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Listing 12.3 Fine-grained session beans
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@Stateless public class ItemManagerBean implements ItemManager { ... Item findByItemId(Long itemId) { ... } } @Stateless public class BidManagerBean implements BidManager { Bid findHighestBid(Item item) { ... }
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ItemManager EJB
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BidManager EJB
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Bid createBid(Item item, Bidder bidder, Double bidPrice) { ... } } @Stateless public class UserManagerBean implements UserManager { Bidder findByUserId(String userId) { } }
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In listing 12.3, we have three fine-grained session beans. UserManagerBean d has a method named findByUserId, ItemManagerBean b has a findItemByItemId method that helps find items by Id, and BidManagerBean c exposes two methods: findHighestBid and createBid, to find the highest bid item and create a new bid, respectively. You can expose these session beans to the client, and expect the client to perform the business logic as follows:
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public class @EJB private @EJB private @EJB private ... ActionBazaarServlet implements GenericServlet { ItemManager itemManager; UserManager userManager; BidManager bidManager;
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public void createBid(String userId, Long itemId, Double bidPrice) throws BidException { Item item = itemManager.findByItemId(itemId); Bid highBid = bidManager.findHighestBid(item); if (bidPrice <= highBid.getBidPrice()) { throw new BidException("Bid Price is lower than the current bid price"); } ... Bid bid = bidManager.createBid(item, bidder, bidPrice); } }
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The problem with this approach is that the client is coupled to the persistence and internals of the business logic layer. It quickly becomes a nightmare to manage the security for so many session beans. Also, some of the real business logic gets scattered in multiple places in the code. A better option is to create a session fa ade like the PlaceBidBean EJB shown here, which uses these operations and exposes PlaceBidBean to the client:
@PersistenceContext(unitName = "actionBazaar", name = "actionBazaar") @Stateless(name = "PlaceBid") public class PlaceBidBean implements PlaceBid {
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public PlaceBidBean() { } public Long addBid(String userId, Long itemId, Double bidPrice) throws BidException { ... Item item = itemEAO.findByItemId(itemId); Bid highBid = itemEAO.findHighestBidForItem(item); if (bidPrice <= highBid.getBidPrice()) { throw new BidException("Bid Price is lower than the current bid price"); } ... return bidEAO.addBid(item, userId, bidPrice); } }
There are several merits to the Session Fa ade pattern. First, it centralizes the real business logic in a single place instead of duplicating it many times. Second, with this pattern the clients don t have to know the internal design of the system. Third, it helps to manage transactions and security centrally instead of managing it for multiple session beans. A final advantage is that it makes the client code much simpler. Take a look at how much the client knowledge is reduced using this approach:
@EJB private PlaceBid placeBid; ... Bid bid = placeBid.addBid(itemId, userId, bidPrice);
If you decide to expose EJBs to remote clients, you will see a dramatic improvement in performance. In addition, such EJBs may be good candidates for being exposed as web services, enhancing reusability and the ability to distribute your enterprise applications. Stateful vs. stateless session fa ade As we saw in the PlaceBid EJB example, most business logic spans only a single method call; therefore, most of your session fa ades will be stateless in nature. You may choose to make your session fa ade stateful if your business requires multiple method invocations as a part of some business process and you must maintain conversational state. A stateful session fa ade also has the benefit of an extended persistence context (discussed in chapter 9) that helps you keep the entities in a managed state that may span across a transaction, and the persistence context is automatically closed when the bean instance is removed. For example, the user registration process is a multistep process. You want to keep the
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entities managed during the registration process and end the persistence context when the user has completed the registration process. A stateful session fa ade may be useful in such situations. This finishes our discussion on EJB design patterns. If you are interested in exploring other design patterns from Sun s blueprints website, please visit http:// java.sun.com/reference/blueprints/index.html. A book about EJB must have an EJB -centric view of the world, mustn t it In other words, you are probably using a Java EE container with EJBs, or you are using a totally different framework that is incompatible with everything related to EJB 3, right These statements are true neither of the authors nor of reality. It is useful to know that many aspects of the EJB 3 specification are both available and compatible with alternate approaches to enterprise Java development. The next section delves into some of the options that you may be interested in implementing on your next project.
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