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Enhanced Component-Based Development

Enhanced Component-Based Development

Component-based development (CBD) has been around for years. Anyone who has been a developer for any length of time has no doubt leveraged some form of reuse in an application.

It may have been as simple as using a Swing component on the form of a client application. As with many areas of technology, software development and the evolution of developer tools is marked not so much by huge, dramatic leaps forward, but rather by constant innovation and improvement. When Sun Microsystems first introduced the Enterprise JavaBean (EJB) specification in 1997, no one thought that managing persistence would be such a big deal. But a big deal it was. Sun listened, and introduced Container-Managed Persistence (CMP) in the EJB 2.0 specification.

More recently, BEA has listened to its customers and provided an integrated development environment for the WebLogic platform that goes a long way toward facilitating effective CBD: BEA WebLogic Workshop. Over the course of this article, I'm going to talk about some of the challenges you face when utilizing components in Enterprise Java development and how the process can be simplified by utilizing the BEA control framework.

Planning
The time to think about how to leverage reuse in your application is at the project-planning phase. As with most aspects of software development, proper planning is essential to success. Once the program requirements are established, the first thing you should do is look in your repository and identify component candidates that can possibly be leveraged. In addition to providing the required functionality, the component must also meet the technical requirements of your application. As you go through the planning stages, be prepared with the answers to some very specific questions: Is the application intended to be Web based? Will a JavaServer Pages application in conjunction with servlets be adequate, or should you also include Web services for scalability and cross-platform flexibility? Are you building a distributed, thin-client app? Will this need the scalability and/or ease of deployment/updating provided by EJBs?

Assuming that your component repository is well documented, you should have no trouble identifying which components will be useful once these questions are answered and the program requirements are established. In many cases, the amount of "glue code" needed to integrate the component into the new project will be minimal. In other cases, there may be a fair amount of work involved. If that work estimate exceeds the amount of time it took to create the component originally, you won't see any reuse cost avoidance and you'll need to either look for another component or create one from scratch.

Another part of the planning process is determining what development environment you will use to create the individual pieces of your application. You have some flexibility here. For instance, when building an n-tier distributed app, you will probably use an IDE such as JBuilder for the client portion. You might use JBuilder for the server-side development as well, or you might decide to go with WebLogic Workshop.

WebLogic Workshop and the BEA Control Framework
BEA WebLogic Server is a powerful platform for Enterprise Java development. BEA WebLogic Workshop is tightly integrated to provide an integrated development and testing environment for Web services (and, soon, Web applications) for the WebLogic platform. As such, it is important for you to consider the added benefits that the BEA control framework provides.

For example, let's assume that as a result of your project planning, you have identified several EJB components that you wish to leverage in your new project. By taking advantage of WebLogic Workshop and the BEA control framework, as well as the inherent flexibility of the J2EE platform, you can create a distributed, scalable, back-end system that supports a variety of client deployments, including a Web application, a thin-client Java application, cross-platform deployment, and more!

To demonstrate this, I'm going to take you through some sample applications that utilize EJBs to provide back-end business logic. We'll also look at how the BEA control framework simplifies the process of integrating these EJBs into our Web services and Web applications.

Note: The sample applications referenced here come packaged with WebLogic Workshop. This allows me to focus on the component-specific aspects within the broader context of a JWS project and enables you to reference the sample projects in your own Workshop environment and experience a working demo.

Entity Beans
Entity beans represent data. They are usually mapped either to records of a relational database or to objects of an object-oriented database. They are transactional in that changes to their state typically occur within the context of a transaction. An instance of an entity bean may be referenced by multiple clients. The container - in this case WebLogic Server - is responsible for ensuring that the data in the bean and the corresponding database are synchronized. Figure 1 shows the sample project, "AccountEJBClient.jws", loaded into WebLogic Workshop. As you can see, Workshop has wrapped the bean as an EJB control and has implemented control interfaces that map to corresponding EJB methods.

If we were using this bean in another environment, we would have to go through a bit of a process in order to use it. First, we would have to look up the EJB in the JNDI registry and obtain the home interface. Next, we would need to obtain an instance of the EJB. Finally, we could invoke its methods. While this is by no means a difficult or complicated process, utilizing the control framework makes instantiating and accessing the bean even simpler. All we have to do is add a new EJB control (see Figure 2), point to the EJB (see Figure 3), and we're done. As you can see, once you've browsed to the EJB, Workshop automatically finds the home and remote interfaces. It takes every method of the bean and creates a corresponding control interface. It creates a corresponding control file (.CTRL) for each control created. Listing 1 shows the control file for this EJB control.

Note: Some controls can be modified by editing the CTRL file. Do not edit an EJB's CTRL file. It is created by Workshop based upon information it obtains from the EJB's code.

Now that the control has been created, it automatically handles the J2EE plumbing such as instantiating and destroying the bean, accessing the methods, and the like. Please remember that you must have a local copy of the EJB's JAR file in the WEB-INF\lib directory of your Workshop project in order for Workshop to discover the EJB's home and remote interfaces.

Listing 2 shows how WebLogic Workshop accesses the EJB and is the code for the createNewAccount operation. It calls the Create method of the EJB control (which wraps the J2EE interfaces of the EJB's Create method). As you can see, Workshop has done the work of specifying the method parameters as well as putting in exception handling. Since this is a simple "Web Service method to EJB method" mapping, there is little else to do (assuming you trust the functionality of the EJB). However, you can add custom validation code here if you wish.

Session Beans
Session beans represent particular business process steps. They capture the interaction or conversation between the user and the system. Session beans come in two flavors: stateful and stateless. Stateful session beans maintain state across multiple remote method invocations by a client on the same client-proxy of the session bean. Every method invocation by the client is routed to the same session bean instance on the server. These are useful when you need to carry out a long conversation with the server and you need the server object to save the conversational state. An example of a stateful session bean is an online shopping cart, where a client might use remote methods to add, remove, or modify the products in the shopping cart.

Stateless session beans do not maintain any state that the client can depend on between multiple remote method invocations. These are useful for situations where the duration of the method execution constitutes a complete unit of work from the client's perspective. For an example of a Stateless Session Bean, open the 'TraderEJBClient.jws' sample project in WebLogic Workshop. As you can see from the JWS operations (see Figure 4), this component has only two methods: Buy and Sell. The EJB control (which wraps the interfaces of the EJB itself) actually has a third interface: Create. In fact, every session and entity EJB contains this method. This is how the instance of the bean is actually created. When dealing with entity beans, this method must be explicitly called - since creating an instance of an entity bean actually creates a new row in the corresponding datasource. However, with WebLogic Workshop it is not necessary to fire off the Create method of session beans. The BEA control framework automatically creates an instance of the bean when any of the methods are called. Even if you do explicitly call the Create method, you might not get a new instance. WebLogic Server may simply return an existing instance of the bean.

Since this is a fine-grained component, I have included both the Buy and Sell method calls in Listing 3. You'll notice that there is very little (if any) validation going on here. One glaring weakness is the lack of any validation that the user actually owns a sufficient quantity of shares to cover the sale. This validation is best added into the client portion of your application, or as part of a larger Web service - one that integrates an entity bean for account management and a stateful session bean to handle an extended conversation with the server (perhaps for transacting multiple stock sales). You want this validation complete before you even instantiate this bean.

Message-Driven Beans
Message-driven Beans (MDBs) enable the development of asynchronous applications. They essentially sit on the server and wait for Java Message Service (JMS) messages. They are anonymous in that no client may hold a reference to an MDB - only the EJB container may access the bean directly. They have neither a home nor a remote interface. As such, they cannot be used inside WebLogic Workshop. There is, however, an alternative. The BEA control framework includes a JMS control that you can add to your Workshop project. It can then listen for callbacks. Figure 5 shows a JWS project for credit reporting that uses a JMS control to receive credit information from the queue. Listing 4 shows the receiveMessage callback handler that processes the callback. This provides the same basic functionality of an MDB from within the Workshop environment.

Summary
Mission-critical applications that depend on tried-and-tested code benefit greatly from component reuse. WebLogic Server - in conjunction with WebLogic Workshop - can make the integration of these components a breeze.

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