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Behind the scenes, the container takes care of several mechanical details to start listening for messages sent to the destination specified by the activation configuration properties. As soon as a message arrives at the destination, the container forwards it to an instance of the MDB.

Instead of implementing a remote or local business interface, MDBs implement the javax.jms.MessageListener interface. The container uses this well-known JMS interface to invoke an MDB. The onMessage method defined by the interface has a single javax.jms.Message parameter that the container uses to pass a received message to the MDB. Believe it or not, this is more or less all you need to know to get by when using MDBs, as long as you have a decent understanding of messaging and JMS. This wraps up this chapter s discussion of the EJB 3 business-tier components. As we mentioned earlier, we ll devote the entirety of the next part of the book to this vital part of the EJB platform. For now, let s move on to the other major part of EJB, the Persistence API.

The Java Persistence API (JPA) is the persistence-tier solution for the Java EE platform. Although a lot has changed in EJB 3 for session beans and MDBs, the changes in the persistence tier have truly been phenomenal. In fact, other than some naming patterns and concepts, JPA has very little in common with the EJB 2 entity bean model. JPA does not follow the container model (which is just not very

well suited to the problem of persistence); instead, it follows an API paradigm similar to JDBC, JavaMail, or JMS. As you ll soon see, the JPA EntityManager interface defines the API for persistence while JPA entities specify how application data is mapped to a relational database. Although JPA takes a serious bite out of the complexity in saving enterprise data, ORM-based persistence is still a nontrivial topic. We ll devote the entire third part of this book to JPA, namely chapters 7 through 10. In almost every step of our ActionBazaar scenario, data is saved into the database using JPA. We won t bore you by going over all of the persistence code for the scenario. Instead, we ll introduce JPA using a representative example and leave you to explore the complete code on your own. You ll see what EJB 3 persistence looks like by revisiting the PlaceBid stateless session bean. As a reminder to how the bidding process is implemented, figure 2.7 depicts the various components that interact with Figure 2.7 PlaceBidServlet invokes the one another when a bidder creates a bid in addBid method of PlaceBid EJB and passes ActionBazaar. a Bid object. The PlaceBidEJB invokes the Recall that the PlaceBidServlet calls persist method of EntityManager to save the Bid entity into the database. When the the PlaceBidBean s addBid method to add a transaction commits, you ll see that a Bid entity into the database. The Placecorresponding database record in the BIDS BidBean uses the JPA EntityManager s pertable will be stored. sist method to save the bid. Let s first take a look at the JPA, and then we ll see the EntityManager in action.

You might have noticed in listing 2.1 that we kept the code to save a bid into the database conveniently out of sight. The PlaceBid EJB s addBid method references the hidden save method to persist the Bid object to the database. Listing 2.8 will fill in this gap by showing you what the save method actually does. The save

method uses the JPA EntityManager to save the Bid object. But first let s take a quick look at the fourth and final kind of EJB the JPA entity. Listing 2.7 shows how the Bid entity looks.