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Once you have a reference to an actual EJB instance, you can use the getHandle() method to retrieve a serializable object that contains information for rebuilding the bean to which it refers. Using an instance of the Handle class, you can store a reference to an EJB, shut down your application, and then restart and restore the conversational state between your client and the EJB. EJB handles can be retrieved only from the EJBObject instance for your EJB. The EJBObject interface is the parent interface to the EJB s remote interface. This means that you can create handles only to remote EJB references, not to local objects. You can also use an EJB handle to pass an EJB reference to a remote object. Since the EJB handle instance is a serializable object, it can be passed over an RMI invocation. This allows you to pass the handle to remote EJBs for callbacks or to any other remote object. Remote objects receiving the handle will not need to know how to look up the EJB they can simply retrieve the EJBObject from the handle and start using it.

3.7 Summary

In this chapter, we examined the various session bean types and how stateless session beans and stateful session beans differ. We looked at the programming rules for both stateless and stateful session beans, and you saw comprehensive examples of both bean types. As you learned, stateless session beans have a simple lifecycle and can be pooled. Stateful beans require instances for each client, and for that reason they can consume a lot of resources. In addition, passivation and activation of stateful beans can impact performance if used inappropriately. You learned about alternatives for using stateful session beans, and that session bean clients can be either local or remote. We showed you that dependency injection simplifies the use of EJB and saves you from having to perform complex JNDI lookups. Finally, we provided some best practices for developing session beans. At this point you have all the ammunition necessary to build the business logic of your application using stateless and stateful session beans. In the next chapter we ll discuss how you can build messaging applications with messagedriven beans.

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5.2 Handling transaction management without the container 5.3 Rolling back the current transaction 5.8 Managing EJB state at transaction boundaries 5.10 Managing EJB state after a rollback

In this chapter we ll take a closer look at developing message-driven beans (MDBs) as well as provide you with an overview of the concepts and technologies these powerful EJB 3 components build on. First we ll introduce you to basic messaging concepts, and then we ll explore the Java Messaging Service (JMS) by creating a message producer. Finally, we ll take a look at MDBs, the EJB 3 answer to messaging. You should gain an understanding of messaging and JMS before diving into MDB for two reasons. First, most MDBs you ll encounter are glorified JMS message consumers implementing JMS interfaces (such as javax.jms.MessageListener) and using JMS components (such as javax.jms.Message). Second, for most solutions with MDB your messaging will involve much more than simply consuming messages. For the simplest of these tasks, such as sending messages, you ll have to understand JMS. This chapter assumes a basic familiarity with JMS and we offer only a brief description of it. If you re comfortable with messaging and JMS, feel free to skip to the sections on MDBs. It is good to reinforce what you know from time to time, though, so you just might want to quickly jog through the first few sections with us anyway.

When we talk about messaging in the Java EE context, what we really mean is the process of loosely coupled, asynchronous communication between system components. Most communication between components is synchronous, such as simple method invocation or Java RMI. In both cases, the invoker and the invocation target have to be present for the communication to succeed. Synchronous communication also means that the invoker must wait for the target to complete the request for service before proceeding. As an analogy, you re communicating synchronously when you (the invoker) call and talk to someone over the phone. But what if the person (the invocation target) isn t available If possible, you leave a voicemail message. The voicemail service makes the communication asynchronous by storing your message so that the receiver can listen to it later and respond. Message-oriented middleware (MOM) enables messaging in much the same way that a voicemail service does by acting as the middleman between a message sender and the receiver so that they don t have to be available simultaneously. In this section, we briefly introduce MOM, show how messaging is used in our ActionBazaar application, and examine popular messaging models.

Figure 4.1 Basic MOM message flow. When the producer sends a message to the software, it is stored immediately and later collected by the consumer. Looks a lot like e-mail, doesn t it