Introduction to EJBs
This is the first of four installments that make up Chapter 4: Introduction to EJBs from the book Sams Teach Yourself J2EE in 21 Days (ISBN:0-672-32384-2) written by Martin Bond, Dan Haywood, Debbie Law, Andy Longshaw, and Peter Roxburgh, published by Sams Publishing.
Day 4: Introduction to EJBs
The study of EJBs is continued on Day 5, "Session EJBs,", Day 6, "Entity EJBs", Day 7, "CMP and EJB QL", Day 8, "Transactions and Persistence", and Day 10, "Message-Driven Beans". As you can see, there is a lot to learn about EJBs, so today serves as a first step on the road to all of this EJB knowledge.
Today, you will
Examine the different types of EJB available
Take a look at how EJBs are applied
Explore the structure of one of the EJBs that forms part of the case study to see how the different parts fit together
Deploy and use some of the EJBs from the case study
Write a simple client for an EJB
First, you need to understand why you would use EJBs.
What Is an EJB?
In a typical J2EE application, Enterprise JavaBeans (EJBs) contain the application's business logic and live business data. Although it is possible to use standard Java objects to contain your business logic and business data, using EJBs addresses many of the issues you would find by using simple Java objects, such as scalability, lifecycle management, and state management.
Beans, Clients, Containers, and Servers
An EJB is essentially a managed component that is created, controlled, and destroyed by the J2EE container in which it lives. This control allows the container to control the number of EJBs currently in existence and the resources they are using, such as memory and database connections. Each container will maintain a pool of EJB instances that are ready to be assigned to a client. When a client no longer needs an EJB, the EJB instance will be returned to the pool and all of its resources will be released. At times of heavy load, even EJB instances that are still in use by clients will be returned to the pool so they can service other clients. When the original client makes another request of its EJB, the container will reconstitute the original EJB instance to service the request. This pooling and recycling of EJB instances means that a few EJB instances, and the resources they use, can be shared between many clients. This maximizes the scalability of the EJB-based application. The EJB lifecycle is discussed further on Days 5 and 6.
The client that uses the EJB instance does not need to know about all of this work by the container. As far as the client is concerned, it is talking to a remote component that supports defined business methods. How those methods are implemented and any magic performed by the container, such as just-in-time instantiation of that specific component instance, are entirely transparent to the client part of the application.
The EJB benefits from certain services provided by the container, such as automatic security, automatic transactions, lifecycle management, and so on. To do this, the EJB must conform to certain rules and implement an appropriate interface that allows the container to manage the component. The EJB is packaged with configuration information that indicates the component's requirements, such as transaction and security requirements. The container will then use this information to perform authentication and control transactions on behalf of the componentthe component does not have to contain code to perform such tasks.
The primary purpose of the container is to control and provide services for the EJBs it contains. When it needs to use some underlying functionality, such as creating a transaction on behalf of a bean, it uses the facilities of the underlying EJB server. The EJB server is the base set of services on top of which the container runs. Different types of EJB will run in different containers, but many different EJB containers can run on a single EJB server. EJB servers are generally delivered as part of a J2EE-compliant application server (examples include BEA WebLogic and IBM WebSphere). You will install and run the application server, which will provide the underlying services required of an EJB server and will host EJB containers.
The EJB Landscape
As you have seen, the J2EE Blueprints (http://java.sun.com/blueprints/enterprise/index.html) define a target architecture for a typical J2EE-based application. In this architecture, EJBs live in the middle tier and are used by other application components that live in the presentation tier. Although it is possible that both of these logical tiers will reside on the same computer, it is most likely that they will reside on different machines. This means that an EJB will usually have to be made available to remote clients.
To offer services to remote clients, EJBs will export their services as RMI remote interfaces. RMI allows you to define distributed interfaces in Java. There are certain caveats on doing this, not only at the implementation level (such as declaring that RemoteExceptions may be thrown when calling a method on an EJB) but also at the design level. Designing remote interfaces is a skill in itself, which will be explored as you progress through topics in this book, such as EJBs and J2EE Patterns.
Because they must use an RMI-based interface to access the functionality of the EJB, the clients of an EJB must have some programming functionality. This means that they are typically either "thick" clients that provide a GUI interface or Web-server components that deliver HTML interfaces to "thin" clients. The different types of client are explored in more detail shortly.
In the other direction, EJBs themselves will make use of data sources, such as databases and mainframe systems, to perform the required business logic. Access to such data and services can be through a JDBC database connection, a J2EE Connector, another EJB, or a dedicated server or class of some form.
While it is quite easy to draw pictures of a 3-tier system containing boxes labelled "EJB," it is important to identify what application functionality should go into an EJB.
At the start of application development, regardless of the precise development process used (Rational Unified Process (RUP), eXtreme Programming (XP), and so on), there is generally some analysis that delivers a Unified Modelling Language (UML) domain model (this identifies the main elements of the business problem to be solved). This can then form the basis of a solution model where the business concepts are mapped into appropriate design-level artefacts, such as components. This is where EJBs come into the design.
The UML model will consist of a set of classes and packages that represent single or grouped business concepts. A class or package can be implemented as an EJB. Generally, only larger individual classes will become EJBs in themselves, because EJBs are intended to be fairly coarse-grained components that incorporate a reasonably large amount of functionality and/or data.
There are generally two types of functionality discovered during analysisdata manipulation and business process flow. The application model will usually contain data-based classes such as Customer or Product. These classes will be manipulated by other classes or roles that represent business processes, such as Purchaser or CustomerManager. There are different types of EJB that can be applied to these different requirements.
Types of EJB
There are three different types of EJB that are suited to different purposes:
Session EJBA Session EJB is useful for mapping business process flow (or equivalent application concepts). There are two sub-types of Session EJB stateless and stateful that are discussed in more detail on Day 5. Session EJBs commonly represent "pure" functionality that is created as it is needed.
Entity EJBAn Entity EJB maps a combination of data (or equivalent application concept) and associated functionality. Entity EJBs are usually based on an underlying data store and will be created based on that data within it.
Message-driven EJBA Message-driven EJB is very similar in concept to a Session EJB, but is only activated when an asynchronous message arrives.
As an application designer, you should choose the most appropriate type of EJB based on the task to be accomplished.
Common Uses of EJBs
So, given all of this, where would you commonly encounter EJBs and in what roles? Well, the following are some examples:
In a Web-centric application, the EJBs will provide the business logic that sits behind the Web-oriented components, such as servlets and JSPs. If a Web-oriented application requires a high level of scalability or maintainability, use of EJBs can help to deliver this.
Thick client applications, such as Swing applications, will use EJBs in a similar way to Web-centric applications. To share business logic in a natural way between different types of client applications, EJBs can be used to house that business logic.
Business-to-business (B2B) e-commerce applications can also take advantage of EJBs. Because B2B e-commerce frequently revolves around the integration of business processes, EJBs provide an ideal place to house the business process logic. They can also provide a link between the Web technologies frequently used to deliver B2B and the business systems behind.
Enterprise Application Integration (EAI) applications can incorporate EJBs to house processing and mapping between different applications. Again, this is an encapsulation of the business logic that is needed when transferring data between applications (in this case, in-house applications).
These are all high-level views on how EJBs are applied. There are various other EJB-specific patterns and idioms that can be applied when implementing EJB-based solutions. These are discussed more on Day 18, "Patterns."
Given this context, common types of EJB client include the following:
A servlet or JSP that provides an HTML-based interface for a browser client
Another EJB that can delegate certain of its own tasks or can work in combination with other EJBs to achieve its own goals
A Java/Swing application that provides a front-end for the business processes encapsulated in the EJB
A CORBA application that takes advantage of the EJB's business logic
An applet that takes advantage of the business logic in a remote EJB so that this business logic does not need to be downloaded to the client
These are common ways that EJBs are applied. What benefits does the use of EJBs give to you as a developer?
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