introduction to mvc and the jakarta struts framework by gyanendra dwivedi software consultant

Introduction to MVC and the Jakarta Struts

FrameworkBy Gyanendra Dwivedi

Software Consultant

Feb 11, 2003 Introduction to Jakarta Struts – Gyanendra Dwivedi

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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:– The majority of modern, high usage websites are

dynamically driven.– People well skilled at presentation (HTML writers) seldom

know how to develop back-end solutions and visa versa.– 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

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What is MVC (cont…)• The View

– 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– The model represents the data in the system. For

instance, a model might represent the properties associated with a user’s profile

• The Controller– 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 versa


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What is MVC (cont…)

User

Controller

Model

View


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What is MVC (cont…)

User

Controller

Model

View


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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.


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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:– User profile– Logon/authentication– Message List– Message


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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 application


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Activity Diagram: Create Profile

Create Profile

Save Profile

Edit Profile[invalid]

[else]

== Create Profile()


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Activity Diagram: Logon

== Logon()

Login Form

[else]

Create Profile()

[new user]


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Activity Diagram: View Msg.

Show Msg. Board

Select Msg.

Display Msg.

== Select Msg()


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Activity Diagram: Edit Msg.

Logon()

Edit Msg.

Save Msg.

[expired session]

[else]

Select Msg()

Create New Msg.

[else]


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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:– Architect the ability to design using the MVC pattern– Developer to exploit the Struts framework when building the

app.– Web designer to be sheltered from learning how to program

http://jakarta.apache.org/struts/


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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 1.1


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Struts (cont…)• The components of MVC in Struts:

– The View is usually defined as a JSP or HTML page.

– The Model is usually defined as a Java object (sometimes called a bean).

– 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.


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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– Ensure your form bean is created when needed– Ensure form submittal directly updates your form object with the

inputted values– Your controller object will be passed the form bean


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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.


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Model: ActionFormimport 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

}


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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...


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Model: ActionForm Validation

public 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 returned and 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 will be set to the original form and any <html:errors> tags which are found are replaced with the contents of the ActionErrors returned from the validate method.


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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 page


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Controller (cont…)

http://myhost/authorize.do

Server configured to pass *.do extensions to org.apache.struts.action.ActionServlet via a web.xml configuration file

ActionServlet object inspects the URI and tries to match it against an ActionMapping located in the struts-config.xml file.

Instance of appropriate Action class is found and it’s perform() method is called

Action object handles the request and returns control to a view based where the user is withinthe flow of the application


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Controller: Sample

public 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()));

}

}

}


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Controller: Forwards• You might be wondering what

mapping.findForward(“success”) means?– The mapping object passed to the Controller’s perform()

method is of type ActionMapping.– When you setup your struts-config.xml file you can

define forward tags that are available via the ActionMapping.findForward() method.

– 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.


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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>


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The View• The view in Struts is

represented by JSP pages• JSP pages allows developers

to write java code, and access server side java objects in a web page

• Although this is good, we will distract ourselves from the reason we are using Struts and MVC.– We want to let our web

page authors create dynamic web pages without knowing how to program

• JSP solves this by introducing the concept of Tag Libraries – Taglibs allow web

designers the convenience of using HTML like tags

– It lets developers program the logic behind the tags

– The attributes of the tags are used as basic parameters that the developers will interpret, which could change the output generated by the tag

* Struts contains a series of taglibs designed to allow developers and web page authors the ability to communication one another to facilitate dynamic web content


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The View: Example

<%@ page language="java" %><%@ taglib uri="/WEB-INF/struts-bean.tld" prefix="bean" %><%@ taglib uri="/WEB-INF/struts-html.tld" prefix="html" %>

<html:html locale="true"><head><title><bean:message key="logon.title"/></title><html:base/></head><body bgcolor="white">

<html:errors/>

<html:form action="/logon.do" focus="username"><table border="0" width="100%">

<tr> <th align="right"> <bean:message key="prompt.username"/> </th> <td align="left"> <html:text property="username" size="16" maxlength="16"/> </td> </tr>

<tr> <th align="right"> <bean:message key="prompt.password"/> </th> <td align="left"> <html:password property="password"

size="16" maxlength="16"/> </td> </tr>

<tr> <td align="right"> <html:submit property="submit"

value="Submit"/> </td> <td align="left"> <html:reset/> </td> </tr>

</table>

</html:form>

</body></html:html>


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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.properties


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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>


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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– If it doesn’t find a class by that name, it tries to find a

MyResources.properties file– 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.classIf it can’t find this class it then looks for:2) mypackage.MyResources_fr.properties


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View i18n (cont…)

MyResources.properties file:

helloworld.title=Hello World!menu.file=Filemenu.file.open=Openmenu.file.close=Closemenu.file.exit=Exitmenu.edit=Editmenu.edit.copy=Copymenu.edit.paste=Paste

MyResources_fr.properties file:

helloworld.title=Allô monde!menu.file=Dossiermenu.file.open=Ouvrirmenu.file.close=Fermermenu.file.exit=Sortiemenu.edit=Rédigermenu.edit.copy=Copiermenu.edit.paste=Pâte


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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"/>– 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.title


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Resources• Struts homepage

– http://jakarta.apache.org/struts/

• Sun’s Servlet Specification– http://java.sun.com/products/servlet/download.html#specs

• Sun’s JSP Specification– http://java.sun.com/products/jsp/download.html

• Blue Stone’s Struts Tutorial– http://developer.bluestone.com/scripts/SaISAPI.dll/

Gallery.class/demos/trailMaps/index.jsp

• My email address for questions:– [email protected]

introduction to mvc and the jakarta struts framework by gyanendra dwivedi software consultant

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