meljun_cortes_jedi slides-web programming-chapter07-mvc i intro and struts

8/3/2019 MELJUN_CORTES_JEDI Slides-Web Programming-Chapter07-MVC I Intro and Struts

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Introduction to MVC and theStruts Framework

Web Programming


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Introduction to Model-View-

Controller Architecture

2Web Programming

An architectural pattern that has proven to be effective indevelopment projects.

Motivation In any application, the part of the code most likely to change is the

portion regarding the user interface. Having your business logictightly coupled with your user interface makes the process of makingchanges to the interface more complex and error-prone.

Solution The MVC pattern provides a solution to these problem by

partitioning the application into Model, View, and Controllercomponents, decoupling them from each other while providing a setof recommendations about their interactions.


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- Renders the models- Requests updates from models

- Sends user gestures to controller- Allows controller to select view

- Encapsulates application state- Responds to state queries- Exposes application functionality- Notifies views of changes

- Defines application behavior- Maps user actions to model updates

- Selects view for response- One for each functionality


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View

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Comprises all the details of user interface implementation.

Only this layer interacts with the user.

Advantages:

Easier to include the presence of a separate design group into thedevelopment team.

Makes it possible to provide for multiple interfaces to the application.


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Controller

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Contains details about program flow/screen transition.

Also responsible for capturing events generated by the userfrom the View layer and possibly updating the Model

components using user-provided data. Advantages:

View components can be designed such that they do not need to beaware of each other.

Updates of th model components are removed from the presentation

layer.


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It is not to say that the MVC pattern comes with all benefitswithout any side-effects. Partitioning the application into

three separate components results in an increase incomplexity. For small applications which do not benefit fromloose coupling of the Model, this may be a stumbling blockusing of this pattern. However, it is probably best to keep inmind that applications often start out small and grow intocomplex systems, so loose coupling should always beaimed for.


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The Model 2 Architecture

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MVC Architecture for the Web

MVC Architecture slightly modified and adapted for the useof web applications

Model 2 applications typically have the following: Front Controller, a servlet controller that:

Provides a single point of access to the rest of the application.

Is responsible for providing central management of application flow.

Can also provide such services as security handling and user management.

Typically uses XML configurations to determine application flow and commandprocessing.

Commonly makes use of helper components that serve as Command objects.


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The Model 2 Architecture

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Implementing such an architecture can be made easierthrough the use of existing third-party frameworks. Theseframeworks provide a lot of the plumbing details (requestforwarding, reading of configuration files, etc.) so that we

can concentrate on more important matters. They alsoprovide additional useful functionality.


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Struts

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An open-sourceframework providedand managed by theApache Software

Foundation.


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MVC and Struts

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Objects provided by Struts for:

Model

View

struts-html Controller

ActionServlet

Action

ActionForm

struts-config.xml


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ActionServlet

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At the center of the controller implementation of the Strutsframework is the ActionServlet.

Serves as a Front Controller servlet and provides a singlepoint of access to the rest of the web application.

Contains the logic behind client request handling.

Knows all details by reading from an XML configuration file,named struts-config.xml.

Which Action to call to handle which request

Which view component should next be called

Provided to us ready-for-use by the Struts framework; onlyneeds to be configured properly.


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web.xml

actionorg.apache.struts.action.ActionServletapplication

ApplicationResourcesconfig/WEB-INF/struts-config.xml...

action*.do


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Action

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All Action objects define a method called execute(), and it isthis method that is called by the ActionServlet to handleclient requests.

The Struts framework only provides developers with thebase Action class; to include Action objects as requesthandlers in their application developers must subclass thisbase class and provide an implementation for the executemethod.


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import org.apache.struts.action.*

public class LoginAction extends Action {public ActionForward execute(ActionMapping mapping,

ActionForm form,HttpServletRequest request,HttpServletResponse response)throws Exception {// cast the generic ActionForm object// to the specific ActionForm implementation// configured for this Action

LoginForm loginForm = (LoginForm)form;

// Retrieve the user -specified data.String loginName = form.getLoginName();String password = form.getPassword();

// Create the business object that will handle the request.UserService service = new UserService();

user = service.login(loginName, password);

// if user does not exist login failed forward// the user to an error pageif (user == null) {return mapping.findForward("failure");}


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// Store the result into session scope for use in the rest of the// application, using a predefined constant

HttpSession session = request.getSession();session.setAttribute(ApplicationConstants.USER_OBJECT, user);

// user has successfully logged in. Forward the user to the// rest of the application.

return mapping.findForward("success");}

}


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Action

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The only activities that an Action should perform are:

Retrieval of user-provided information from the associatedActionForm bean.

Translating form data into parameters required by the business

objects that implement the functionality.

Retrieve the result from the operation of the business object andfrom there determine the next view the user should be forwarded to.

Optionally, store the data results of the business operation into thesession or request objects for use by the rest of the application.


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Action

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The framework will only instantiate a single copy of theAction objects and use it to facilitate all requests. Thismeans that we must always:

Code the Action to be thread-safe.

Make sure that we always use local variables not instance variables.


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Action

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Action instances are able to instruct the ActionServlet whichview component to delegate the response to by returninginstances of ActionForward objects. Actions have access tothese ActionForward objects through the use of an

ActionMapping object which encapsulates the data of logicalpath mappings for each Action. These mappings are readfrom the configuration file by the ActionServlet, which is thenresponsible for passing the necessary ActionMapping to theAction. So, to instruct the ActionServlet to pass control to a

logical mapping named success, our Action performed thefollowing statement:

return mapping.findForward("success");


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ActionForm

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Instances of this class are used to facilitate the retrieval ofdata from user-populated forms to the Action instanceshandling those form events.

Each instance of an ActionForm represents a form or aseries of forms.

They define properties that correspond to the form elements of theform(s) they represent, and expose them using publicly accessiblesetters and getters. Actions which then need the data from the formssimply call on the ActionForm instance's getter methods.

Struts provides the base class definition; developers havethe responsibility of creating their own implementations.


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import org.apache.struts.action.*;

public class LoginForm extends ActionForm {private String loginName;private String password;

public String getLoginName() {return loginName;

}

public void setLoginName(String loginName) {this.loginName = loginName;}

public String getPassword() {return password;

}

public void setPassword(String password) {this.password = password;}

}


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ActionForm

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Some things to remember when coding ActionForms:

Define properties (with associated get and set methods) for eachelement in the form it will represent.

Do NOT place any business logic in the ActionForm. They are

meant merely to transfer data between View and Controllercomponents and as such is not suited for business logic.

Optionally include a validate method to perform validation of databefore control even passes to the Action.


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struts-config.xml

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Serves as the configuration file for the components of theStruts framework.

Here, we are able to define which action is called for whichrequest, which form component to use for each action, andthe mapping of logical names to actual paths, among otherthings.


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struts-config.xml

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Example


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struts-config.xml

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(Example continued)

Defines the XML file as a configuration file for use by the Struts framework.Excluding this line, or mistyping it, will result in errors when the application loads.

The root element of the configuration file.

Marks the start and end of definitions of ActionForm instances.

Defines an ActionForm instance that can be used by the application. It has twoattributes:

name: The logical name to be associated with the ActionForm class.

type: The fully qualified class name of the ActionForm class.


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struts-config.xml

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(Example continued)

Marks the start and end definitions of actions and their mappings

Defines an instance of an Action object for use by the application. Most actionelements will implement the following attributes:

path: The context-relative path to be used by this Action. Any request to this path results in the definedAction being called.

type: The fully qualified class name of the Action class.

name: The name of the element to use with this action.

scope: The context scope where our ActionForm can be accessed. This dictates where the ActionServlet

will store the instance of the ActionForm.


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struts-config.xml

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(Example continued)

Actions can have zero to many forward elements. This element defines a logicalmapping between a name and an actual path in our application. It has thefollowing attributes:

name: The logical name of the forward which can be used by the Action instance

path: The path to the view component associated with this forward


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Controller

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Summary of things to do for the controller layer:

For one-time setup:

Configure the ActionServlet in your app's deployment descriptor

For each form handler to be added to the application:

Create an ActionForm object that will represent all data gathered from the form

Create an Action object which, in its execute method, defines how the form willbe handled

Create a configuration entry for the ActionForm object in struts-config.xml, insidethe section

Create a configuration entry for the Action object in struts-config.xml, inside the


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View

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Struts can make use of any presentation layer technology,though in most cases JSPs and/or HTML is used.

Struts provide for this layer a set of tag libraries that allowsus to make use of Struts features for automatic form

population and validation.


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struts-html

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Mimics a lot of functionality we have come to expect fromstandard HTML tags but provides extra features.

Most often used when creating forms to be used for theapplication.

Example:

Login Page

Login Page
User Name :
Password :


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struts-html

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(Example Continued)

This was the form used in our previous example.

Notice how standard HTML elements like , , , and

were replaced by tags from the struts-html library.


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struts-html

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Renders an HTML form.

Each form tag is associated with an action mapping defined by theaction attribute. The value inside this action attribute specifies the

path of the action mapping.

Looking back at the configuration file that we used for the previousexample:

......

Note that the value in the path attribute matches that of the value in the actionattribute in the tag. This indicates that this form, when submitted,will be presented to the LoginAction for handling.


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struts-html

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( continued)

One of the conditions for a valid html:form tag is that each of thefields it contains must be defined in the ActionForm specified for theparticular action mapping.

Other possible attributes: method: Can either be POST or GET. Defines the HTTP method to be used

when submitting the form.


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struts-html

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Renders a standard HTML text input field.

Attributes:

property: Specifies which property in the associated ActionForm it is bound to.

size: Defines the size of the text field to be displayed.

maxlength: Defines the maximum length of the value user can input.

Renders a standard HTML password field.

Shares the attributes of .


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struts-html

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html:text and html:password are bound to properties of theActionForm instance associated with the form. It means thatthe values entered by the user into these fields willautomatically be set into the bound properties in the

ActionForm object. If there are previous existing values inthe ActionForm object (the form has already been accessedbefore), the form fields will automatically be prepopulatedwith values from the ActionForm object.


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struts-html

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Other tags in the struts-html tag library:

Renders an HTML hidden form field.

Sample usage:

Renders a HTML radio check box.

Sample usage:


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struts-html

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(Other tags continued)

,

html:select is used to render a drop-down list box. The options for this list box arespecified using html:option tags.

Sample usage:0 Label1 Label2 Label

,

Are used to render checkbox field and textarea field, respectively.


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View

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Summary of things to do for the View layer:

For one-time setup:

Configure the tag libraries for use in the application's deployment descriptor.

Place the JAR files containing the implementation for the tag libraries in the

appliaction's WEB-INF/lib directory .


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View

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(Summary continued)

For each form to be created:

Add a taglib directive to the JSP page to enable use of the struts-html tag library.

In place of the standard tag, use . Specify in its action

attribute the path of the Action that will handle the form. In place of the HTML field tags (, etc), make use of the tags

included in the struts-html tag library that perform the same functionality(, etc).

Make sure that all input fields present in the form definition are present asproperties in the ActionForm object associated with this request. It is NOTnecessary that all properties in the object be displayed as fields, but it is required

that all fields are present as properties. Remember to close the tag.


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Looking at Things as a Whole

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Example: A User Logging In

Before the user ever enters the site, the ActionServlet takes in theconfiguration file and reads the details. So, when the user accessesthe login form, the framework already knows the associatedActionForm that will store its details and the Action that will handleform submission.

When the login page loads, the struts-html tags that we have usedattempts to render the HTML fields. If the ActionForm for this formdoes not exist, the page will not display. If there are more fields inthe form than there are properties in the ActionForm to back them

up, the page will also not display. If the ActionForm does exist, thetags will see if there were any previous values stored in theActionForm. If there are, the form fields are prepopulated with data.If not, the form fields are left empty and the user will see a blankform.


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Looking at Things as a Whole

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(Example continued)

When the form is submitted, the values in the form fields areautomatically set into the ActionForm object by the StrutsFramework. This object is then passed into the appropriate Actionhandler, along with the ActionMapping object that reflect mappingdetails as specified in the configuration file.

The Action object performs its handling, then tells the ActionServletwhere to go next by specifying one of the forwards configured in themapping. The ActionServlet then redirects the user to that page.

meljun_cortes_jedi slides-web programming-chapter07-mvc i intro and struts

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