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Building business logic with session beans
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Now that we ve looked at the basic structure of session beans, we ll outline relatively simple programming rules for a session bean.
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3.1.3 Understanding the programming rules Like all EJB 3 beans, session beans are POJOs that follow a small set of rules. The following summarizes the rules that apply to all types of session beans:
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As we discussed earlier in section 3.1.2, a session bean must have at least one business interface. The session bean class must be concrete. You cannot define a session bean class as either final or abstract since the container needs to manipulate it. You must have a no-argument constructor in the bean class. As we saw, this is because the container invokes this constructor to create a bean instance when a client invokes an EJB. Note that the compiler inserts a default noargument constructor if there is no constructor in a Java class. A session bean class can subclass another session bean or any other POJO. For example, a stateless session bean named BidManager can extend another session bean PlaceBidBean in the following way:
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@Stateless public BidManagerBean extends PlaceBidBean implements BidManager { ... }
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The business methods and lifecycle callback methods may be defined either in the bean class or in a superclass. It s worth mentioning here that annotation inheritance is supported with several limitations with EJB 3 session beans. For example, the bean type annotation @Stateless or @Stateful specified in the PlaceBidBean superclass will be ignored when you deploy the BidManagerBean. However, any annotations in the superclasses used to define lifecycle callback methods (more about that later in this section) and resource injections will be inherited by the bean class. Business method names must not start with ejb. For example, avoid a method name like ejbCreate or ejbAddBid because it may interfere with EJB infrastructure processing. You must define all business methods as public, but not final or static. If you are exposing a method in a remote business interface of the EJB, then make sure that the arguments and the return type of the method implement the java.io.Serializable interface.
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You ll see these rules applied when we explore concrete examples of stateless and stateful session beans in sections 3.2 and 3.3, respectively.
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Getting to know session beans
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Now that we ve looked at the basic programming rules for the session beans, let s discuss the fundamental reasons behind splitting them into two groups.
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3.1.4 Conversational state and session bean types
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Earlier, we talked about stateful and stateless session beans. However, we have so far avoided the real differences between them. This grouping of bean types centers on the concept of the conversational state. A particular business process may involve more than one session bean method call. During these method calls, the session bean may or may not maintain a conversational state. This terminology will make more sense if you think of each session bean method call as a conversation, or exchange of information, between the client and the bean. A bean that maintains conversational state remembers the results of previous exchanges, and is a stateful session bean. In Java terms, this means that the bean will store data from a method call into instance variables and use the cached data to process the next method call. Stateless session beans don t maintain any state. In general, stateful session beans tend to model multistep workflows, while stateless session beans tend to model generalpurpose, utility services used by the client. The classic example of maintaining conversational state is the e-commerce website shopping cart. When the client adds, removes, modifies, or checks out items from the shopping cart, the shopping cart is expected to store all the items that were put into it while the client was shopping. As you can imagine, except for the most complex business processes in an application, most session bean interactions don t require a conversational state. Putting in a bid at ActionBazaar, leaving buyer or seller feedback, and viewing a particular item for bid are all examples of stateless business processes. As you ll soon see, however, this does not mean that stateless session beans cannot have instance variables. Even before we explore any code, common sense should tell us that session beans must cache some resources, like database connections, for performance reasons. The critical distinction here is client expectations. As long as the client does not need to depend on the fact that a session bean uses instance variables to maintain conversational state, there is no need to use a stateful session bean.
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