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Listing 2.7 Bid entity
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package ejb3inaction.example.persistence; import import import import import import import import java.io.Serializable; java.sql.Date; javax.persistence.Column; javax.persistence.Entity; javax.persistence.Id; javax.persistence.Table; javax.persistence.GenerationType; javax.persistence.GeneratedValue;
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@Entity Marks POJO as entity @Table(name="BIDS") Specifies table mapping public class Bid implements Serializable { private Long bidID; private Long itemID; private Long bidderID; private Double bidAmount; private Date bidDate;
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@Id @GeneratedValue(strategy=GenerationType.AUTO) @Column(name="BID_ID") public Long getBidID() { return bidID; } public void setBidID(Long bidID) { this.bidID = bidID; } @Column(name="ITEM_ID") public Long getItemID() { return itemID; } public void setItemID(Long itemID) { this.itemID = itemID; } @Column(name="BIDDER_ID") public Long getBidderID() { return bidderID; }
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public void setBidderID(Long bidderID) { this.bidderID = bidderID; } @Column(name="BID_AMOUNT") public Double getBidAmount() { return bidAmount; } public void setBidAmount(Double bidAmount) { this.bidAmount = bidAmount; } @Column(name="BID_DATE") public Date getBidDate() { return bidDate; } public void setBidDate(Date bidDate) { this.bidDate = bidDate; } }
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You probably have a good idea of exactly how object-relational mapping in JPA works just by glancing at listing 2.7, even if you have no familiarity with ORM tools such as Hibernate. Think about the annotations that mirror relational concepts such as tables, columns, and primary keys. The @Entity annotation signifies the fact that the Bid class is a JPA entity b. Note that Bid is a POJO that does not require a business interface, unlike session and message-driven beans. The @Table annotation tells JPA that the Bid entity is mapped to the BIDS table C. Similarly, the @Column annotations F indicate which Bid properties map to which BIDS table fields. Note that entities need not use getter- and setter-based properties. Instead, the field mappings could have been placed directly onto member variables exposed through nonprivate access modifiers. (You ll learn more about access via entity properties and fields in chapter 7.) The @Id annotation is somewhat special. It marks the bidID property as the primary key for the Bid entity D. Just like a database record, a primary key uniquely identifies an entity instance. We have used the @GeneratedValue annotation with strategy set to GenerationType.AUTO E to indicate that the persistence provider should automatically generate the primary key when the entity is saved into the database.
Persisting data with EJB 3 JPA
NOTE
If you have used EJB 2 you may remember that it was almost rocket science to generate primary key values with container-managed persistence (CMP) entity beans. With EJB 3 JPA, the generation of primary keys is a snap; you have several options, such as table, sequence, identity key, and so on. We ll discuss primary-key generation in chapter 8.
The Bid entity could have been related to a number of other JPA entities by holding direct object references (such the Bidder and Item entities). EJB 3 JPA allows such object reference based implicit relationships to be elegantly mapped to the database. We ve decided to keep things simple for now and not dive into this topic quite so early; we ll discuss entity relationship mapping in chapter 8. Having looked at the Bid entity, let s now turn our attention to how the entity winds up in the database through the PlaceBid bean.
2.5.2 Using the EntityManager
You ve probably noticed that the Bid entity doesn t have a method to save itself into the database. The JPA EntityManager performs this bit of heavy lifting by reading ORM configuration and providing entity persistence services through an API-based interface.
NOTE
The EntityManager knows how to store a POJO entity into the database as a relational record, read relational data from a database, and turn it into an entity; update entity data stored in the database; and delete data mapped to an entity instance from the database. As you ll see in chapters 9 and 10, the EntityManager has methods corresponding to each of these CRUD (Create, Read, Update, Delete) operations, in addition to support for the robust Java Persistence Query Language (JPQL).
As promised earlier, listing 2.8 shows how the PlaceBid EJB uses EntityManager API to persist the Bid entity.
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