Medical web site - 13.1 Implementing the Storefront Service Using EJB Even
13.1 Implementing the Storefront Service Using EJB Even though this chapter is specific to EJB, the intent is still to keep the focus on Struts. With that in mind, the discussion of EJB implementation details will be kept to a minimum. EJB is a complex topic, but the nature of several design patterns geared toward the interaction between EJBs and their clients makes this an easier task than you might first think. After all, an overriding goal of this chapter is to demonstrate how to design an application so that your Struts classes aren’t impacted by the choice to use an EJB implementation of the model. You already have a head start on some of the central issues here, having seen how the model component of a web application can be hidden behind a service interface. In particular, you’ve seen through the Storefront example how easy it is to swap a debug model with a full implementation that accesses a database when this design approach is followed. Throughout this chapter, the Storefront example will be used to illustrate how an EJB application tier can be used with a Struts application. If it weren’t for the remote nature of accessing an EJB from a client application, this implementation choice wouldn’t make any difference to you. However, the distributed aspects of EJB must be taken into account when your web-tier classes access this type of model. What you’ll see in the remainder of this chapter are some recommendations on how best to implement the code needed to interface with the application tier. Key to this discussion is an approach for isolating the code that handles what’s unique about EJB so that your action classes aren’t affected. 13.1.1 A Quick EJB Overview The EJB specification defines three types of beans: entity, session, and message-driven. Each type of bean has a different purpose within an EJB application. Entity beans provide transactional access to persistent data and are most often used to represent rows stored in one or more related tables in a database. For example, you might implement CustomerBean, ItemBean, and OrderBean entity classes, among others, to support the Storefront application. These entity beans would incorporate the functionality provided by the corresponding business object classes in the example application. When entity beans are used, they take on the primary role of supplying the application model. They are expected to provide both the required data persistence operations and the business logic that governs the data they represent. Because the model should be reusable across applications in an enterprise, entity beans need to be independent of the types of clients that ultimately access the application tier. Session beans are often described as extensions of the client applications they serve. They can implement business logic themselves, but more often their role is to coordinate the work of other classes (entity beans, in particular). They are more closely tied to their clients, so the precaution of keeping entity beans reusable doesn’t apply to session beans to the same extent. The application tier is primarily considered to be the application model, but session beans are also referred to as “controllers” because of the coordination they do. This is especially true when session-bean methods are used to implement transactions that touch multiple business objects. Session beans can be implemented as either stateless or stateful. A stateless session bean maintains no state specific to its client, so a single instance can efficiently be shared among many clients by the EJB container. A stateful bean instance, on the other hand, is assigned to a specific client so that state can be maintained across multiple calls. Holding state in the application tier can simplify the client application logic, but it makes it more difficult to scale to a large number of clients. Typical web applications maintain their client state in
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