birt barcode tool Saving a bid record using the EJB 3 JPA in Java

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Listing 2.8 Saving a bid record using the EJB 3 JPA
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package ejb3inaction.example.buslogic; ... import javax.persistence.PersistenceContext; import javax.persistence.EntityManager; ... @Stateless
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A first taste of EJB
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public class PlaceBidBean implements PlaceBid { @PersistenceContext(unitName="actionBazaar") Injects instance of private EntityManager entityManager; EntityManager ... public Bid addBid(Bid bid) { System.out.println("Adding bid, bidder ID=" + bid.getBidderID() + ", item ID=" + bid.getItemID() + ", bid amount=" + bid.getBidAmount() + ".");
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return save(bid); } private Bid save(Bid bid) { entityManager.persist(bid); return bid; } }
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Persists entity instance
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The true magic of the code in listing 2.8 lies in the EntityManager interface. One interesting way to think about the EntityManager interface is as an interpreter between the object-oriented and relational worlds. The manager reads the ORM mapping annotations like @Table and @Column on the Bid entity and figures out how to save the entity into the database. The EntityManager is injected into the PlaceBid bean through the @PersistenceContext annotation b. Similar to the name parameter of the @Resource annotation in listing 2.5, the unitName parameter of the @PersistenceContext annotation points to the persistence unit specific to ActionBazaar. A persistence unit is a group of entities packaged together in an application module. You ll learn more about persistence units in chapters 9 and 11. In the save method, the EntityManager persist method is called to save the Bid data into the database C. After the persist method returns, a SQL statement much like the following is issued against the database to insert a record corresponding to the bid:
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INSERT INTO BIDS (BID_ID, BID_DATE, BIDDER_ID, BID_AMOUNT, ITEM_ID) VALUES (52, NULL, 60, 20000.50, 100)
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It might be instructive to look back at listing 2.7 now to see how the EntityManager figures out the SQL to generate by looking at the object-relational mapping annotations on the Bid entity. Recall that the @Table annotation specifies that the bid record should be saved in the BIDS table while each of the @Column annotations in listing 2.7 tells JPA which Bid entity field maps to which column in the BIDS table. For example, the bidId property maps to the BIDS.BID_ID column, the bidAmount property maps to the BIDS.BID_AMOUNT column, and so on. As we
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discussed earlier, the @Id and @GeneratedValue value annotations specify that the BID_ID column is the primary key of the BIDS table and that the JPA provider should automatically generate a value for the column before the INSERT statement is issued (the 52 value in the SQL sample). This process of translating an entity to columns in the database is exactly what object-relational mapping and JPA is all about. This brings us to the end of this brief introduction to the EJB 3 Java Persistence API and to the end of this whirlwind chapter. At this point, it should be clear to you how simple, effective, and robust EJB 3 is, even from a bird s-eye view.
2.6 Summary
As we stated in the introduction, the goal of this chapter was not to feed you the guru pill for EJB 3, but rather to show you what to expect from this new version of the Java enterprise platform. This chapter introduced the ActionBazaar application, a central theme to this book. Using a scenario from the ActionBazaar application, we showed you a cross section of EJB 3 functionality, including stateless session beans, stateful session beans, message-driven beans, and the EJB 3 Java Persistence API. You also learned some basic concepts such as deployment descriptors, metadata annotations, and dependency injection. We used a stateless session bean (PlaceBidBean) to implement the business logic for placing a bid for an item in an auctioning system. To access the bean, we built a very simple servlet client that used dependency injection. We then saw a stateful session bean (PlaceOrderBean) that encapsulated the logic for ordering an item, and we built an application client that accesses the PlaceOrderBean. We saw an example of an MDB, OrderBillingMDB, that processes a billing request when a message arrives on a JMS queue. Finally, we built an entity for storing bids and used the EntityManager API to persist the entity to the database. Most of the rest of this book roughly follows the outline of this chapter. 3 revisits session beans; chapter 4 discusses messaging, JMS and MDBs; chapter 5 expands on dependency injection and discusses such topics as interceptors and timers; and chapter 6 explores transactions and security management in EJB. s 7 through 10 are devoted to a detailed exploration of the Persistence API. Finally, chapters 11 through 16 cover advanced topics in EJB. In the next chapter, we ll shift to a lower gear and dive into the details of session beans.
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