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By Sams Publishing July 29, 2002 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 EJBsJ2EE provides different types of components for different purposes. Today, you will start to look at one of the principal types of component in J2EE—Enterprise JavaBeans (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
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 ServersAn 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 component—the 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 LandscapeAs 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. Discovering EJBsWhile 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 analysis—data 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 EJBThere are three different types of EJB that are suited to different purposes:
As an application designer, you should choose the most appropriate type of EJB based on the task to be accomplished. Common Uses of EJBsSo, given all of this, where would you commonly encounter EJBs and in what roles? Well, the following are some examples:
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:
These are common ways that EJBs are applied. What benefits does the use of EJBs give to you as a developer? Why Use EJBs?Despite the recommendations of the J2EE Blueprints, the use of EJBs is not mandatory. You can build very successful applications using servlets, JSPs or standalone Java applications. As a general rule of thumb, if an application is small in scope and is not required to be highly scalable, you can use J2EE components, such as servlets, together with direct JDBC connectivity to build it. However, as the application complexity grows or the number of concurrent users increases, the use of EJBs makes it much easier to partition and scale the application. In this case, using EJBs gives you some significant advantages. Hiding ComplexityEarly middleware environments, such as "raw" CORBA, require the application developer to write a lot of code that interacts with the CORBA environment and facilitates the connectivity and registration process. Such code can be likened to the plumbing that pipes water around a house. It needs to be there but, as the user of a sink or shower, you do not want to be intimately involved with it. In J2EE application terms, business developers want to write business code, not "plumbing" code. The EJB model tries to reduce such interaction to a minimum by using the following mechanisms:
Separation of Business Logic from UI and Data AccessOne of the key facets of applying EJBs is that they allow business functionality to be developed and then deployed independently of the presentational layer. You might, for example, create an application with a user interface built using Java's Swing API. This application might then provide access to some business functionality for the employees working on the company's internal network. If the underlying business functionality is implemented using EJBs, a different user interface could take its place without having to redevelop the entire application. A Web-based interface that used servlets would make the application available from the Internet without having to change a single line of code in the business functionality. Figure 4.1 is a UML component diagram that shows this. (More information on UML can be found in Appendix A, "An Introduction to UML," on the accompanying CD-ROM.) It can sometimes be difficult to distinguish between the functionality that an application provides and the user interface that is used to invoke that functionality. This is not unexpected because many common applications—such as a word-processor—are single-tier; the presentational logic and the business functionality are a single entity. On the other hand, consider programming a video recorder. Most modern video recorders can be programmed either directly on the console or through a remote control unit. Either user interface will accomplish the task of recording your favorite TV show, but there is only a single "application."
Figure 4.1: An application implemented using EJBs can have more than one user interface. Consider another example. In most supermarkets, a cashier is responsible for scanning the items in your shopping cart and then requesting a payment for the total. However, some supermarkets also offer a trust system, whereby the customer scans the items with a mobile scanner as they place the item into the shopping cart. To pay for the goods in the shopping cart, the customer simply swipes his or her own card, and then leaves with the goods. Again, there is a single application (to purchase shopping items) but two different interfaces—the cashier's till and the customer's mobile scanner. To implement a distributed application using EJBs, make sure you have distinguished between the user interface and the underlying business function. The EJB itself is concerned only with the latter of these. Container ServicesThe container provides various services for the EJB to relieve the developer from having to implement such services, namely
All of these container services are covered in more detail as the book progresses. Now that you know why you would want to use an EJB and how to apply it, you can examine the inner workings of an EJB to understand how all the parts fit together. Tuesday's installment: What's in an EJB?About the AuthorAndy Longshaw is a consultant, writer and educator specializing in J2EE, XML, Web-based technologies and components, particularly the design and architecture decisions required to use these technologies successfully. Andy has been explaining technology for most of the last decade as a trainer and in conference sessions. A wild rumor suggests that some people have managed to stay awake in these sessions. Despite being well educated and otherwise fairly normal, Andy still subjects himself, and his family, to "trial by unpredictability" by watching Manchester City FC far more often than is healthy. Andy and the other authors work for Content Master Ltd., a technical authoring company in the United Kingdom specializing in the production of training and educational materials. For more information on Content Master, please see their web site at www.contentmaster.com Source of this material
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