WebSphere Studio Application Developer Version 5 Programming Guide part 33 ppsx

294 WebSphere Studio Application Developer Version 5 Programming Guide What is Struts? Struts is an open source framework for building Web applications according to the model-view-controller (MVC) architectural pattern. Struts is part of the Jakarta project, sponsored by the Apache Software Foundation. By using Struts you can get a clean separation between the presentation and business logic layers of your application. Struts also speeds up Web application development by providing an extensive JSP tag library, parsing and validation of user input, error handling, and internationalization support. The scope of this chapter is not to teach you the Struts framework in detail, but to show how to use Application Developer’s Struts tools for building a Struts Web application. To learn more about the Struts framework, please refer to the official Jakarta project Struts home page and the official Struts user guide at: http://jakarta.apache.org/struts http://jakarta.apache.org/struts/userGuide/introduction.html There is also a number of very good Struts tutorials available on the Internet. MVC architectural pattern The model-view-controller architectural pattern was conceived in the mid-1980's by developers of the Smalltalk-80 GUI library. It has since then been extensively applied in most object oriented user interface and application control toolkits, as is the case with Struts, and has been improved to respond to specific platform requirements, such as J2EE. It has also been generalized as a pattern for implementing the separation of concerns among application layers in general, and not only the three originally proposed layers. According to the MVC pattern, a software component (an application, a module, and so forth) should separate its business logic (the model) from its presentation (the view). There are many reasons for this requirement, among which are these facts:  You may have more than one view of the same model. If both the business logic and its presentation were built together, adding an additional view would cause considerable disruptions and increase the component's complexity. A good example of a model with two views would be a banking application that can be accessed through both the Internet and mobile phones. Note: Application Developer 5.0 includes support for Struts Version 1.02 and Version 1.1 beta 2. At the time of writing, the latest version of the Struts framework is version 1.1 Release Candidate 1. Chapter 10. Developing Struts applications 295  You do not want to have to change the model every time you modify the view. The view is definitely dependent on the model, for it presents specific aspects of the model to the user. It makes no sense, though, to have the model depend on the view. Building both together dramatically increases the chances of this happening, and of you having to change the model every time you implement a small change to the view. This separation can be achieved through the layering of the component into:  The model layer, responsible for implementing the business logic.  The view layers, each responsible for rendering the user interface (be it graphical or not) to a specific client type and in a specific fashion. With these two layers, we can implement the business logic and present it to the user. That solves only half of the problem. We would also like to be able to interact with the model. The implementation of this interaction is better left to a third layer, called controller. In the following sections we discuss some of the properties of each of these layers. Model The model layer manages the application domain’s concepts, both behavior and state. It responds to requests for information about its state and responds to instructions to change its state. Just like any software component, the model should have a well-defined and an as simple as possible public interface. This is usually achieved through the use of a facade. The intent of facades is to provide a simple and unified interface to the otherwise complex model that lies behind it. By doing so, we reduce the dependencies between the model classes and its clients. Less dependencies mean more freedom to adapt to new requirements. As an example, consider an appliance with which you are certainly familiar: a television set. You will probably agree that changing TV channels is quite a simple task. Now ask yourself these questions:  Do you know how the television set represents channels internally?  Do you know exactly what goes on when you change TV channels?  Do you think you should know these things? Note: Facade is a documented design pattern. For more information, refer to Design Patterns: Elements of Reusable Object-Oriented Softwar e. 296 WebSphere Studio Application Developer Version 5 Programming Guide Chances are that you have answered no to all of the questions above. What is the advantage of you not knowing how TVs represent their data and implement their operations? That way you can buy a new TV, which does these things internally in a completely different way, and you could still change channels. Figure 10-1 shows the model layer with its encapsulated business domain objects and the exposed facade object. Figure 10-1 Model layer Please note that the model does not have any dependences on views or controllers. View The view layer implements a rendering of the model. The responsibility of the view is to know what parts of the model’s state are relevant for the user, and to query the model for that information. The view retrieves the data from the model or receives it from the controller, and displays it to the user in a way the user expects to see it. Controller The controller’s responsibility is to capture user events and to determine which actions each of these events imply, depending on both the user’s and the application’s state. This usually involves verifying pre- and post-conditions. These actions can then be translated to messages to the model and view layers, as appropriate. Model Façade Chapter 10. Developing Struts applications 297 Dependencies between MVC layers Figure 10-2 shows the dependencies allowed in the MVC pattern. Note that the less dependencies your layers have, the easier it will be for the layers to respond to requirement changes. Figure 10-2 Dependencies allowed in the MVC pattern So, to summarize, the MVC pattern is really about separation. MVC framework with Struts Struts provides these components to develop applications using MVC (Figure 10-3): Model Struts does not provide model classes. The business logic must be provided by the Web application developer as JavaBeans or EJBs. View Struts provides action forms to create form beans that are used to pass data between the controller and view. In addition, Struts provides custom JSP tag libraries that assist developers in creating interactive form-based applications using JSPs. Application resource files hold text constants and error message, translated for each language, that are used in JSPs. Controller Struts provides an action servlet (controller servlet) that populates action forms from JSP input fields and then calls an action class where the developer provides the logic to interface with the model. A B <=> A depends on B Controller View Model 298 WebSphere Studio Application Developer Version 5 Programming Guide Figure 10-3 Struts components in the MVC architecture A typical Struts Web application is composed of these components:  A single servlet (extending org.apache.struts.action.ActionServlet) implements the primary function of mapping a request URI to an action class. Before calling the action class, it populates the form bean associated to the action with the fields from the input JSP. If specified, the action servlet also requests the form bean to validate the data. It then calls the action class to carry out the requested function. If form bean validation fails, control is returned to the input JSP so the user can correct the data. The action servlet is configured by an XML configuration file that specifies the environment and the relationship between the participating components.  Multiple JSPs that provide the end-user view. Struts comes with an extensive tag library to make JSP coding easier. The JSPs display the information prepared by the actions and requests new information from the user.  Multiple action classes (extending org.apache.struts.action.Action) that interfaces with the model. When an action has performed its processing, it returns an action forward object which determines the view that should be called to display the response. The action class prepares the information required to display the response, usually as a form bean, and makes it available to the JSP. Usually the same form bean that was used to pass information to the action is used also for the response, but it is also common to have special view beans tailored for displaying the data. An action forward has properties for its name, address (URL) and a flag specifying if a forward : ActionForm View Controller Model Action Action Action Action configuration file Model Application Resources ActionServlet Tag libraries : JSP Struts Support Chapter 10. Developing Struts applications 299 or redirect call should be made. The address to an action forward is usually hard coded in the action servlet configuration file but can also be generated dynamically by the action itself.  Multiple action forms (extending org.apache.struts.action.ActionForm) to hold the data retrieved from the JSPs. The action forms are generic Javabeans with getters and setters for the input fields available on the JSPs. Usually there is one form bean per Web page, but you can also use more coarse-grained form beans holding the properties available on multiple Web pages (this fits very well for wizard-style Web pages). If data validation is requested (a configurable option) the form bean is not passed to the action until it has successfully validated the data. Therefore the form beans can act as a sort of firewall between the JSPs and the actions, only letting valid data into the system.  One application resource file per language supported by the application holds text constants and error messages and makes internationalization easy. Figure 10-4 shows the flow of information for an interaction in a Struts Web application. Figure 10-4 Struts request sequence A request from a Web browser reaches the Struts ActionServlet. If the action that will handle the request has a form bean associated with it, Struts creates the form bean and populates it with the data from the input form. It then calls the validate method of the form bean. If validation fails, the user is returned to the input page to correct the input. If validation succeeds, Struts calls the action’s execute method. The action retrieves the data from the form bean and performs the appropriate logic. Actions often call session EJBs to perform the business : Web user (Browser) : ActionServlet : Action : ActionForm : JSP HTTP setXxx() validate() execute() forward() getXxx() getXxx() setXxx() "forward" 300 WebSphere Studio Application Developer Version 5 Programming Guide logic. When done, the action either creates a new form bean (or other appropriate view bean) or reuses the existing one, populates it with new data, and stores it in the request (or session) scope. It then returns a forward object to the Struts action servlet, which forwards to the appropriate output JSP. The JSP uses the data in the form bean to render the result. Application Developer support for Struts Application Developer provides specific support for Struts-based Web applications:  A Web project can be configured for Struts. This adds the Struts run time (and dependent JARs), tag libraries, and action servlet to the project, and creates skeleton Struts configuration and application resources files. Application Developer 5.0 provides support both for Struts 1.02 and 1.1 beta 2, selectable when setting up the project.  A set of component wizards to define action form classes, action classes with action forwarding information, and JSP skeletons with the tag libraries included.  A configuration file editor to maintain the control information for the action servlet.  A graphical design tool to edit a graphical view of the Web application from which components (forms, actions, JSPs) can be created using the wizards. This graphical view is called a Web diagram. The Web diagram editor provides top-down development (developing a Struts application from scratch), bottom-up development (that is, you can easily diagram an existing Struts application that you may have imported) and meet-in-the-middle development (that is, enhancing or modifying an existing diagrammed Struts application).  Web Structure view that provides a hierarchical (tree-like) view of the application. This view shows the artifacts, such as Web pages and actions, and you can expand the artifacts to see their attributes. For example, an action can be expanded to see its forwards, which can then be expanded to see its links. This is useful for understanding specific execution paths of your application. The Web Structure view is available in the Web perspective.  Page Designer support for rendering the Struts tags, making it possible to properly view Web pages that use the Struts JSP tags. This support is customizable using Application Developer’s Preferences settings. Note: In Struts version 1.0 the execute method of the Action class was called perform. Chapter 10. Developing Struts applications 301  Validators to validate the Struts XML configuration file and the JSP tags used in the JSP pages. Developing a Web application using Struts To show how to use the Application Developer Struts tools, we rewrite the RedBank Web application we developed in “Sample application: RedBank” on page 180 using the Struts framework. The application is a banking application that allows a customer to enter his or her customer number, select an account to work with, and then to perform four different transactions: list the transactions for the selected account, deposit, withdraw, and transfer money. The back-end of the application is, for sake of simplicity, implemented as simple JavaBeans. We will implemented the back-end using EJB technology in Chapter 12, “Developing EJB applications” on page 373. Because the Struts framework provides us with the controller and view parts of a Web application, but not the model, we will reuse the model from the RedBank application and simply show you how to replace its controller and view with Struts. To create our Web application with Struts:  We create a new Web project with Struts 1.1 beta 2 support.  We import the parts of the existing RedBank application that we will reuse.  We create a Web diagram where we lay out all the JSPs, form beans, and actions that make up the application, and connect them to define the application flow.  We implement the components one by one.  Finally, we do some customization using the Struts configuration file editor and then launch the application in the built-in WebSphere Test Environment. Note: Because this chapter focuses on Application Developer’s Struts tools and wizards (more than the architecture and best practices of a Struts application) we try to use the Struts tools and wizards as much as possible when creating our application. After having used the wizards to create some components (JSPs, form beans, actions) you may be able to create new components even quicker by copying and pasting from your existing components than by using all the wizards. 302 WebSphere Studio Application Developer Version 5 Programming Guide Creating a Struts Web project Before we can start implementing the Struts application we create a Web project with Struts support:  Create a new Web project by selecting File -> New -> Web project and enter ItsoProGuideStrutsWeb as the name of the project. Select Add Struts support and then click Next (Figure 10-5). Figure 10-5 Creating a Web project with Struts support  Add the project to the existing ItsoProGuide enterprise application project. Leave ItsoProGuideStrutsWeb as the context root and click Next (Figure 10-6). Note: If you create a Web project as part of creating an Enterprise Application Project you will not be given the option to add Struts support at that time. You will have to add the Struts support afterwards by selecting Properties from the Web project’s context menu and check the Add Struts support option there. Chapter 10. Developing Struts applications 303 Figure 10-6 Web project settings  On the Module Dependencies page, select the ItsoProGuideJava.jar project to create a classpath reference to the model that we reuse and click Next (Figure 10-7). Figure 10-7 Specify module dependencies . 294 WebSphere Studio Application Developer Version 5 Programming Guide What is Struts? Struts is an open source framework for building Web applications according to the. where the developer provides the logic to interface with the model. A B <=> A depends on B Controller View Model 298 WebSphere Studio Application Developer Version 5 Programming Guide Figure. using all the wizards. 302 WebSphere Studio Application Developer Version 5 Programming Guide Creating a Struts Web project Before we can start implementing the Struts application we create a Web