barcode 128 generator vb.net 7: Enterprise JavaBeans and the EJB Container Model in Java

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7: Enterprise JavaBeans and the EJB Container Model
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CERTIFICATION OBJECTIVE 7.03
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Recognize Appropriate Uses for Entity, Stateful Session, and Stateless Session Beans
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In an enterprise environment, application use may grow to a point at which systems based on such things as Java servlets and Hypertext Markup Language (HTML) are not scalable to provide the required performance. At this point, a distributed solution can provide the scalability needed to meet changing demands. EJB allows the application to be distributed onto as many servers as required.
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When to Use Entity and Session JavaBeans
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The following details some appropriate scenarios for using EJBs:
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n Use entity beans to persist data. An entity bean is a sharable enterprise data
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resource that can be accessed and updated by multiple users.
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n Use stateful session beans when any one of the following conditions is true;
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otherwise use stateless session beans:
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n The session bean must retain data in its member variables across method
invocations.
n The state of the bean needs to be initialized when the session bean is instantiated. n The session bean must retain information about the client across multiple
method invocations.
n The session bean is servicing an interactive client whose presence must be
known to the applications server or EJB container.
CERTIFICATION OBJECTIVE 7.04
Distinguish Between Stateful and Stateless Session Beans
Now that you know a little bit about the EJB architecture, session beans, and entity beans, this objective breaks down each of these components in detail and covers using session beans and the differences between stateless session beans and stateful
Distinguish Between Stateful and Stateless Session Beans
session beans. You ll learn how to define stateless and stateful session bean classes, add methods to them, define the session bean interface, create a remote interface, create a home interface, and create deployment descriptors. We ll talk about the steps required to compile, assemble, and deploy stateless session beans and how to call them from a client.
Using Session Beans
Building a session bean can be quite simple once you have mastered a few basic steps. These steps are explained later in this section by walking you through an example of a session bean that provides validation for fields passed to it in a hash table. As mentioned, session beans can either be stateful or stateless. With stateful beans, the EJB container saves internal bean data during and in-between method calls on the client s behalf. With stateless beans, the clients may call any available instance of an instantiated bean for as long as the EJB container has the ability to pool stateless beans. This enables the number of instantiations of a bean to be reduced, thereby reducing required resources.
Stateless Session Beans
A session bean represents work performed by a single client. That work can be performed within a single method invocation, or it may span multiple method invocations. If the work does span more than one method, the object must retain the user s object state across the method calls, and a stateful session bean would therefore be required. Generally, stateless beans are intended to perform individual operations automatically and don t maintain state across method invocations. They re also amorphous, in that any client can use any instance of a stateless bean at any time at the container s discretion. They are the lightest in weight and easiest to manage of the various EJB component configurations.
Stateful Session Beans
Stateful session beans maintain state both within and between transactions. Each stateful session bean is therefore associated with a specific client. Containers are able to save and retrieve a bean s state automatically while managing instance pools (as opposed to bean pools) of stateful session beans.
7: Enterprise JavaBeans and the EJB Container Model
Stateful session beans maintain data consistency by updating their fields each time a transaction is committed. To keep informed of changes in transaction status, a stateful session bean implements the SessionSynchronization interface. The container calls methods of this interface while it initiates and completes transactions involving the bean. Session beans, whether stateful or stateless, are not designed to be persistent. The data maintained by stateful session beans is intended to be transitional. It is used solely for a particular session with a particular client. A stateful session bean instance typically can t survive system failures and other destructive events. While a session bean has a container-provided identity (called its handle), that identity passes when the client removes the session bean at the end of a session. If a client needs to revive a stateful session bean that has disappeared, it must provide its own means to reconstruct the bean s state.
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