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the arrow connecting Bid and Item points in both directions in figure 7.3. This is what is meant by a bidirectional relationship or association between Bid and Item as opposed to a unidirectional relationship or association between Seller and BillingInfo. Typically, objects are nouns and relationships are verbs such as has, is part of, is member of, belongs to, and so on. Multiplicity, or cardinality As you can probably infer from figure 7.3, not all relationships are one-to-one. That is, there may be more than one object on either side of a relationship. For example, a Category can have more than one Item. Multiplicity or cardinality refers to this multifaceted nature of relationships. The multiplicity of a relationship can be:

Combining email-sending code with a message-driven bean provides the asynchronous behavior that is ideal for enterprise applications. Listing 6.22 contains a message-driven bean that sends email using property values passed to it via a JMS message.

One-to-one Each side of the relationship may have at most only one object. An Employee object can have only one ID card and an ID card can only be assigned to one employee. One-to-many A particular object instance may be related to multiple instances of another. For example, an Item can have more than one Bid. Note that, taken from the point of view of a Bid, the relationship is said to be manyto-one. For example, many Bids can be placed by a Bidder in figure 7.3.

In section 1.2.3, we mentioned that the persistence provider s job is to provide standardized JPA services. Let s explore how it does that. Instead of following the JVM-like container model, JPA follows a model similar to APIs, like JDBC. JPA provides persistence services such as retrieving, adding, modifying, and deleting JPA entities when you explicitly ask for them by invoking EntityManager API methods. The provider terminology comes from APIs such as JDBC and JNDI too. If you ve worked with JDBC, you know that a provider is essentially the vendor implementation that the JDBC API uses under the covers. Products that provide JPA implementation are persistence providers or persistence engines. JBoss Hibernate and Oracle TopLink are two popular JPA providers.

staticinitialization(classexpr) An execution of a static block in a class that matches classexpr adviceexecution() An execution of an advice code block

import javax.jms.*; public class EmailBean implements MessageDrivenBean, MessageListener { private MessageDrivenContext ctx; public void onMessage( Message msg ) { MapMessage map = (MapMessage) msg; try { sendEmail( map.getProperty( "Recipient" ), map.getProperty("message" ) ); } catch(Exception ex) { ex.printStackTrace(); } } private void sendEmail(String recipient, String text) throws Exception { Session mailSession = null; javax.mail.Message msg = null; try{ System.out.println( "Sending Email to: " + rcpt ); mailSession = (Session) ctx.lookup("BookMailSession"); msg = new MimeMessage(mailSession); msg.setFrom(); msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse( recipient , false)); msg.setSubject("Important Message"); msg.setText(text); Sends the email Transport.send(msg); System.out.println("Sent Email to: "+rcpt); } catch(Exception e){ e.printStackTrace(); } } }

Since JPA is completely pluggable and separable, the persistence provider and container in an EJB 3 solution need not come from the same vendor. For example, you could use Hibernate inside a BEA WebLogic container if it suits you better, instead of the Kodo implementation WebLogic ships with. But without services, what good are containers In the next section, we explore the services concept critical to EJB.

The first thing that should cross your mind while evaluating any technology is what it really gives you What s so special about EJB Beyond a presentation-layer technology like JSP, JSF, or Struts, couldn t you create your web application using just the Java language and maybe some APIs like JDBC for database access The plain answer is that you could if deadlines and cutthroat competition were not realities Indeed, before anyone dreamed up EJB this is exactly what people did What the resulting long hours proved is that you tend to spend a lot of time solving very common system-level problems instead of focusing on the real business solution These bitter experiences emphasized the fact that there are common solutions that can be reused to solve common development problems This is exactly what EJB brings to the table.