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Listing 5.5 The javax.ejb.Timer interface
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public interface javax.ejb.Timer { public void cancel(); public long getTimeRemaining(); public java.util.Date getNextTimeout(); public javax.ejb.TimerHandle getHandle();
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Scheduling: the EJB 3 timer service
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public java.io.Serializable getInfo(); }
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The cancel method is particularly useful in canceling a timer prior to its expiration. You can use this method to stop timers prematurely. In our bid-monitoring example, we can use this method to stop the chain of recurring callbacks when bidding on the item is over. It is vital to invoke the cancel method for recurring Timers when they are no longer needed. Otherwise, the EJB will spin in an infinite loop unnecessarily. This is a subtle, common, and easy mistake to make. The getTimeRemaining method can be used on either a single-use or interval timer. The return value of this method indicates the remaining time for the timer to expire, in milliseconds. You might find that this method is rarely used. The getNextTimeout method indicates the next time a recurring Timer will time out, as a java.util.Date instead of a long time offset. Similar to the getTimeRemaining method, this method is useful in the rare instance that you might need to determine whether to cancel a Timer based on when it will fire next. The getHandle method returns a Timer handle. javax.ejb.TimerHandle is a serialized object that you can store and then use to obtain information about the Timer (by using the getTimer method available through TimerHandle). This is a relatively obscure method that we ll leave for you to explore on your own if you want to. You have already seen the getInfo method in action. This method is extremely useful in writing nontrivial timeout functions and accessing extra processing information attached to the Timer by the bean method creating the Timer. Let s now discuss situations where EJB Timers are an appropriate fit.
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EJB timers and transactions
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EJB Timers are transactional objects. If the transaction that a timer is triggered under rolls back for some reason (e.g., as a result of a runtime exception in the timeout method), the timer creation is undone. In addition, the timeout method can be executed in a transactional context. You can specify a transactional attribute for the timeout method Required or RequiresNew and the container will start a transaction before invoking the timeout method. If the transaction fails, the container will make sure the changes made by the failed method do not take effect and will retry the timeout method. We ll talk about EJB transactions in much greater detail in the next chapter.
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5.4.3 When to use EJB timers
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Clearly, although EJB timers are relatively feature-rich, they are not intended to go toe-to-toe against full-fledged scheduling solutions like Flux or Quartz. However, under some circumstances they are sufficient if not ideal. Like almost all other technology choices, this decision comes down to weighing features against needs for your specific situation and environment. Merits of timers Here are some of the merits of using EJB 3 timers:
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Timers are part of the EJB specification. Hence, applications using EJB timers will remain portable across containers instead of being locked into the nonstandard APIs of job schedulers like Quartz. Since the EJB timer service comes as a standard part of a Java EE application server, using it incurs no additional cost in terms of time or money. No extra installation or configuration is required as would be the case for an external job scheduler, and you won t need to worry about integration and support. The timer is a container-managed service. No separate thread pools or user threads are required for it, as would be the case with an external scheduler. For the same reasons, the EJB timer service is likely to have better out-of-the-box performance than third-party products. Transactions are fully supported with timers (see the sidebar titled EJB timers and transactions ), unlike external job schedulers, in which you may need to do extra setup for supporting JTA. By default, EJB timers are persisted and survive EJB lifecycles and container restarts. The same cannot be said of all third-party schedulers.
Limitations for timers The following are the primary limitations of EJB timers:
EJB timers are meant for long-running business processes and not real-
time applications where precision timing is absolutely critical. Commercial schedulers may provide much better guarantees in terms of precision than the EJB 3 timer service.
EJB timers lack support for extremely flexible cron-type timers, blackout dates, workflow modeling for jobs, and so on. These advanced features are commonly available with external job schedulers.
Summary
There is no robust GUI admin tool to create, manage, and monitor EJB 3 timers. Such tools are generally available for third-party job schedulers.
This concludes our analysis of EJB 3 timers and marks the end of this chapter. In general, you should attempt to use EJB 3 timers first. Resort to third-party schedulers only if you run into serious limitations that cannot be easily overcome. Although robust schedulers are a compelling idea, in general they are complex and should not be used frivolously. However, there are many complex, scheduling-intensive applications where robust schedulers are a must, especially in industries like banking and finance.
5.5 Summary
In this chapter, we covered a few advanced concepts common to all EJB types:
The EJB object acts as a proxy between clients and container where you can use EJBContext to access container runtime information and services. Interceptors are lightweight AOP features in EJB 3 for dealing with crosscutting concerns such as logging and auditing. You can use interceptors at the EJB module level, class level, or method level.
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