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Since multiple instances of BIDS records would refer to the same record in the ITEMS table, the BIDS table will hold a foreign key reference to the primary key of the ITEMS table. In our example, this foreign key is BID_ITEM_ID, and it refers to the ITEM_ID primary key of the ITEMS table. This database relationship between the tables is shown in figure 8.6. In listing 8.9 the many-to-one relationship is expressed in ORM using @JoinColumn annotations. In effect, a @JoinColumn annotation s job is to specify a primary/foreign key relationship in the underlying data model. Note that the exact @JoinColumn specification could have been repeated for both the Bid.item and Item.bids persistent fields on either side of the relationship. In @ManyToOne C, the name element specifies the foreign key, BID_ITEM_ID, and the referencedColumnName element specifies the primary key, ITEM_ID. From the Item entity s perspective, this means the persistence provider would figure out what Bid instances to put in the bids set by retrieving the matching BIDS_ITEM_ID in the BIDS table. After performing the join, the JPA provider will see what BIDS records are retrieved by the join and populate them into the bids set. Similarly, when it is time to form the item reference in the Bid entity, the persistence provider would populate the @JoinColumn-defined join with the available BID_ITEM_ID foreign key, retrieve the matched ITEMS record, and put it into the item field. Instead of repeating the same @JoinColumn annotation, we have used the mappedBy element B we mentioned but did not describe in chapter 7.

Many enterprise applications are dependent on other systems for completing a workflow. Actually, enterprise applications many times represent steps within an enterprise workflow. In these situations, data represented by your entity beans may also be shared by (or be important to) other applications. These outside applications might need to know when the data is changed in order to begin their steps in the workflow. Sending notifications is best accomplished using a JMS implementation accessed from the application server running your EJB container.

Figure 8.6 The one-to-many relationship between the Item and Bid entities is formed through a foreign key in the BIDS table referring to the primary key of the ITEMS table.

Listing 9-21. A Simple Client Program for the Stats MBean package aop.management.jmx.simple; public class JMXExample { private static Stats statistics = new Stats(); public static void sendOrder(float amount) { if (amount>0) { statistics.incOrders(); statistics.addAmount(amount); } else { statistics.setStatus("KO"); try { Thread.sleep(200); } catch (InterruptedException e) { } statistics.setStatus("OK"); } }

The persistence provider will generate deployment-time errors if you specify @JoinColumn on both sides of a bidirectional one-tomany relationship.

You can specify this element in the OneToMany element on the Item.bids variable as follows:

To notify outside listeners of entity changes, you will use a JMS publisher. Before looking at the bean source, let s examine the object used to publish messages into a JMS system (listing 3.27). It publishes provided messages to a particular JMS topic created in the application server (see your vendor s documentation for setting up JMS topics in your application server). This publisher provides access to a JMS topic in a Weblogic application server.

public class Item { ... @OneToMany(mappedBy="item") protected Set<Bid> bids; ... }

public static void main(String[] str) throws Exception { Injector injection = new Injector(); injection.start(); } } The sendOrder method is used by the Injector instance to simulate orders. If the passed amount is positive, statistics are updated. Otherwise, an error is generated, and the status attribute is set to KO for 200 milliseconds before being reset to OK . The Injector class shown in Listing 9-22 is a Java thread that generates arbitrary orders. More precisely, it generates ten random orders. Every five orders, an invalid order (with an amount of 1000) is generated. Listing 9-22. A Thread for the Random Generation of Orders package aop.management.jmx.simple; public class Injector extends Thread { public void run() { float amount = 0; for (int i=1;i<=10;i++) { try {

The mappedBy element is essentially pointing to the previous relationship field Bid.item with the @JoinColumn definition. In a bidirectional one-to-many relationship, the owner of the relationship is the entity side that stores the foreign key that is, the many side of the relationship. The persistence provider will know to look it up appropriately when resolving Bid entities. In general, you have to use the mappedBy element wherever a bidirectional relationship is required. If you do not specify the mappedBy element with the @OneToOne annotation, the persistence provider will treat the relationship as a unidirectional relationship. Obviously, this would not be possible if the one-tomany relationship reference were unidirectional since there would be no owner of the relationship to refer to. Unfortunately, JPA does not support unidirectional one-to-many relationships using a foreign key on the target table and you cannot use the following mapping if you have a unidirectional one-to-many relationship between Item and Bid:

public class JMSPublisher { private TopicConnection private TopicSession private TopicPublisher private Topic private TopicConnectionFactory private String

@OneToMany(cascade=CascadeType.ALL) @JoinColumn(name="ITEM_ID", referencedColumnName="BID_ITEM_ID") protected Set<Bid> bids;