Using Timers

The EJB Timer service is a container-managed service that provides methods to allow callbacks to be scheduled for time-based events.

You can use the Timer service only for stateless session and message-driven beans.

There are four TimerService.createTimer() methods, which create a timer with a different type of configuration. The types are:

single-event timer that expires once;
interval timer that expires repeatedly at a specified interval.

When the time expires the Timer service calls the bean's ejbTimeout() method or a callback method annotated with @javax.ejb.Timeout.

This graphic is explained in the accompanying text.

An enterprise bean typically creates a timer within the scope of a transaction. If the transaction is then rolled back, the timer creation is rolled back.

Procedure

This is an example of an e-mail account manager with a timer. If you do not access the e-mail for 30 days, the account is removed.

First you create a single-event timer that expires 30 days after the e-mail account is created. Every time the user accesses the account, the timer is cancelled and set again.

You also have a timeout callback method, which is called when the timer expires, this means that the user has not accessed the account for more than 30 days.

Creating the Timer

To create a timer, you use the appropriate createTimer() method.

Syntax Syntax

@Stateless

public class EmailManagerFacadeBean implements EmailManagerFacadeRemote {

   private final long THIRTY_DAYS = 1000 * 60 * 60 * 24 * 30;



   @Resource

   private TimerService timerService;



   public void createEmailAccount(String emailAddress) {

      // Source code for creating the actual e-mail account

      // ......



      // Set e-mail account expiration in 30 days if not accessed

      timerService.createTimer(THIRTY_DAYS, emailAddress);

   }

End of the code.

Canceling the Timer

To cancel the timer, you call the timer's cancel method.

Syntax Syntax

   public List<String> getMessages(String emailAddress) {



      // cancel the timer when the e-mail account is accessed

      Collection<Timer> timers = timerService.getTimers();

      for (Timer timer : timers) {

         if (emailAddress.equals(timer.getInfo())) {

            timer.cancel();

         }

      }



      // create a new timer that expires in 30 days

      timerService.createTimer(THIRTY_DAYS, emailAddress);



      // Source code for fetching the email messages

      // ........

   }

End of the code.

Creating the Timeout Callback Method

To register an enterprise bean class of a stateless or message-driven bean with the Timer service for timer callbacks, you have to provide a timeout callback method using the @Timeout annotation. You can use this annotation only once in a bean class.

Syntax Syntax

   @Timeout

   public void removeEmailAccount(Timer timer) {

      String emailAddress = (String) timer.getInfo();



      // Source code for removing the e-mail account

      // ........

   }

}

End of the code.

Annotation Reference

Name	Use	Target
`@Timeout`	Use it to designate a timeout method.	METHOD