Java™ Application Development on Linux - Dator

22.2 EJBs: You Don’t Know Beans?495right, the EJB container (server) makes code that uses your interfaces, alongwith its own code, to do the infrastructure work of EJBs.Talking about all these pieces of an EJB can be confusing, too. Sometimesit is helpful to think of an EJB as a single class; sometimes it’s better to thinkof it as a family of classes that act together pretending to be a single bean thatis distributed across several hosts. This can make it a bit confusing when talkingabout an EJB—do we mean the family of interacting classes or do we mean thesingle class that provides the application functionality that we want?The names of EJB classes and EJB interfaces (which we will extend andimplement) don’t help much either—they can be confusing, too. For example,we will extend EJBObject, but not to write an EJB session bean; no, we extendSessionBean for that, but EJBObject is the name for the remote interface.Go figure.A bit of perspective may help here. The names Remote, Local, and Homeare used as modifiers on these classes. Local means “on the same host as thebean.” But Home and Remote don’t offer much of a clue. The home interfaceis what we get from a lookup; it produces remote objects (objects which implementthe remote interface). A remote object is what our application uses as ifit were a Java object doing what we need, even though its application-specificactivity will happen on a bean somewhere else on the network.Let’s look at a very very simple example, to see the pieces in action.22.2.1 SessionBeanLet’s write a stateless session bean that will compute the time value of money.Why that? Well, two reasons. First, we already have an SAMoney class with asave() method for computing some values; and second, we need some simple,stateless, but somewhat computationally intensive task to make for a halfwayreasonable example.The real guts of an EJB, the core of the application functionality—in ourexample, the computation of the time value of money—is the session (or entity)bean. For our session bean we begin by implementing the SessionBeaninterface, which means that we need to define these methods:public void setSessionContext(SessionContext context) { }public void ejbCreate() { }public void ejbRemove() { }public void ejbActivate() { }public void ejbPassivate() { }