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In listing 8.10, the Category entity refers to its parent through the PARENT_ID foreign key pointing to the primary key value of another record in the CATEGORY table. Since multiple subcategories can exist under a single parent, the @JoinColumn annotation specifies a many-one relationship b. We ll conclude this section on mapping domain relationships by dealing with the most complex relationship mapping: many-to-many.

You know her, she knows you, but she wants to eat him, and everyone s okay with this Timon from The Lion King

As we mentioned in section 8.1.1, a many-to-many relationship in the database world is implemented by breaking it down into two one-to-many relationships stored in an association or join table. In other words, an association or join table allows you to indirectly match up primary keys from either side of the relationship by storing arbitrary pairs of foreign keys in a row. This scheme is shown in figure 8.7. The @JoinTable mapping in EJB 3 models this technique. To see how this is done, take a look at the code in listing 8.11 for mapping the many-to-many relationship between the Item and Category entities. Recall that a Category can contain multiple Items and an Item can belong to multiple Category entities.

Figure 8.7 Many-to-many relationships are modeled in the database world using join tables. A join table essentially pairs foreign keys pointing to primary keys on either side of the relationship.

EJBs are hard workers that can perform many activities. In addition to working with data (covered in chapter 3), EJBs can be used to retrieve environment vari-

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@Entity @Table(name="CATEGORIES") public class Category implements Serializable { @Id @Column(name="CATEGORY_ID") protected Long categoryId; @ManyToMany @JoinTable(name="CATEGORIES_ITEMS", joinColumns= @JoinColumn(name="CI_CATEGORY_ID", referencedColumnName="CATEGORY_ID"), inverseJoinColumns= @JoinColumn(name="CI_ITEM_ID", referencedColumnName="ITEM_ID")) protected Set<Item> items; ... } @Entity @Table(name="ITEMS") public class Item implements Serializable { @Id @Column(name="ITEM_ID") protected Long itemId; ... @ManyToMany(mappedBy="items") Subordinate many-to-many

protected Set<Category> categories; ... }

ables, access the file system, send email, and more. This chapter also covers two important improvements added to EJBs with the 2.1 specification: the EJB timer service, and the use of EJBs as web service endpoints. You will find the following topics in this chapter:

The @JoinTable annotation s name element specifies the association or join table, which is named CATEGORIES_ITEMS in our case b The CATEGORIES_ITEMS table contains only two columns: CI_CATEGORY_ID and CI_ITEMS_ID The CI_CATEGORY_ ID column is a foreign key reference to the primary key of the CATEGORIES table, while the CI_ITEM_ID column is a foreign key reference to the primary key of the ITEMS table The joinColumns and inverseJoinColumns elements indicate this Each of the two elements describes a join condition on either side of the many-tomany relationship The joinColumns element describes the owning relationship between the Category and Item entities, and the inverseJoinColumns element describes the subordinate relationship between them Note the distinction of the owning side of the relationship is purely arbitrary Just as we used the mappedBy element to reduce redundant mapping for one-tomany relationships, we are using the mappedBy element on the Item.

try { ut.begin(); } catch (Exception e) { throw new EJBException("transferFunds: " + e.getMessage()); } try { String fromType= fromAccount.getType(); BigDecimal fromBalance= fromAccount.getBalance(); if (DomainUtil.isCreditAccount(fromType)) { BigDecimal fromNewBalance= fromBalance.add(amount); if (fromNewBalance.compareTo(fromAccount.getCreditLine()) == 1) throw new InsufficientCreditException(); executeTx(amount,descr,fromAccountId,fromNewBalance,fromAccount); } else { BigDecimal fromNewBalance= fromBalance.subtract(amount); if (fromNewBalance.compareTo(bigZero) == -1) throw new InsufficientFundsException(); executeTx(amount.negate(),descr,fromAccountId,fromNewBalance,fromAccount); } String toType= toAccount.getType(); BigDecimal toBalance= toAccount.getBalance(); if (DomainUtil.isCreditAccount(toType)) { BigDecimal toNewBalance= toBalance.subtract(amount); executeTx(amount.negate(),descr,toAccountId,toNewBalance,toAccount); } else { BigDecimal toNewBalance= toBalance.add(amount); executeTx(amount, descr, toAccountId, toNewBalance, toAccount); } ut.commit(); } catch (Exception ex) { try { ut.rollback(); } catch (Exception e) { throw new EJBException("transferFunds: " + e.getMessage()); } throw new EJBException("transferFunds: " + ex.getMessage()); } // transferFunds