barcode in vb.net 2008 Stateful vs. Stateless Session Beans in Java

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Stateful vs. Stateless Session Beans
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A stateful session bean will maintain a conversational state with a client. The state of the session is maintained for the duration of the conversation between the client and the stateful session bean. When the client removes the stateful session bean, its session ends and the state is destroyed. The transient nature of the state of the stateful session bean should not be problematic for either the client or the bean, because once the conversation between the client and the stateful session bean ends, neither the client nor the stateful session bean should have any use for the state. A stateless session bean will not maintain conversational states for specific clients longer than the period of an individual method invocation. Instance variables used by a method of a stateless bean may have a state, but only for the duration of the method invocation. After a method has finished running either successfully or unsuccessfully, the states of all its instance variables are dropped. The transient nature of this state gives the stateless session bean beneficial attributes, such as the following:
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Any stateless session bean method instance that is not currently invoked is equally available to be called by an EJB container or application server to service the request of a client. This allows the EJB container to pool stateless bean instances and increase performance. Because stateless session beans are able to service multiple clients, they tend to be more scalable when applications have a large number of clients. When compared to stateful session beans, stateless session beans usually require less instantiation.
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Distinguish Between Stateful and Stateless Session Beans
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An EJB container will never move a stateless session bean from RAM out to a secondary storage, which it may do with a stateful session bean; therefore, stateless session beans may offer greater performance than stateful session beans.
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Since no explicit mapping exists between multiple clients and stateless bean instances, the EJB container is free to service any client s request with any available instance. Even though the client calls the create() and remove() methods of the stateless session bean, making it appear that the client is controlling the life cycle of an EJB, it is actually the EJB container that is handling the create() and remove() methods without necessarily instantiating or destroying an EJB instance.
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Defining the Session Bean Class (Prior to EJB 3.0)
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The session bean class must be declared with the public attribute. This attribute enables the container to obtain access to the session bean. Java gives a developer the ability to extend a base class and inherit its properties. This ability pertains to session beans as well, allowing developers to take full advantage of any objectoriented legacy code that they may wish to reuse. The following is an example of a session bean (prior to EJB 3.0) extending a base class:
public class ValidateInputBean extends TradingBaseClass implements SessionBean { ... }
Session Bean Interface (Prior to EJB 3.0)
Session beans are held to the Java Platform EE (prior to EJB 3.0) specification that requires all session beans to implement the javax.ejb.SessionBean interface. This requirement forces session beans to contain the following methods:
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ejbActivate() ejbPassivate() ejbRemove() setSessionContext(SessionContext)
7: Enterprise JavaBeans and the EJB Container Model
A minimum sample of how a bean class must look is shown here:
public class ValidateInputBean extends TradingBaseClass implements SessionBean { public void ejbActivate () throws EJBException {..} public void ejbPassivate () throws EJBException {..} public void ejbRemove () throws EJBException {..} protected SessionContext m_context; public void setSessionContext (SessionContext sc) throws EJBException { m_context = sc; } }
Clients of a session bean may either be remote or local, depending on what interfaces are implemented. Remote clients access a session bean via their remote and remote home interfaces (javax.ejb.EJBObject and javax.ejb.EJBHome, respectively). Remote clients have the advantage of being location independent. They can access a session bean in an EJB container from any Remote Method Invocation-Internet Inter-ORB Protocol (RMI-IIOP) compliant application, including non-Java programs such as CORBAbased applications. Because remote objects are accessed through standard Java RMI APIs, objects that are passed as method arguments are passed by value. This means that a copy of the object being passed is created and sent between the client and the session bean. Local clients access a session bean via their local and local home interfaces (javax .ejb.EJBLocalObject and javax.ejb.EJBLocalHome, respectively). A local client is location dependent. It must reside inside the same JVM as the session bean with which it interfaces. Local clients can have objects passed as arguments to methods by reference. Doing this avoids the overhead of creating copies of objects sent between clients and session beans. Certain applications will perform considerably better without this overhead. The enterprise bean provider should be aware that both the client and the session bean can change common objects. Both local and remote home interfaces (javax.ejb.LocalHome and javax.ejb.EJBHome, respectively) provide an interface to the client, allowing the client to create and remove session objects. However, session objects are more commonly removed by using the remove() method in the EJBObject interface. Neither local nor remote clients access session beans directly. To gain access to session bean methods, they use a component interface to the session bean. Instances of a session bean s remote interface are called session EJBObjects, while instances of a session bean s local interface are called session EJBLocalObjects.
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