment payment);
}(For JavaBean components we can annotate the component implementation class if we like.)
The use of asynchronicity is transparent to the bean class:
@Stateless
@Name("paymentHandler")
public class PaymentHandlerBean implements PaymentHandler
{
public void processPayment(Payment payment)
{
//do some work!
}
}And also transparent to the client:
@Stateful
@Name("paymentAction")
public class CreatePaymentAction
{
@In(create=true) PaymentHandler paymentHandler;
@In Bill bill;
public String pay()
{
paymentHandler.processPayment( new Payment(bill) );
return "success";
}
}The asynchronous method is processed in a completely new event context and does not have access to the session or conversation context state of the caller. However, the business process context is propagated.
Asynchronous method calls may be scheduled for later execution using the @Duration, @Expiration and @IntervalDuration annotations.
@Local
public interface PaymentHandler
{
@Asynchronous
public void processScheduledPayment(Payment payment, @Expiration Date date);
@Asynchronous
public void processRecurringPayment(Payment payment,
@Expiration Date date,
@IntervalDuration Long interval)'
}@Stateful
@Name("paymentAction")
public class CreatePaymentAction
{
@In(create=true) PaymentHandler paymentHandler;
@In Bill bill;
public String schedulePayment()
{
paymentHandler.processScheduledPayment( new Payment(bill), bill.getDueDate() );
return "success";
}
public String scheduleRecurringPayment()
{
paymentHandler.processRecurringPayment( new Payment(bill), bill.getDueDate(),
ONE_MONTH );
return "success";
}
}Both client and server may access the Timer object associated with the invocation. The Timer object shown below is the EJB3 timer when you use the EJB3 dispatcher. For the default ScheduledThreadPoolExecutor, the returned object is Future from the JDK. For the Quartz dispatcher, it returns QuartzTriggerHandle, which we will discuss in the next section.
@Local
public interface PaymentHandler
{
@Asynchronous
public Timer processScheduledPayment(Payment payment, @Expiration Date date);
}@Stateless
@Name("paymentHandler")
public class PaymentHandlerBean implements PaymentHandler
{
@In Timer timer;
public Timer processScheduledPayment(Payment payment, @Expiration Date date)
{
//do some work!
return timer; //note that return value is completely ignored
}
}@Stateful
@Name("paymentAction")
public class CreatePaymentAction
{
@In(create=true) PaymentHandler paymentHandler;
@In Bill bill;
public String schedulePayment()
{
Timer timer = paymentHandler.processScheduledPayment( new Payment(bill),
bill.getDueDate() );
return "success";
}
}Asynchronous methods cannot return any other value to the caller.
The Quartz dispatcher (see earlier on how to install it) allows you to use the @Asynchronous, @Duration, @Expiration, and @IntervalDuration annotations as above. But it has some powerful additional features. The Quartz dispatcher supports three new annotations.
The @FinalExpiration annotation specifies an end date for the recurring task.
// Defines the method in the "processor" component
@Asynchronous
public QuartzTriggerHandle schedulePayment(@Expiration Date when,
@IntervalDuration Long interval,
@FinalExpiration Date endDate,
Payment payment)
{
// do the repeating or long running task until endDate
}
... ...
// Schedule the task in the business logic processing code
// Starts now, repeats every hour, and ends on May 10th, 2010
Calendar cal = Calendar.getInstance ();
cal.set (2010, Calendar.MAY, 10);
processor.schedulePayment(new Date(), 60*60*1000, cal.getTime(), payment);
Note that the method returns the QuartzTriggerHandle object, which you can use later to stop, pause, and resume the scheduler. The QuartzTriggerHandle object is serializable, so you can save it into the database if you need to keep it around for extended period of time.
QuartzTriggerHandle handle =
processor.schedulePayment(payment.getPaymentDate(),
payment.getPaymentCron(),
payment);
payment.setQuartzTriggerHandle( handle );
// Save payment to DB
// later ...
// Retrieve payment from DB
// Cancel the remaining scheduled tasks
payment.getQuartzTriggerHandle().cancel();
The @IntervalCron annotation supports Unix cron job syntax for task scheduling. For instance, the following asynchronous method runs at 2:10pm and at 2:44pm every Wednesday in the month of March.
// Define the method
@Asynchronous
public QuartzTriggerHandle schedulePayment(@Expiration Date when,
@IntervalCron String cron,
Payment payment)
{
// do the repeating or long running task
}
... ...
// Schedule the task in the business logic processing code
QuartzTriggerHandle handle =
processor.schedulePayment(new Date(), "0 10,44 14 ? 3 WED", payment);
The @IntervalBusinessDay annotation supports invocation on the "nth Business Day" scenario. For instance, the following asynchronous method runs at 14:00 on the 2nd business day of each month. By default, it excludes all weekends and US federal holidays until 2010 from the business days.
// Define the method
@Asynchronous
public QuartzTriggerHandle schedulePayment(@Expiration Date when,
@IntervalBusinessDay NthBusinessDay nth,
Payment payment)
{
// do the repeating or long running task
}
... ...
// Schedule the task in the business logic processing code
QuartzTriggerHandle handle =
processor.schedulePayment(new Date(),
new NthBusinessDay(2, "14:00", WEEKLY), payment);
The NthBusinessDay object contains the configuration of the invocation trigger. You can specify more holidays (e.g., company holidays, non-US holidays etc.) via the additionalHolidays property.
public class NthBusinessDay implements Serializable
{
int n;
String fireAtTime;
List <Date> additionalHolidays;
BusinessDayIntervalType interval;
boolean excludeWeekends;
boolean excludeUsFederalHolidays;
public enum BusinessDayIntervalType { WEEKLY, MONTHLY, YEARLY }
public NthBusinessDay ()
{
n = 1;
fireAtTime = "12:00";
additionalHolidays = new ArrayList <Date> ();
interval = BusinessDayIntervalType.WEEKLY;
excludeWeekends = true;
excludeUsFederalHolidays = true;
}
... ...
}
The @IntervalDuration, @IntervalCron, and @IntervalNthBusinessDay annotations are mutually exclusive. If they are used in the same method, a RuntimeException will be thrown.
Component-driven events may also be asynchronous. To raise an event for asynchronous processing, simply call the raiseAsynchronousEvent() method of the Events class. To schedule a timed event, call the raiseTimedEvent() method, passing a schedule object (for the default dispatcher or timer service dispatcher, use TimerSchedule). Components may observe asynchronous events in the usual way, but remember that only the business process context is propagated to the asynchronous thread.
Seam makes it easy to send and receive JMS messages to and from Seam components.
To configure Seam's infrastructure for sending JMS messages, you need to tell Seam about any topics and queues you want to send messages to, and also tell Seam where to find the QueueConnectionFactory and/or TopicConnectionFactory.
Seam defaults to using UIL2ConnectionFactory which is the usual connection factory for use with JBossMQ. If you are using some other JMS provider, you need to set one or both of queueConnection.queueConnectionFactoryJndiName and topicConnection.topicConnectionFactoryJndiName in seam.properties, web.xml or components.xml.
You also need to list topics and queues in components.xml to install Seam managed TopicPublishers and QueueSenders:
<jms:managed-topic-publisher name="stockTickerPublisher"
auto-create="true"
topic-jndi-name="topic/stockTickerTopic"/>
<jms:managed-queue-sender name="paymentQueueSender"
auto-create="true"
queue-jndi-name="queue/paymentQueue"/>Now, you can inject a JMS TopicPublisher and TopicSession into any component:
@In
private TopicPublisher stockTickerPublisher;
@In
private TopicSession topicSession;
public void publish(StockPrice price) {
try
{
topicPublisher.publish( topicSession.createObjectMessage(price) );
}
catch (Exception ex)
{
throw new RuntimeException(ex);
}
}Or, for working with a queue:
@In
private QueueSender paymentQueueSender;
@In
private QueueSession queueSession;
public void publish(Payment payment) {
try
{
paymentQueueSender.send( queueSession.createObjectMessage(payment) );
}
catch (Exception ex)
{
throw new RuntimeException(ex);
}
}You can process messages using any EJB3 message driven bean. Message-driven beans may even be Seam components, in which case it is possible to inject other event and application scoped Seam components.
Seam Remoting lets you subscribe to a JMS topic from client-side JavaScript. This is described in Chapter 21, Remoting.