how to generate barcode in vb.net 2008 An EJB 3 based solution in Java

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2.2.2 An EJB 3 based solution
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Figure 2.2 shows how the ActionBazaar scenario in the previous section can be implemented using EJB 3 in a traditional four-tier layering scheme. For our purposes, the presentation tier is essentially an amorphous blob that generates business-tier requests in response to user actions. If you examine the scenario in figure 2.2, you ll see that only two processes are triggered by the user adding a bid to an item and ordering items won. One more process might be apparent: the background billing process to charge the order, triggered by order confirmation. If you guessed that the billing process is triggered through a message,
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A first taste of EJB
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you guessed right. As you can see in figure 2.3, the bidding and ordering processes are implemented as session beans (PlaceBidBean and PlaceOrderBean) in the business logic tier. On the other hand, the billing process is implemented as an MDB (OrderBillingMDB) since it is triggered by a message sent from the PlaceOrderBean instead of a direct user request. All three of the processes persist data. The PlaceBidBean needs to add a bid record to the database. Similarly, the PlaceOrderBean must add an order record. Alternatively, the OrderBillingMDB updates the order record to reflect the results of the billing process. These database changes are performed through two entities in the JPA-managed persistence tier the Bid and Order entities. While the PlaceBidBean uses the Bid entity, the PlaceOrderBean and OrderBillingMDB use the Order entity.
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Figure 2.3 The ActionBazaar scenario implemented using EJB 3. From the EJB 3 perspective, the presentation layer is an amorphous blob that generates business-tier requests. The business-logic tier components match up with the distinct processes in the scenario putting a bid on an item, ordering the item won, and billing the user. The billing MDB is triggered by a message sent by the order confirmation process. The business-tier components use JPA entities to persist application state into the database.
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Building business logic with session beans
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Recall that although JPA entities contain ORM configuration, they do not persist themselves. As you ll see in the actual code solutions, the business-tier components have to use the JPA EntityManager API to add, delete, update, and retrieve entities as needed. If your mental picture matches up with figure 2.3 pretty closely, it is likely the code we are going to present next will seem intuitive too, even though you don t know EJB 3. In the following sections, we explore each of the EJB 3 component types using our scenario. Without further ado, we can now begin our whirlwind tour of EJB 3 component types, starting with the session beans in the business-logic tier.
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2.3 Building business logic with session beans
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Session beans are meant to model business processes or actions, especially as perceived by the system user. This is why they are ideal for modeling the bidding and ordering processes in our scenario. Session beans are the easiest but most versatile part of EJB. Recall that session beans come in two flavors: stateful and stateless. We ll take on stateless session beans first, primarily because they are simpler. You ll then discover how you can add statefulness to the ActionBazaar application by using a stateful session bean. Along the way, we introduce you to an example of a session bean client in a web tier, and then build a standalone Java client for a session bean.
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2.3.1 Using stateless beans
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Stateless session beans are used to model actions or processes that can be done in one shot, such as placing a bid on an item in our ActionBazaar scenario. The addBid bean method in listing 2.1 is called from the ActionBazaar web tier when a user decides to place a bid. The parameter to the method, the Bid object, represents the bid to be placed. The Bid object contains the ID of the bidder placing the bid, the ID of the item being bid on, and the bid amount. As we know, all the method needs to do is save the passed-in Bid data to the database. In a real application, you would see more validation and error-handling code in the addBid method. Since the point is to show you what a session bean looks like and not to demonstrate the ber geek principles of right and proper enterprise development, we ve conveniently decided to be slackers. Also, as you ll see toward the end of the chapter, the Bid object is really a JPA entity.
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