Types of EJB in Java

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Types of EJB
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There are three basic bean types supported by the current EJB 20 specification:
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Session beans are associated with a specific business action, particularly one requested during an interactive session For example, the logic for totaling up an order might be encoded in a session bean As their name implies, session beans are the primary application interface for synchronous, interactive sessions Entity beans are associated with an application object that requires persistent storage For example, order and customer objects themselves could each be represented using entity beans Session beans and message-driven beans typically interact with entity beans during execution when persistent data needs to be managed Message-driven beans are associated with a specific business action, particularly one that's necessary for application integration or periodic batch processing For example, all in-store orders might be batch-processed after each business day through a messagedriven bean Currently, message-driven beans are accessible only via the JMS
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Figure 8-2 shows the general relationship between the different EJB types Notice that some clients invoke the application via session beans, some via message beans Regardless, these "interfacing" beans then interact with each other and possibly with a set of entity beans that represent persistent data Figure 8-2 General role of EJB types and possible interbean relationships
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A large application implemented with EJBs usually involves several EJB types To understand how application data and functionality requirements map into these types, let's consider an example
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Throughout this chapter, we'll discuss EJBs in the context of a simple employer benefitsprocessing system The goal is a system that allows members to iteratively choose benefits In addition, an alternative interface enables enrollments to be processed in batch mode Our sample application has the following requirements:
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Members can select and deselect benefits The current benefit listing for any member can be obtained Batch enrollment of member benefit elections is possible
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Notice how these features can be neatly mapped into the EJB types I introduced earlier Iterative member enrollment is the main business task, so it can be handled by a session bean let's call it BenefitEnroller Batch enrollment is a business task, but this kind of bulk, offline processing is better suited for a message-driven bean we'll call it BatchBenefitEnroller Finally, the key objects involved members, benefits, and member benefits need to be persisted and are thus well suited to entity beans so we'll call them Member, Benefit, and MemberBenefits, accordingly The ease of mapping our application requirements into EJB types is no accident The types made available by the specification encompass the ability to execute core business logic and to manage persistent application data, and nearly every application demand can fall into one of these two categories From this perspective, message-driven beans can be viewed as more of an optimization than anything else Certainly, we can get by with just session and entity beans, but, since crosscompany integrated and batch processing tasks are so prevalent in the business world, messagedriven beans are a natural extension that results in better integration and more efficient processing
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Now let's explore the details of each EJB type We'll first look at how beans are designed and then focus on examples of their implementation Based on their purpose and capabilities, we'll see that our rough mapping of requirements to beans is in fact reasonable
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Session Beans
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Session beans correspond to business tasks primarily related to interactive sessions When an end user or client wants to execute some action, such as appending to an order, he routes his request to a session bean It's sometimes helpful to think of a session bean roughly as the device through which a logical connection to an application is established between client and server Session EJBs come in two flavors: stateful and stateless As the name implies, stateful session beans maintain state during communication with a client More specifically, they retain the values of their instance variables between client requests This state disappears when the client and bean session ends (ie, the client terminates it) Obviously, since the bean is maintaining state between client requests, it's important that the client continue to converse with the same bean Theoretically, there should be as many stateful session beans as there are concurrent sessions, since each session will have a client that needs its own state to be managed According to the J2EE spec, stateful session beans may be periodically written to persistent storage
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Stateless session beans don't maintain state between requests and therefore can be used to process requests from any client Since they're not associated with any one client, the number of stateless session beans does not necessarily have to equal the number of concurrent sessions Sessions frequently consist of inactivity, so it's possible for only a few stateless session beans to be required to handle application requests for many clients They're thus inherently more scalable from the bean perspective than stateful session beans From my description, stateless session beans seem like an obvious win-win situation Pooling beans instead of issuing one per client interaction seems an obvious way to improve scalability and conserve resources However, keep in mind that sessions often require state management For example, online retail applications usually need virtual shopping carts The state must be stored somewhere: at the client (ie, via cookies), in rewritten URLs, in server-side memory, or in the database There's just no getting around that Designing an efficient solution for state management is one of the challenges facing an application architect We'll discuss some options at the end of this chapter
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