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Enterprise JavaBeans Model Author: Gopalan Suresh Raj A basic EJB architecture is shown in the Figure below and consists of:
Now, let us examine the primary components of the EJB architecture in greater detail: EJB servers The EJB server provides an organized framework or execution environment for EJB containers to run in. It makes available system services for multiprocessing, load-balancing, and device access for EJB containers. The EJB server also makes EJB containers running within them visible to the outside world. The EJB server may also provide vendor specific features like an optimized data access interface, additional CORBAServices, SSL support etc. The EJB server is also required to provide JNDI-accessible naming service and transaction management services. In some respects, the EJB server is analogous to CORBA’s Object Transaction Monitor (OTM). The OTM too provides an execution framework for running server side CORBA components. EJB containers An EJB container acts as the interface between an enterprise Bean and low-level, platform-specific functionality that supports the Bean. In essence, the EJB container is an abstraction that manages one or more EJB classes while making the required services available to EJB classes through standard interfaces as defined in the EJB specification. The container vendor is also free to provide additional services implemented at either the container or the server level. An EJB client never accesses a Bean directly. Any Bean access is done through the methods of the container-generated classes, which in turn invoke the Bean's methods. Having the container interpose on all Bean invocations allows the container to manage transactions, load Bean instances if necessary, and in general to do all the wonderful things that EJBs do. There are two types of containers: Session containers that may contain transient, non-persistent EJBs whose states are not saved at all and entity containers that contain persistent EJBs whose states are saved between invocations. The EJB Home interface and Home object Factory methods for locating, creating and removing instances of EJB classes are defined in the EJB Home interface. The Home object is the implementation of the Home interface. The EJB developer defines the Home interface for his Bean. The EJB container vendor provides tools that automatically generate the implementation code for the home interface defined by the EJB developer. The Remote interface and EJBObject The remote interface lists the business methods available for the enterprise Bean. The EJBObject is the client’s view of the enterprise Bean and implements the remote interface. While the enterprise Bean developer defines the remote interface, the container vendor provides the tools necessary to generate the implementation code for the corresponding EJBObject. However, it should be noted that the EJB container is still responsible for managing the EJBObject. Each time the client invokes the EJBObject’s methods, the EJB container first handles the request before delegating it to the enterprise Bean The Enterprise JavaBean The real EJB Bean itself is contained within an EJB container, and should never be directly accessed by anyone but the container. Although direct access may be possible, it is inadvisable as that breaks the contract between the Bean and the container The EJB container should mediate all EJB Bean accesses. For this reason, the EJB Bean developer does not implement the remote interface within the EJB Bean itself. The implementation code for the remote interface is generated automatically by tools that the container vendor provides. This prevents inadvertent direct accesses from clients or other beans The EJB Clients EJB clients locate the specific EJB container that contains the enterprise Bean through the Java Naming and Directory Interface (JNDI). They then make use of the EJB container to invoke Bean methods. The EJB client only gets a reference to an EJBObject instance and never really gets a reference to the actual EJB Bean instance itself. When the client invokes a method, the EJBObject receives the request and delegates it to the corresponding Bean instance while providing any necessary wrapping functionality. The client uses the Home object to locate, create or destroy instances of an EJB class. It uses the EJBObject instance, to invoke the business methods of a Bean What do you think of this article? Have your say about the article. You can make your point about the article by mailing [email protected] (If you haven't allready joined, you can join by going to https://www.onelist.com/community/dev-java). You can also write a review. We will publish the best ones here on this article. Send your review to [email protected]. Please include the title of the article you are reviewing. Further Reading The Enterprise JavaBeans Series:
Author: Gopalan Suresh Raj Gopalan has his own site at Author Central (visit him. He also maintains his own site at https://www.execpc.com/~gopalan/) - Contribute to iDevResource.com and you can have one too! © Copyright 1997-2000 Gopalan Suresh Raj. Reproduced with Permission |
Enterprise Java Beans
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