Introduction to MVC and the Jakarta Struts Framework        By Gyanendra Dwivedi         Software Consultant
What is MVC • The Model View Controller design pattern is a technique used to   separate Business logic/state (the Model) ...
What is MVC (cont…) • The View     o   The view is how data is represented to the user. For instance the         view in a...
What is MVC (cont…)                               User                    View               Controller                   ...
What is MVC (cont…)                                  User               View                                  Controller  ...
Benefits of MVC •   Promotes Modularity •   Multiple views •   Abstraction •   Allows application to be defined in a flow-...
Designing an MVC App. • Let’s try designing an application from scratch • The application will be a message board where us...
Activity Diagrams • I find the UML activity diagrams to be the most useful construct   for describing program flow. • Acti...
Activity Diagram: Create Profile         Create Profile                                                             Create...
Activity Diagram: Logon                 Login FormCreateProfile()               [new user]                                ...
Activity Diagram: View Msg.          Show Msg.            Board                                                           ...
Activity Diagram: Edit Msg.               Select Msg()                   Logon()                                     [expi...
Jakarta Struts • In order to take advantage of the MVC design pattern in your   applications a considerable framework must...
Struts (cont…) • Struts takes the grunt work out of developing an MVC based web   app. • The Struts framework provides a p...
Struts (cont…) • The components of MVC in Struts:     o The View is usually defined as a JSP or HTML page.     o The Model...
Struts: The Model • To define a model in Struts is simple. Just create a java class   and provide some functions. • Your m...
The Model (cont…) • Struts will only handle models in an automatic fashion if you   extend org.apache.struts.action.Action...
Model: ActionForm  import javax.servlet.http.HttpServletRequest;  import javax.servlet.http.HttpServletResponse;  import o...
Model: Action Form (cont…) • When this “LogonForm” is associated with a controller, it will be   passed to the controller ...
Model: ActionForm Validationpublic ActionErrors validate(ActionMapping mapping,HttpServletRequest request) {if (“”.equals(...
The Controller • The controller is the switch board of MVC. • It directs the user to the appropriate views by providing th...
Controller (cont…)http://myhost/authorize.doServer configured to pass *.do extensions toorg.apache.struts.action.ActionSer...
Controller: Samplepublic class LogonAction extends Action {public ActionForward perform(ActionMapping mapping, ActionForm ...
Controller: Forwards • You might be wondering what mapping.findForward(“success”)   means?     o The mapping object passed...
Controller: ActionMapping<action-mappings><action path="/logon"type="org.apache.struts.example.LogonAction"name="logonForm...
The View    •   The view in Struts is             •   JSP solves this by introducing        represented by JSP pages      ...
The View: Example<%@ page language="java" %>                                                   <tr><%@ taglib uri="/WEB-IN...
View: Internationalization • Commonly referred to as i18n (I <followed by 18 letters> n) • The ability to maintain a web a...
View: i18n (cont…) • When the resource bundle class loader tries to load   MyStrings_fr, it will look for a MyStrings_fr.p...
View: i18n (cont…) • In the previous example, when the ActionServlet is loaded it will   read it’s “application” parameter...
View i18n (cont…)MyResources.properties file: MyResources_fr.properties                             file:helloworld.title=...
View i18n (cont…) • In order for web developers to get at the string resources we use   a special Struts tag called <bean:...
Resources • Struts homepage     o   http://jakarta.apache.org/struts/ • Sun’s Servlet Specification     o   http://java.su...
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  • MVC promotes modularity by sperating business rules, from program flow and User interface. An important part of web applications is presenting different views of data. Although the views may be different, the Model (or data) would stay the same. This is the heart of MVC. (e.g. Stock Data in chart/numbered) Abstraction is another key point of MVC, it allows developers the ability to expose an API which shields from database changes or other implementation changes. Also, web developers don’t have to worry about how the underlying components work, just simply how to access them. Architects will like the fact that they can produce a Use case or Activity diagram and have their developers quickly institute the program flow they have mapped out.
  • I like to think of an activity diagram as a web page. Each action state in the diagram can be thought of as a webpage along a certain trail. Some pages may have links to other trails, these links are called Activity states and they typically represent a separate activity diagram.
  • Action States and Activity State.
  • &lt;html:form action=“/logon.do” focus=“username”&gt; When the submit button is pressed, the input of this form is sent to logo.do. That is dispatched to the ActionServlet, which then looks up the mapping to the LogonAction object. &lt;html:text property=“username” …/&gt; This will look up the form object associated with this page and will display the value of that property in an html text input box.
  • Go over example: web.xml: 1) database servlet, initializes a hashed database the is present throughout the application 2) Defines the action servlet and associates it with the .do extension. struts-config.xml: 1) Set’s up the form beans, name property for action==formbean name 2) Set’s up any global forwards 3) Actions: path is the URI which will tell the ActionServlet to pass the request to this controller type is the class name of the controller which services these requests name is the name of the form bean we setup to handle input scope
  • 72185-26528-StrutsMVC

    1. 1. Introduction to MVC and the Jakarta Struts Framework By Gyanendra Dwivedi Software Consultant
    2. 2. What is MVC • The Model View Controller design pattern is a technique used to separate Business logic/state (the Model) from User Interface (the View) and program progression/flow (the Control). • This pattern is very useful when it comes to modern web development: o The majority of modern, high usage websites are dynamically driven. o People well skilled at presentation (HTML writers) seldom know how to develop back-end solutions and visa versa. o Separating business rules from presentation is good no matter what environment you develop in be it web or desktop.Feb 11, 2003 Introduction to Jakarta Struts – Gyanendra Dwivedi
    3. 3. What is MVC (cont…) • The View o The view is how data is represented to the user. For instance the view in a web application may be an HTML page, an image or other media • The Model o The model represents the data in the system. For instance, a model might represent the properties associated with a user’s profile • The Controller o The controller is the glue between the model and the view. It is responsible for controlling the flow of the program as well as processing updates from the model to the view and visa versaFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    4. 4. What is MVC (cont…) User View Controller ModelFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    5. 5. What is MVC (cont…) User View Controller ModelFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    6. 6. Benefits of MVC • Promotes Modularity • Multiple views • Abstraction • Allows application to be defined in a flow-chart, use-case or activity diagram which is more easily transferred to implementation.Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    7. 7. Designing an MVC App. • Let’s try designing an application from scratch • The application will be a message board where users can view messages and post messages • The application will require that a user has an account setup with the message board before posting a message, but an account is not required for viewing a message. • The basic components we must implement for this message board are: o User profile o Logon/authentication o Message List o MessageFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    8. 8. Activity Diagrams • I find the UML activity diagrams to be the most useful construct for describing program flow. • Activity diagrams are much like flow charts, except a few new constructs are added such as describing concurrency as well as recursive elements. • Each node of the Activity Diagram can be thought of as a call to a “Controller” asking it to take you to the next step in the applicationFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    9. 9. Activity Diagram: Create Profile Create Profile Create Edit Profile [invalid] == Profile() [else] Save ProfileFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    10. 10. Activity Diagram: Logon Login FormCreateProfile() [new user] == Logon() [else]Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    11. 11. Activity Diagram: View Msg. Show Msg. Board Select Select Msg. == Msg() Display Msg.Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    12. 12. Activity Diagram: Edit Msg. Select Msg() Logon() [expired session] [else] Create New Msg. Edit Msg. [else] Save Msg.Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    13. 13. Jakarta Struts • In order to take advantage of the MVC design pattern in your applications a considerable framework must be in place. • Today we are going to learn about such a framework named Struts • Struts is an open source MVC framework developed by the Apache Jakarta project group. • Struts allows JSP/Servlet writers the ability to fashion their web applications using the MVC design pattern. • By designing your web application using Struts you allow: o Architect the ability to design using the MVC pattern o Developer to exploit the Struts framework when building the app. o Web designer to be sheltered from learning how to program http://jakarta.apache.org/struts/Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    14. 14. Struts (cont…) • Struts takes the grunt work out of developing an MVC based web app. • The Struts framework provides a plethora of canned objects which can facilitate fundamental aspects of MVC, while allowing you to extend further as need suites • Struts allows you to configure a lot of the default framework objects through xml configuration files. • Our examples will use the Struts framework using the Apache Tomcat Servlet/JSP container. http://jakarta.apache.org/tomcat/ • Tomcat 3.2.1 provides a reference implementation of the Servlet specification 2.2 and JSP specification 1.1Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    15. 15. Struts (cont…) • The components of MVC in Struts: o The View is usually defined as a JSP or HTML page. o The Model is usually defined as a Java object (sometimes called a bean). o The Controller is defined by a Java object which extends the org.apache.struts.action.Action class. The Action class is at the heart of the Struts framework.Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    16. 16. Struts: The Model • To define a model in Struts is simple. Just create a java class and provide some functions. • Your model classes should be coded independent of the Struts framework to promote maximum code reuse by other applications (i.e. if you have to reference javax.servlet.* class in your model, you are doing something wrong) • Struts provides some default Model components, the most important of which is ActionForm. • If you create a Model class by extending the Struts ActionForm class, Struts will o Ensure your form bean is created when needed o Ensure form submittal directly updates your form object with the inputted values o Your controller object will be passed the form beanFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    17. 17. The Model (cont…) • Struts will only handle models in an automatic fashion if you extend org.apache.struts.action.ActionForm • Most likely you will have already built model object such as Customer, Employee, etc… • Because you don’t want to tie your existing model objects to the Struts framework by having to extend ActionForm, Struts provides a org.apache.struts.util.PropertyUtils class that has a static method called copyProperties • When your controller receives a form, it can simply call PropertyUtils.copyProperties(myModel, form) to copy all form properties to your original model object.Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    18. 18. Model: ActionForm import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.struts.action.ActionForm; import org.apache.struts.action.ActionMapping; import org.apache.struts.upload.FormFile; public class LogonForm extends ActionForm { protected String userName; protected String password; public void setUserName(String userName) { this.userName = userName; } public void setPassword(String password) { this.password = password; } //There would also be getters for these properties }Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    19. 19. Model: Action Form (cont…) • When this “LogonForm” is associated with a controller, it will be passed to the controller whenever it’s service is requested by the user. • JSP pages acting as the view for this LogonForm are automatically updated by Struts with the current values of the UserName and Password properties. • If the user changes the properties via the JSP page, the LogonForm will automatically be updated by Struts • But what if the user screws up and enters invalid data? ActionForms provide validation… • Before an ActionForm object is passed to a controller for processing, a “validate” method can be implemented on the form which allows the form to belay processing until the user fixes invalid input as we will see on the next slide...Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    20. 20. Model: ActionForm Validationpublic ActionErrors validate(ActionMapping mapping,HttpServletRequest request) {if (“”.equals(this.userName)) {ActionErrors aes = new ActionErrors();aes.add(new ActionError(“error.username.invalid”));return aes;}}Typically what will happen is Struts will see that errors are being returnedand will forward the user to a jsp page that has been setup as the “failure”page.Usually, the errors result from bad input on a form, so the failure page willbe set to the original form and any <html:errors> tags which are found arereplaced with the contents of the ActionErrors returned from the validatemethod. Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    21. 21. The Controller • The controller is the switch board of MVC. • It directs the user to the appropriate views by providing the view with the correct model • The task of directing users to appropriate views is called “mapping” in struts. • Luckily, the Struts framework provides a base object called org.apache.struts.action.ActionServlet. • The ActionServlet class uses a configuration file called struts- config.xml to read mapping data called action mappings • The ActionServlet class reads the incoming URI and tries to match the URI against a set of action mappings to see if it can find a controller class which would be willing to handle the request • This process is described in a diagram on the following pageFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    22. 22. Controller (cont…)http://myhost/authorize.doServer configured to pass *.do extensions toorg.apache.struts.action.ActionServletvia a web.xml configuration fileActionServlet object inspects the URI and tries to match itagainst an ActionMapping located in the struts-config.xml file.Instance of appropriate Action class is found and it’s perform() method iscalled Action object handles the request and returns control to a view based where the user is within the flow of the applicationFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    23. 23. Controller: Samplepublic class LogonAction extends Action {public ActionForward perform(ActionMapping mapping, ActionForm form,HttpServletRequest request,HttpServletResponse response) {LogonForm myForm = (LogonForm) form;if (myForm.getUserName().equals(“john”) &&myForm.getPassword().equals(“doe”)) {//return a forward to our success pagereturn mapping.findForward(”success”);} else {ActionErrors errors = new ActionErrors();errors.add("password",new ActionError("error.password.required"));this.saveErrors(errors); //Action implements this method//go back to the page that made the requestreturn (new ActionForward(mapping.getInput()));}}} Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    24. 24. Controller: Forwards • You might be wondering what mapping.findForward(“success”) means? o The mapping object passed to the Controller’s perform() method is of type ActionMapping. o When you setup your struts-config.xml file you can define forward tags that are available via the ActionMapping.findForward() method. o In the previous example, our ActionMapping object would have been loaded with values from the <action-mapping> section defined for the LogonAction controller in struts-config.xml.Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    25. 25. Controller: ActionMapping<action-mappings><action path="/logon"type="org.apache.struts.example.LogonAction"name="logonForm"scope="request"input="/logon.jsp"></action><forward name="success” path="/msgBoard.jsp"/></action-mappings>Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    26. 26. The View • The view in Struts is • JSP solves this by introducing represented by JSP pages the concept of Tag Libraries • JSP pages allows developers to o Taglibs allow web designers write java code, and access the convenience of using server side java objects in a HTML like tags web page o It lets developers program • Although this is good, we will the logic behind the tags distract ourselves from the o The attributes of the tags are reason we are using Struts and used as basic parameters MVC. that the developers will o We want to let our web page interpret, which could authors create dynamic web change the output generated pages without knowing how by the tag to program* Struts contains a series of taglibs designed to allow developers and web pageauthors theability to communication one Introduction tofacilitate dynamic web content Feb 11, 2003 another to Jakarta Struts - Gyanendra Dwivedi
    27. 27. The View: Example<%@ page language="java" %> <tr><%@ taglib uri="/WEB-INF/struts-bean.tld" prefix="bean" %> <th align="right"><%@ taglib uri="/WEB-INF/struts-html.tld" prefix="html" %> <bean:message key="prompt.password"/> </th><html:html locale="true"> <td align="left"><head> <html:password property="password"<title><bean:message key="logon.title"/></title> size="16" maxlength="16"/><html:base/> </td></head> </tr><body bgcolor="white"> <tr><html:errors/> <td align="right"> <html:submit property="submit"<html:form action="/logon.do" focus="username"> value="Submit"/><table border="0" width="100%"> </td> <td align="left"><tr> <html:reset/><th align="right"> </td><bean:message key="prompt.username"/> </tr></th><td align="left"> </table><html:text property="username" size="16" maxlength="16"/></td> </html:form></tr> </body> </html:html> Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    28. 28. View: Internationalization • Commonly referred to as i18n (I <followed by 18 letters> n) • The ability to maintain a web app in several different languages • Based on the language setup on the client side, the web app will auto-magic-ally • This is achieve in Struts through Java Resource Bundles • ResourceBundles are classes that support, among other things, String data. • Instead of having to make a ResourceBundle class for each language supported by your web app, resource bundles can be described as text files • If you had a set of strings in the French language you might make a file named MyStrings_fr.propertiesFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    29. 29. View: i18n (cont…) • When the resource bundle class loader tries to load MyStrings_fr, it will look for a MyStrings_fr.properties file and create the class based on the properties in the file • Part of the Struts framework states that when setting up the ActionServlet in the web.xml file, if you add an initialization parameter named “application” this should contain the class name of your resource file: <servlet> <servlet-name>action</servlet-name> <servlet-class> org.apache.struts.action.ActionServlet </servlet-class> <init-param> <param-name>application</param-name> <param-value>myPackage.MyResources</param-value> </init-param> </servlet>Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    30. 30. View: i18n (cont…) • In the previous example, when the ActionServlet is loaded it will read it’s “application” parameter to figure out that it should load string resources from mypackage.MyResources o If it doesn’t find a class by that name, it tries to find a MyResources.properties file o It then figures out what language the client is using. If it is different from the default, it looks up the 2 character ISO language code and tries to look for a class or .properties file by appending the ISO code to the class:  If the client is French it would look up: 1) mypackage.MyResources_fr.class If it can’t find this class it then looks for: 2) mypackage.MyResources_fr.propertiesFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    31. 31. View i18n (cont…)MyResources.properties file: MyResources_fr.properties file:helloworld.title=Hello World!menu.file=File helloworld.title=Allô monde!menu.file.open=Open menu.file=Dossiermenu.file.close=Close menu.file.open=Ouvrirmenu.file.exit=Exit menu.file.close=Fermermenu.edit=Edit menu.file.exit=Sortiemenu.edit.copy=Copy menu.edit=Rédigermenu.edit.paste=Paste menu.edit.copy=Copier menu.edit.paste=Pâte Feb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    32. 32. View i18n (cont…) • In order for web developers to get at the string resources we use a special Struts tag called <bean:message/> • e.g. <bean:message key=”helloworld.title"/> o This tag will be replaced with text from the appropriate resource bundle associated with this web app (via the “application” init param for the ActionServlet in web.xml) for the key logon.titleFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
    33. 33. Resources • Struts homepage o http://jakarta.apache.org/struts/ • Sun’s Servlet Specification o http://java.sun.com/products/servlet/download.html#specs • Sun’s JSP Specification o http://java.sun.com/products/jsp/download.html • Blue Stone’s Struts Tutorial o http://developer.bluestone.com/scripts/SaISAPI.dll/Gallery.class/de mos/trailMaps/index.jsp • My email address for questions: o craiger@tataryn.netFeb 11, 2003 Introduction to Jakarta Struts - Gyanendra Dwivedi
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