Java™ Application Development on Linux - Dator

21.2 Expanding to EJBs477as we go along. For now, be aware that the container will start, stop, instantiate,destroy, and provide network services to the EJBs it contains.21.2.2 Bean TypesEnterprise JavaBeans come in more than one variety. Let’s take a look at those.21.2.2.1 Session BeansA session bean is a reusable component that represents a collection of servermethods. The intention is that each such method represents some businessprocess, such as addCustomer(), createShoppingCart(), and so on.Session beans are thus organized around business processes. Actually, asession bean is not much more than a facade that collects a business processAPI into a single class. But remember what EJBs give you—networked serverbasedfunctionality, load balancing, clustering features, reliability, failover, andthe ability to handle increased demand by merely adding more Bean instancesand server hardware. The power comes not from how EJBs extend the language,but from how they automate and hide infrastructure.Session beans come in two major varieties: stateless and stateful. Let’s takea look at what they offer and the differences between the two.Stateless Session BeansWhat they are. A stateless session bean is one where each method call iscompletely independent of any other method call, whether by one or manyclients.An EJB application designed this way has certain advantages. Since a singleBean instance can be switched from client to client on demand (because nostate information is kept between method invocations), a small number of Beaninstances can handle a large number of clients. Compare this with statefulsession beans described below.Why you would use them. In a sense, this is the optimum bean. If you candesign your application to use nothing but stateless session beans (perhapsbacked by entity beans), then you have a maximally flexible, extensible, andadaptable enterprise application—one that can be easily scaled from a singleapplication server to a very large cluster. Why? Because the lack of state informationmeans that any single Bean instance can serve any given client at any