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Session bean
- 1. Enterprise Java Unit- 4 Chapter-2 Working with Session Beans • Session Bean • Types of Session Bean • Remote and Local Interface • Lifecycle of Enterprise Beans • Packaging Enterprise Beans Prof. Sandeep Vishwakarma
- 2. SESSION BEANS • A Session Bean implements and encapsulates a business task that can be invoked programmatically by a client over local, remote, or web service client views. • The session bean performs work (such as calculations or database access) for its client by executing business tasks inside the server. • The application container or EJB container manages the life cycle of the session bean instances. It notifies the instances when bean action may be necessary, and it provides a full range of services to ensure that the session bean implementation is scalable and can support a large number of clients.
- 3. TYPES OF SESSION BEANS
- 4. STATELESS SESSION BEAN • Stateless Session Beans are business objects that do not have state associated with them. • The @Stateless annotation is used to mark the class as Stateless Session Bean. • Access to a single bean instance is still limited to only one client at a time and concurrent access to the bean is prohibited. • Instance variables can be used but the contents of those instance variables are not guaranteed to be preserved across different client method calls. • Instances of Stateless Session beans are typically pooled because all stateless bean instances are equivalent when they are not involved in servicing a client-invoked method.
- 5. Examples • Sending an e-mail to customer • Adding an item to the database • Getting a particular list of items from the database • Any operation which can be completed in one single shot.
- 6. STATELESS SESSION BEAN LIFE CYCLE
- 7. STATELESS SESSION BEAN LIFE CYCLE STEPS • A session bean instance’s life starts when a client obtains a reference to a stateful session bean instance through dependency injection or JNDI lookup. • Stateless Session Bean is never passivated because it has only two stages. • The EJB Container creates and maintains a pool of stateless session beans, beginning its lifecycle . • The container performs any dependency injection and then invokes the method annotated @PostConstruct . • At the end of the life cycle , the EJB Container calls the method annotated @PreDestroy , if it exists. The bean instance is then ready for garbage collection
- 8. STATEFUL SESSION BEAN • Stateful Session Beans are business objects with @Stateful annotation . • Stateful Session Beans keep track of which calling client they are dealing with throughout a session. • A session bean is not shared and access to the bean instance is strictly limited to only one client at a time. • When the client terminates, its session bean appears to terminate and is no longer associated with the client. • When the client invokes a method on the bean marked with @Remove, it signals the end of the session with the bean. • If the client removes the bean, the session ends and the state disappears.
- 9. Examples • Shopping cart can be implemented by Stateful session bean where the list of items added to the cart by a user is stored in an instance variable of session bean. When a new item is added to the cart all the previous items will still be maintained by the bean until the session ends.
- 10. STATEFUL SESSION BEAN LIFE CYCLE
- 11. STATEFUL SESSION BEAN LIFE CYCLE STEPS • A session bean instance’s life starts when a client obtains a reference to a stateful session bean instance through dependency injection or JNDI lookup. • The container performs any dependency injection and then invokes the method annotated @PostConstruct ,if any. The bean is now ready to have its business methods. • While in the ready stage, EJB Container may decide to deactivate or passivate. • EJB Container call @PrePassivate then call method annotated @PostActivate. • At the end of the life cycle , the client invoke a method annotated @Remove and EJB Container calls method anootated @PreDestroy if any. The bean instance is then ready for garbage collection
- 12. SINGLETON SESSION BEAN • Available from EJB 3.1, Singleton Session Beans are business objects having a global shared state within a JVM. • The @Singleton annotation is used to mark the class as Singleton Session Bean. • They are instantiated once per application and exist for the lifecycle of the application. • In cases where the container is distributed over many virtual machines, each application will have one bean instance of the Singleton for each JVM. • It maintains its state between client invocations but that state is not required to survive container shutdown or crash. • A Singleton session bean is intended to be shared and supports concurrent access by clients.
- 13. Examples • Loading a global daily price list that will be the same for every user might be done with a singleton session bean, since this will prevent the application having to do the same query to a database over and over again. • Application hit counter
- 14. SINGLETON SESSION BEAN LIFECYCLE
- 15. • A singleton session bean instance’s life starts when the container invokes the newInstance method on the session bean class to create the singleton bean instance. • Next, the container performs any dependency injection as specified by the metadata annotations on the bean class or by the deployment descriptor. • If singleton is annotated with @Startup annotation then container performs any dependency injection and invokes @PostConstruct ,if any. The singleton bean instance is now ready to have a business method by the client . • At the end of the life cycle , the EJB Container calls the method annotated @PreDestroy if exists. and EJB Container calls method anootated @PreDestroy if any. The singleton bean is now ready for garbage collection
- 16. What is a Message-Driven Bean? • A message driven bean is a type of enterprise bean, which is invoked by EJB container when it receives a message from queue or topic. Message driven bean is a stateless bean and is used to do task asynchronously. • A message-driven bean is an enterprise bean that allows Java EE applications to process messages asynchronously. This type of bean normally acts as a JMS message listener, which is similar to an event listener but receives JMS messages instead of events.
- 17. Characteristics of Message-driven beans • They execute upon receipt of a single client message. • They are invoked asynchronously. • They are relatively short-lived. • They do not represent directly shared data in the database, but they can access and update this data. • They can be transaction-aware. • They are stateless.
- 18. Message-Driven Bean Life Cycle
- 19. Message-Driven Bean Life Cycle steps • A message-driven bean instance’s life starts when the container invokes newInstance on the message-driven bean class to create a new instance. • Next, the container injects the bean’s MessageDrivenContext, if applicable, and performs any other dependency injection as specified by metadata . • The container then calls the bean’s PostConstruct lifecycle callback methods, if any. • The message-driven bean instance is now ready to be delivered a message sent to its associated destination or endpoint by any client. • When the container no longer needs the instance , the container invokes the PreDestroy lifecycle callback methods for it, if any. This ends the life of the message-driven bean instance.