birt barcode tool Scheduling: the EJB 3 timer service in Java

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Scheduling: the EJB 3 timer service
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As the figure demonstrates, an EJB method can register a time-driven callback with the container timer service. When the time interval specified by the EJB expires, the timer service invokes the timeout method pointed to by the EJB. We ll show you how this works with a simple example next.
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5.4.2 Using the timer service
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Let s explore the features of the EJB 3 timer service by adding a timer to the PlaceBid EJB we introduced in chapter 2. We ll add a timer in the addBid method to check the status of the newly placed bid every 15 minutes. Although we won t code it, another compelling use case is to create a timer when an item is added for bidding. Such a timer could be triggered when the auction time expired and would determine the winning bidder. We ll leave the implementation of this timer as an exercise for you. Among other things, the timer we ll implement will notify the bidder via e-mail if they have been outbid. We have omitted most of the code that is not absolutely necessary to explain timer functionality in listing 5.3. The complete code is included in the downloadable code samples if you are interested in exploring further.
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Listing 5.3 Using the EJB 3 timer service
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public class PlaceBidBean implements PlaceBid { ... @Resource TimerService timerService; Timer service injected ... public void addBid(Bid bid) { ... Code to add the bid ... timerService.createTimer(15*60*1000, 15*60*1000, bid); ... Timer created } Timeout method ... @Timeout public void monitorBid(Timer timer) { Bid bid = (Bid) timer.getInfo(); ... Code to monitor the bid ... } }
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We use EJB 3 resource injection to gain access to the timer service b. In the addBid method, after we add the bid we schedule a timer service callback to occur every 15 minutes C. The newly added Bid is attached as timer information when the timer is registered. At regular intervals, the monitorBid method is
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Learning advanced EJB concepts
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called by the timer service, which is designated with the @Timeout annotation D. The monitorBid method retrieves the Bid instance attached as timer information and monitors the bid. We ll explore EJB timer services details using listing 5.3 as a jumping-off point in the next few sections, starting with ways to get access to the EJB 3 timer service. Accessing the timer service As you just saw in listing 5.3, the EJB timer service can be injected into a Java EE component using the @Resource annotation. Alternatively, you can get access to the container timer service through the EJB context:
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@Resource SessionContext context; ... TimerService timerService = context.getTimerService();
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Which method you choose is largely a matter of taste. In general, if you are already injecting an EJB context, you should avoid injecting the timer service as well, in order to avoid redundant code. Instead, you should use the getTimerService method as in the previous code. However, if you aren t using the EJB context for anything else, it makes perfect sense to simply inject the TimerService as we did in listing 5.3. Next let s take a closer look at the injected timer service. Using the TimerService interface In listing 5.3, we use the TimerService interface to register a Timer. As you ll soon see, a Timer is simply a Java EE representation of a scheduled task. The createTimer method used in listing 5.3 is one of four overloaded methods provided in the TimerService interface to add Timers. The one we used specified that the Timer should initially trigger in 15*60*1000 milliseconds (15 minutes), repeat every 15*60*1000 milliseconds (15 minutes), and added a Bid instance as Timer information:
timerService.createTimer(15*60*1000, 15*60*1000, bid);
Let s take a look at the complete definition of the TimerService interface to get a clearer picture of the range of options available (listing 5.4).
Listing 5.4 Specification for the TimerService interface is used to create either single-event or recurring timers
public interface javax.ejb.TimerService { public Timer createTimer(long duration, java.io.Serializable info);
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