Java™ Application Development on Linux - Dator

21.2 Expanding to EJBs479Here, items like the identity of the customer and the list of items in thecart must be preserved between method invocations. Obviously, it is possibleto present such an interface through a stateless bean by creating some sort of asession identifier token and passing that in to every method, thus allowing thestateless session bean to save this data to a database and then load it back, butthe primary advantage of a stateful session bean is that this work is done foryou through the setting of the bean’s context.So, the primary advantage of stateful session beans is that the server sidecan keep track of client data for you. The primary disadvantage is that thecontainer will try its best to keep an instance of the Bean around for everyclient, so it must sometimes swap an idle instance out to make room for an activeinstance, which is an expensive operation. But—and this is important tokeep in mind—it is much less expensive than reading and writing this data onevery call to a stateless bean! You have to understand what is happening underthe hood if you want to produce an optimal design. If you need state betweenmethod calls, a stateful bean is likely to be the most effective way to go.21.2.2.2Entity BeansWhat they are. An entity bean is often described as an object that representsa row in a database table. This is the most typical case, but it isn’t always so.We have worked on a J2EE application where the entity bean represented anXML document in the filesystem.The general idea is that enterprise applications tend to work on lists ofsimilar things: customers, employees, locations, accounts, servers, inventoryitems, and so on. An entity bean is an object that represents a single item insuch a list. In other words, it is an interface to a data item. And, yes, in practicethere is one entity bean class for a table and one instance of the class foreach row.Obviously, a J2EE container doesn’t maintain an in-memory instance forevery row of every table. In fact, you can think of both entity beans and sessionbeans as ways to automate keeping the optimal balance between in-memoryinstances for speed and data storage for memory utilization.Entity beans can be written to manage the persistent storage itself, usingcode added to the bean implementation by the bean author (this is known asbean-managed persistence, or BMP), or they may be written to allow thecontainer to automatically manage the data in the underlying database for you(this is known as container-managed persistence, or CMP). Which you use may