Java™ Application Development on Linux - Dator

21.2 Expanding to EJBs475to its status. Those methods are defined in the bean class that the implementationclass extends. The classes one extends to implement a bean are:• SessionBean• EntityBean• MessageBeanPlease forgive us right now for admitting that we will not cover messagebeans in this book beyond a brief description in the next section. For detailson message beans, take a look at Chapter 8 of Enterprise JavaBeans ComponentArchitecture by Gail Anderson and Paul Anderson (ISBN 0-13-035571-2).Not all of the control methods need to be implemented in all cases, butthe full set is not that large or difficult to understand. They correspond to important“life events” in lifetime of the bean. The primary events are: creation,destruction, passivation, activation, persist to DB, restore from DB, and contextswitching.Creation. A bean is created when its constructor is called. As we shall learn,the calling of the constructor has absolutely no connection with a client request.For session beans, the container maintains a pool of instances to handle clientrequests. For entity beans, a bean (most commonly) represents a database table,and the setEntityContext() method is used to move that bean from row(instance) to row as needed.In practice, this usually means that a number of instances of the bean arecreated when the application server is started. Additional instances are createdas the demand for this bean (number of clients, number of calls per unit time)increases.Destruction. As you know, Java objects do not have explicit destructors,but when a bean instance is destroyed, an explicit method in the Bean class iscalled to permit shutdown operations to take place.This is quite distinct from activation and passivation. Activation and passivationare operations carried out when the application server needs to shoveaside some Bean instances that may still be needed by clients in order to makeroom for an active request that requires more than the available systemresources.Passivation and activation. As we said above, passivation involves thecontainer asking the beans that are still potentially needed to step aside to allow