Java™ Application Development on Linux - Dator

474Chapter 21Introduction to Enterprise JavaBeansThat confessed, we still think our example is the best way to address ourtopic. Why? Because EJBs are a large topic, worthy of several books (and weencourage you to read some of them, such as Enterprise JavaBeans ComponentArchitecture by Gail Anderson and Paul Anderson (ISBN 0-13-035571-2). Wesimply cannot teach you all about EJBs in our book. Our goal here is to introducethe concepts, provide a working example, and explain how to deploy andmaintain an EJB system on Linux using JBoss. Believe us, we’ll have enoughto cover to achieve that modest goal.21.2.1 EJB ConceptsIn a way, the use of the term Enterprise JavaBean, with its echo of the olderterm JavaBean, is unfortunate. Apart from the similar intention of creatingreusable components, the two technologies have little in common.21.2.1.1 The Life Cycle of an Enterprise JavaBeanMost of the books on EJBs that we have seen start with the simplest type ofbean and work their way up in complexity. We’re going to dare to be different,because the most complex case isn’t that hard, and once you understand it, thefunctions of all the other types of EJBs become obvious. They simply don’timplement what they do not need.For the moment, assume that an EJB is a simple class that provides a setof methods to clients. The methods represent business functions that clientswant the server to carry out.Implementing an Enterprise JavaBean requires implementing three Javasource files:• An object that represents the bean implementation• An interface called the home interface that represents the interface betweenthe bean and the EJB container• An interface called the remote interface which represents the methods thata client may call on the beanWe’ll get into the details of these interfaces (and the sometimes obtusereasons behind) a little later on. For now, we will concentrate on theimplementation.The implementation class contains methods that are there only to allowthe container to control the bean and to inform the bean of impending changes