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As we mentioned, EJB components live in a container. Together, the components, or EJBs, and the container can be viewed as a framework that provides valuable services for enterprise application development. Although many people think EJBs are overkill for developing relatively simple web applications of moderate size, nothing could be further from the truth. When you build a house, you don t build everything from scratch. Instead, you buy materials or even the services of a contractor as you need it. It isn t too practical to build an enterprise application from scratch either. Most server-side applications have a lot in common, including churning business logic, managing application state, storing and retrieving information from a relational database, managing transactions, implementing security, performing asynchronous processing, integrating systems, and so on. As a framework, the EJB container provides these kinds of common functionality as out-of-the-box services so that your EJB components can use them in your applications without reinventing the wheel. For instance, let s say that when you built the credit card module in your web application, you wrote a lot of complex and error-prone code to manage transactions and security access control. You could have avoided that by using the declarative transaction and security services provided by the EJB container. These services, as well as many others you ll learn about in section 1.3, are available to the EJB components when they are deployed in the EJB container, as you can see in figure 1.2. This means writing high-quality, feature-rich applications much faster than you might think. The container provides the services to the EJB components in a rather elegant new way: metadata annotations are used to preconfigure the EJBs by specifying the type of services to add when the container deploys the EJBs. Java 5 introduced

/** * @ejb.bean * name="MessageBean" * type="MDB" * message-selector="<![CDATA messageType = 'buyerRequest']]>" */ public class MessageBean implements MessageDrivenBean, MessageListener {

Design patterns are one approach for reusability, which is one of the key concepts of objectoriented programming. The idea of a pattern is not unique to programming; in fact, the idea stems from a similar concept that is used in architecture and town planning, an idea formalized by architect Christopher Alexander. The ideas were adapted to object-oriented programming by Ward Cunningham and Ken Beck in their article Using Pattern Languages for Object-Oriented Programs, which was presented in 1987 at a conference in Orlando, Florida. As stated previously, design patterns offer generic solutions to recurrent problems in specific contexts. The context in which a design pattern is used is important because a problem may be solved differently depending on the context. A good quality design pattern must be the abstraction of a concrete and well-tested solution. A design pattern must be well documented if it is to be used efficiently. Several catalogues of design patterns are available, the GoF book being the most well known. Each catalogue is split into sections that contain descriptions of the design patterns.

package ejb3inaction.example.buslogic; import javax.ejb.Stateless; import ejb3inaction.example.persistence.Bid; @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); } ... } ... package ejb3inaction.example.buslogic; import javax.ejb.Local; import ejb3inaction.example.persistence.Bid; @Local public interface PlaceBid { Bid addBid(Bid bid); }

Notice also the change in the type attribute for this example. Instead of session or entity, its value is MDB, indicating that this class is a message-driven EJB.

The first thing that you have probably noticed is how plain this code looks. The PlaceBidBean class is just a plain old Java object (POJO) and the PlaceBid interface is a plain old Java interface (POJI). There is no cryptic EJB interface to implement, class to extend, or confusing naming convention to follow. In fact, the only notable features in listing 2.1 are the two EJB 3 annotations: @Stateless and @Local:

With this documentation, almost anyone can use these design patterns and allow best practices to be applied easily in application development; however, it is still necessary to understand the possible applications of a design pattern in order to use it correctly.

@Stateless The @Stateless annotation tells the EJB container that PlaceBidBean is a stateless session bean. This means that the container automat-