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3.1.2 Session beans: the basics
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Although we briefly touched on session beans in the previous chapter, we didn t go into great detail about developing them. Before we dive in, let s revisit the code in chapter 2 to closely examine some basic traits shared by all session beans.
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Building business logic with session beans
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The anatomy of a session bean Each session bean implementation has two distinct parts one or more bean interfaces and a bean implementation class. In the PlaceBid bean example from chapter 2, the bean implementation consisted of the PlaceBid interface and the PlaceBidBean class, as shown in figure 3.1. All session beans must be divided into these two parts. This is because clients cannot have access to the bean implementation class directly. Instead, they must use session beans through a business interface. Nonetheless, interface-based programming is a sound idea anyway, especially when using dependency injection. Figure 3.1 Parts of the Interface-based programming is the practice of PlaceBid session bean. Each not using implementation classes directly whenever session bean has one or more interfaces and one possible. This approach promotes loose coupling implementation class. since implementation classes can easily be swapped out without a lot of code changes. EJB has been a major catalyst in the popularization of interface-based programming; even the earliest versions of EJB followed this paradigm, later to form the basis of DI. The session bean business interface An interface through which a client invokes the bean is called a business interface. This interface essentially defines the bean methods appropriate for access through a specific access mechanism. For example, let s revisit the PlaceBid interface in chapter 2:
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@Local public interface PlaceBid { Bid addBid(Bid bid); }
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Since all EJB interfaces are POJIs, there isn t anything too remarkable in this code other than the @Local annotation specifying that it s a local interface. Recall that a business interface can be remote or even web service-accessible instead. We ll talk more about the three types of interfaces in section 3.2.3. The interesting thing to note right now is the fact that a single EJB can have multiple interfaces. In other words, EJB implementation classes can be polymorphic, meaning that different clients using different interfaces could use them in completely different ways.
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Getting to know session beans
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The EJB bean class Just like typical OO programming, each interface that the bean intends to support must be explicitly included in the bean implementation class s implements clause. We can see this in the code for the PlaceBidBean from chapter 2:
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@Stateless public class PlaceBidBean implements PlaceBid { ... public PlaceBidBean() {} public Bid addBid(Bid bid) { System.out.println("Adding bid, bidder ID=" + bid.getBidderID() + ", item ID=" + bid.getItemID() + ", bid amount=" + bid.getBidAmount() + "."); return save(bid); } ... }
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The PlaceBidBean class provides the concrete implementation of the addBid method required by the PlaceBid interface. Session bean implementation classes can never be abstract, which means that all methods mandated by declared business interfaces must be implemented in the class. Note that EJB implementation classes can have nonprivate methods that are not accessible through any interface. Such methods can be useful for creating clever unit-testing frameworks and while implementing lifecycle callback, as you ll learn in section 3.2.5. Also, an EJB bean class can make use of OO inheritance. You could use this strategy to support a custom framework for your application. For example, you could put commonly used logic in a parent POJO class that a set of beans inherits from.
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Unit-testing your session beans
It is clear that session beans are POJOs. Since EJB annotations are ignored by the JVM, session beans can be unit-tested using a framework like JUnit or TestNG without having to deploy them into an EJB container. For more information on JUnit, browse www.junit.org. On the other hand, since several container-provided services such as dependency injection cannot be used outside the container, you cannot perform functional testing of applications using EJBs outside the container at least not easily.
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