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Building business logic with session beans
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instantiate the placeBid variable with the EJB named PlaceBid before the variable is available for use. Recall our discussion in section 2.1.2 that DI can be viewed as the opposite of JNDI lookup. Recall also that JNDI is the container registry that holds references to all container-managed resources such as EJBs. Clients gain access to session beans like our PlaceBid EJB directly or indirectly through JNDI. In EJB 2, you would have to manually populate the placeBid variable using JNDI lookup code that looks like the following:
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Object ejbHome = new InitialContext().lookup("java:comp/env/PlaceBid"); PlaceBidHome placeBidHome = (PlaceBidHome) PortableRemoteObject.narrow(ejbHome, PlaceBidHome.class); PlaceBid placeBid = placeBidHome.create();
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It isn t easy to fully appreciate DI until you see code like this. EJB 3 DI using the @EJB annotation reduces all this mechanical JNDI lookup code to a single statement! In a nontrivial application, this can easily translate to eliminating hundreds of lines of redundant, boring, error-prone code. You can think of EJB 3 DI as a high-level abstraction over JNDI lookups. Understanding statelessness An interesting point about the PlaceBid stateless bean is that as long as calling the addBid method results in the creation of a new bid record each time, the client doesn t care about the internal state of the bean. There is absolutely no need for the stateless bean to guarantee that the value of any of its instance variables will be the same across any two invocations. This property is what statelessness means in terms of server-side programming. The PlaceBid session bean can afford to be stateless because the action of placing a bid is simple enough to be accomplished in a single step. The problem is that not all business processes are that simple. Breaking a process down into multiple steps and maintaining internal state to glue together the steps is a common technique to present complex processes to the user in a simple way. Statefulness is particularly useful if what the user does in a given step in a process determines what the next step is. Think of a questionnaire-based setup wizard. The user s input for each step of the wizard is stored behind the scenes and is used to determine what to ask the user next. Stateful session beans make maintaining server-side application state as easy as possible.
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A first taste of EJB
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2.3.3 Using stateful beans
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Unlike stateless session beans, stateful session beans guarantee that a client can expect to set the internal state of a bean and count on the state being maintained between any number of method calls. The container makes sure this happens by doing two important things behind the scenes. Maintaining the session First, the container ensures that a client can reach a bean dedicated to it across more than one method invocation. Think of this as a phone switchboard that makes sure it routes you to the same customer service agent if you call a technical support line more than once in a given period of time (the period of time is the session ). Second, the container ensures that bean instance variable values are maintained for the duration of a session without your having to write any session maintenance code. In the customer service example, the container makes sure that your account information and call history in a given period of time automatically appear on your agent s screen when you call technical support. This automagic maintenance of session state is a huge leap from having to fiddle with the HTTP session, browser cookies, or hidden HTML form variables to try to accomplish the same thing. As we ll see in the coming code samples, you can develop stateful beans as if you are developing in a Hello World application, not a web application with verbose code to maintain session state. The ActionBazaar ordering process is a great example for stateful session beans since it is broken up into four steps, each of which correspond to a screen presented to the user:
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Adding items to the order. If the user started the ordering process by clicking the Order Item button on the page displaying an item won, the item is automatically added to the order. The user can still add additional items in this step. Specifying shipping information, including the shipping method, shipping address, insurance, and so on. Adding billing information, such as credit card data and the billing address. Confirming the order after reviewing the complete order, including total cost.
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